Septarian Concretion from Khambhat

My friend Bhushan Panse, who is a geology enthusiast and an avid rock and mineral collector, handed this specimen to me over a coffee meeting. He had bought it from a mineral supplier from Khambhat, Gujarat.

I commented that it is a septarian concretion. These hard ellipsoidal or oval shaped lumps form in mud and silt layers by the precipitation of calcite  around a nucleus. Khambhat and many other parts of Gujarat are underlain by Mesozoic and Cenozoic age sedimentary rocks. The process of concretion formation would have taken place at shallow burial depths when these sediments were still porous and water saturated. Mineral deposition in pore spaces often takes place in concentric layers. The calcium carbonate comes from saturated marine pore water or is derived from shells as they start dissolving during shallow burial. Notice the rust to brown color of the concretion. It is likely due to the presence of iron oxide and hydroxides which formed in the pore spaces from the iron contained in clay minerals.

The term Septarian Concretion refers to the radiating cracks or Septaria (derived from Septum). Cracks come in a variety of shapes. There are radiating cracks as seen in this specimen. These cracks are wider near the center and taper outwards. Other concretions may show concentrically oriented cracks, or overlapping sigmoidal shapes. Cracks may intersect, pointing to multiple cracking events. They are filled with either calcite or silica. The crystals filling these cracks are sometimes broken and displaced, and cracks may contain mud and silt. These features indicate a variety of stresses at play in concretions interiors. 

There are many ideas on how these cracks form. They have been interpreted as shrinkage cracks due to desiccation and hardening of mud. Dehydration during chemical transformation of clay minerals is another explanation.  A third hypothesis links the formation of cracks to gas expansion released during putrefaction of organic material. 

Sedimentologist Brian Pratt has offered another novel explanation. He proposed that these cracks result due to shaking of sediment during synsedimentary earthquakes. Shaking during ground motion results in variable stress fields in the interior of the concretion forming a large variety of crack geometries. These concretions may be preserving signals of  seismicity affecting that sedimentary basin!

Here is his compilation of the large variation in septarian concretion cracks from various sedimentary basins across Canada.

 Source: B. Pratt: Septarian concretions: internal cracking caused by synsedimentary earthquakes

A geologist friend who worked with the Geological Survey of India suggested another intriguing explanation. Parts of the region near Khambhat experienced explosive volcanic activity towards the waning phases of Deccan Volcanism. Ash expelled from volcanoes can coat small broken lava fragments forming lumps known as  'áccretionary lapilli'. Aggregations of ash and pyroclastic material if larger than 64 mm are known as volcanic bombs. This concretion fits the size range of a bomb. The dark fragments in the center of the concretion do resemble a fine grained igneous rock. A closer examination under a microscope is needed for a confirmation of its origin.

It is fun to examine hand specimens that friends collect from various part of the world and try to identify the rocks and minerals. But often a clear cut answer is not possible due to the need for additional information from a higher resolution or the chemical makeup. But a guessing game over coffee is always welcome. 

Geodes, nodules, and concretions found in volcanic and sedimentary rocks are mystery objects. You never know what you will see inside when you break open one of these lumps. There may be an array of perfectly faceted purple amethyst crystals and multicolored calcite. Or a trapped fossil. Or a crack network filled with bright and shiny calcite and quartz. These crystal rich interiors give us important information on the composition of fluids which react with rock at many different times during their geologic history. This water rock interaction is of interest to mineralogists and  economic geologists who want to understand the history of fluid flow through sedimentary basins and the conditions that lead to the concentration and deposition of metals. 

Geological investigation at all scales inform us about how the earth works. One can stand and gape at great mountain ranges and wonder about the movement of tectonic plates. But you can also crack open a rather dull colored lump from a shale and marvel at its insides, all telling a story of groundwater flow and chemical reactions, and who knows, past earthquakes as well. 

Liesegang Banding In Proterozoic Badami Sandstone

The Chalukya era (6th-8th CE) rock cut caves and sculptures at Badami in Karnataka are an archeological wonder. But there is plenty of geology there to admire. In January 2020, I spent some time wandering through Badami. The sandstone layers are 900 million years old river deposits. I wrote a long post about them, explaining the primary sedimentary structures that one can observe in these rocks, and what they tell us about the water depths and currents during deposition of the sediment. 

But these primary structures, i.e. sedimentary layer orientations that form during deposition, are not the only interesting features of these rocks. Chemical reactions in these sediments after their burial has overprinted an intriguing fabric on to the rock.

In the picture a very distinct dark and light banding is seen in one of the Badami rock surfaces. This is Liesegang banding. 

The dark bands are rich in iron oxide. The lighter bands have little or no iron oxide. Such banding forms by the mobilization of ions from one location in the sediment and their precipitation at another. Ions diffuse along a concentration gradient in the water filled pore spaces. Robert A. Berner's book, Early Diagenesis: A Theoretical Approach, has a good explanation for the formation of Liesegang banding. I am reproducing that below.

"Mobilization of different components of a substance can occur at two or more different locations. The best example of this is the formation of Liesegang banding.In Liesegang banding we have the interdiffusion of two dissolved ions which cab react with one another to form a relatively insoluble solid. The two ions can come from different sources and when their concentrations at a given site build  up, via diffusion, to sufficiently high values, precipitation of the insoluble solid occurs. This precipitation suddenly lowers concentration in the neighborhood of the solid, and as a result the diffusion profiles become altered. Continued interdiffusion results in a new build-up in concentration and precipitation at another site. Depending on the geometry of the situation, this process may result in Liesegang rings (3-dimensional), tubes (2-dimensional), or layers (1-dimensional). A common example of Liesegang phenomenoa are rhythmic bands of iron oxides often found in sandstones. In this case precipitation is most likely brought about by the interdiffusion of dissolved Fe++ (from an anoxic) source) and dissolved O2 (from an oxic source). Where the Fe++ and O2 meet, Liesegang banding occurs".

The iron (Fe++) would already have been present in the sediment perhaps in discrete grains of pyrite (FeS2), or trapped in carbonaceous plant debris.   Rainfall fed groundwater is the common source of oxygen.  As pyrite gets oxidized it releases Fe++ and sulphur ions. The ferrous ions get oxidized to ferric ions (Fe+++). These then nucleate to form iron oxide or hydroxides. Rapid diffusion of ions towards a growing crystal will eventually lower the concentration of ferric ions in the region surrounding the grain to below the nucleation threshold, at which point crystal growth stops. This threshold is reached at a different location where pyrite oxidation is releasing a fresh supply of Fe++. At this new location the concentration of ferric ions build up again to levels where they start nucleating into iron oxide. This migration of zones of dissolution (of pyrite) , diffusion, and nucleation results in the distinct banding. I've summarized this explanation from a paper by P. Ortoleva and colleagues on redox (reduction-oxidation) front propagation and formation of mineral banding.

Formation of redox fronts during the burial of a sedimentary rock can be economically important. For example, a certain type of sandstone hosted uranium deposit known as 'roll-front' occur where oxidizing fluids containing dissolved uranium meet reduced components such as pyrite or organic matter. 

Here is another close up of these Liesegang bands. They have a ring or a tube like geometry. The cross bedding indicated by the arrow is a primary structure formed by the movement of sand sculpted into ripples or waves on the river bed. The Liesegang bands have been imprinted over the cross beds subsequently. 

The chemical reactions that occur in sediment after their deposition are of great interest to geologists.  They play a large role in the reorganization of porosity and permeability through the dissolution and re-precipitation of minerals.Throughout the history of a sedimentary basin, fluids move through these pore networks mobilizing elements, and under favorable conditions, enriching them at particular locations. Geologists prospecting for metal and hydrocarbon deposits want to understand this process.

Permian Seafloor Gardens Of Glass

In Metazoa:Animal Minds and the Birth of Consciousness, author Peter Godfrey-Smith describes the Hexactinellida, a group of sponges that construct hard parts made of silicon dioxide as a support for its soft tissue. In an earlier post I had written briefly about amorphous varieties of silica. The Hexactinellidae's skeleton is made up of opal, denoted by the chemical formula SiO2.nH2O. Sponges put together their skeleton using a variety termed opal-A , the A indicating amorphous. Over geologic time the amorphous opal-A often transforms by expelling water and re configuring the geometry arrangement of silicon and oxygen atoms to opal-CT and chalcedony, both silicon dioxide varieties showing the first glimmer of a crystalline structure.

Hexactinellida are popularly called the glass sponges because of their transparent silica frame. The basic elements of this skeleton are tiny rods or spicules which are joined to form dagger, star or snowflake like shapes. These then group together to form a hard mesh that supports the soft tissue. Upon death, the silica skeleton disintegrates, leaving a carpet of spicules on the sea floor. 

The sketches below are from Godfrey-Smith's book. They are drawings by Rebecca Gelernter of  sponges collected on the Challenger expedition of the 1870's.

One fascinating function of these glass elements could be as collectors of light. Sponges often have colonies of photosynthetic organisms like diatoms living inside them. The speculation is that the glass channels light energy into the interior of the sponge body, which the diatoms use as a power source for photosynthesis.

Glass gardens on the sea floor is an evocative way to describe these sponge communities. And occasionally in geologic history these gardens have proliferated on a scale that is simply hard to imagine. Some time back I read a very interesting paper by Edward J. Matheson and Tracy D. Frank on Late Permian age (~260 million years old) sedimentary rocks deposited on the northwestern shores of the supercontinent Pangea. Different sedimentary rock types were deposited in this long lived basin. One distinct layer, termed the Tosi Chert, contains significant amounts of chalcedony and chert. A closer examination revealed that these two silicon dioxide minerals were derived from a siliceous sponge precursor.

Scattered through these Permian rocks are 'ghosts' of spicules. The Tosi Chert was once a glass sponge garden colonizing a gently sloping sea floor.  It was staggering in scale. These sponge meadows extended over 75,000 sq km. To the east of these sponge habitats lay an arid Laurentian desert, Laurentia being the northern continent which had joined the southerly placed Gondwana to form the supercontinent Pangea. To the west was the subtropical epicontinental Phosphoria Sea. An epicontinental sea is a shallow sea that floods the interiors of continents during times of a global sea level high. Since siliceous sponges were the dominant benthos these depositional systems are called glass ramps, the latter term indicating a uniformly sloping sea bed. The paleogeographic map below shows the position and range of the  'spicule belt' (in orange) on the northwestern edge of Pangea.  The pale pink area is the desert.

Source: An epeiric glass ramp: Permian low-latitude neritic siliceous sponge colonization and its novel preservation (Phosphoria Rock Complex) Edward J. Matheson and Tracy D. Frank

The Tosi sponge communities lived during a time of sea level rise. The sedimentary variation within the Tosi Chert indicates that sponges occupied environments  ranging from subtidal settings to near shore tidal flats. In the open ocean subtidal regions the sediment was mostly sponge debris. Nearer to the shore the environments were more variable. Calcium carbonate mineralizing organsims such as molluscs lived in patchy zones. Abiogenic ooids formed in some areas. In other regions, currents transported quartz detritus from adjacent areas.  Wind blown silt size mica and iron oxide particles sourced from the eastern deserts mixed with the biogenic sediment. Landward, in shallow ponds and depressions, layers of gypsum precipitated from saline waters. 

These environments of deposition of the Tosi Member are depicted in the block graphic below. 

Source: An epeiric glass ramp: Permian low-latitude neritic siliceous sponge colonization and its novel preservation (Phosphoria Rock Complex) Edward J. Matheson and Tracy D. Frank

These conditions persisted for hundreds of thousands of  years. Eventually, sea level began to fall and the sponge communities began to die out. Calcareous biota replaced the silica sponges. The glass gardens were buried under layers of lime sediment.

Like an artist dismantling a patiently constructed exhibit of installation art, nature relentlessly ground up the delicate glass sponges and transformed them into rock. But this change took its own interesting route. 

As sea level dropped, a mosaic of tidal flats and lagoons developed. In the arid climate, high rates of evaporation resulted in the development of hypersaline magnesium rich brines. These denser pools of water percolated downwards through the shallow buried silica rich sediment. The magnesium calcium carbonate mineral dolomite started precipitating within the sponge rich sediment. Along with dolomite, the calcium sulphate mineral gypsum formed at places. 

The dolomite rich sediment then underwent another transformation. The opal skeletons of the sponges started dissolving. The released silica however did not diffuse away in to the open sea. Rather, the high amounts of released silica created zones of silica supersaturation within the pore spaces of the sediment resulting in the precipitation of chalcedony and chert. Silica got redistributed within the Tosi sediment package, first dissolving and then reprecipitating a few millimeters away. The new silica minerals were not spread evenly but formed compact masses giving the evolving rock a nodular appearance.    Here and there the original shapes of the sponge spicules were preserved, although they were no longer made up of opal, having being replaced by chalecdony and chert. 

The photomicrographs show examples of dolomite and silica nodule replacement of the original sponge skeletal debris. The pale area in the image to the left is a chert nodule with a diffuse boundary that gives way to a darker dolomite matrix. The image to the right shows a bioturbated dolomite rock with some chert replacement. Tiny lath shaped particles are ghosts of sponge spicules.

 Source: An epeiric glass ramp: Permian low-latitude neritic siliceous sponge colonization and its novel preservation (Phosphoria Rock Complex) Edward J. Matheson and Tracy D. Frank

Today the Tosi Chert is not that attractive or spectacular rock to look at. It is a few meters thick, has a grey to red to purple color and is made up mainly of  silica nodules and dolomite with minor amounts of quartz, anhydrite and gypsum. Layers of limestone, lithified from patchy molluscan and ooid sediment, interfinger with silica rich strata.

Calcium carbonate secreting organisms have been the most prolific biogenic sediment producers in Phanerozoic shallow marine settings. Siliceous sponges more commonly occur in deeper water and high latitude settings.  Occasionally though,  siliceous sponges did take over the shallow marine domain. The extensive Mid-Late Permian Pangean sponge belt is an example of such ecological opportunism, where silica rich sea water and nutrient availability resulted in prolific growth and persistence of sponge communities over vast areas of the northwestern Laurentian margin. Those majestic glass gardens, perhaps harboring photosynthetic symbionts are now gone, transformed to dull looking rock, but look closely and the ghosts of those long dead sponges are waiting to tell you their story.

Cretaceous Cauvery Basin Stratigraphy

In the second year of my bachelor's degree course, a few of us friends had gone fossil hunting near the town of Ariyalur in Tamil Nadu. Ariyalur sits on Cretaceous age sediments deposited in a basin that formed as India broke away from Antarctica and Australia. The basin got filled slowly over time, by sediments brought in by rivers, as well as in the marine realm, as the sea episodically kept encroaching on to the continent interior. 

Before leaving for the trip we had approached Dr. V.D.Borkar, a research scientist with the Agarkar Research Institute in Pune, to help us plan the fossil collection. He very generously lent us maps and gave us a detailed idea of the villages to travel to and nearby field locations. 

All in all it was a fun field trip. We roamed the countryside around Ariyalur and collected plenty of fossils. In our collection were plant impressions on clay, ammonoids, belemnites, echinoids, coral fragments, and a variety of bivalves. The non geology highlight was the absolutely delicious vegetarian thali meal served in the canteen next to the town bus station! We used to gorge on it everyday, twice a day.

At that time I didn't have a good understanding of stratigraphy and even sedimentary geology. As it happened I did not grasp the broader implications of the distribution of particular fossils and the arrangement of strata that I was observing in the field. 

Its never too late to update yourself! The past month I have been reading three papers on the Cretaceous outcrops around Ariyalur which focus on basin development and stratigraphic evolution. In simpler language, stratigraphic evolution means the patterns by which basins fill up. A closer look reveals that basins are not made up of uniform continuous layers (layer cake stratigraphy) of one sediment type succeeding another, but rather there is lateral interfingering of different types of sediment, controlled by sediment distribution patterns, water energy, and basin topography.  

There are exogenous influences too. A long term drop in sea level will result in a particular arrangement of strata known as 'progradation', formed for example when deltas build out in to the sea. This may be followed by a long term sea level rise forming an overlay of a different sedimentary pattern, called  'retrogradation'. In this case as the sea encroaches on land, coarser sediments that are deposited closer to the shore get buried under deeper water fine grained sediments  A sedimentary section from base to the top (older to younger) reveals in its sediment characteristics these changing environmental conditions.

Documenting these patterns in not as esoteric an exercise as it may seem to some. Such analysis is very keenly taken up during petroleum exploration.  One may find during outcrop mapping that coarse sand deposits (potential petroleum reservoirs) occur at repeated intervals and are juxtaposed against finer organic rich mud rocks (potential hydrocarbon source rocks). This then may become a guide for optimizing detailed exploration strategy in areas of the basin where strata are buried and can't be observed directly. Just such a situation occurs in the Cretaceous Cauvery Basin. The sediments around Ariyalur is one of the main accessible outcrops. But further to the east, these sedimentary layers continue under the sea bed of the Bay of Bengal. A well documented and well understood outcrop provides an analogue for the unseen portions of the basin.

These three papers clarified to me much of the Cretaceous stratigraphy that I had failed to understand in my college days.

Here are the links:

1) Cretaceous tectonostratigraphy and the development of the Cauvery Basin, southeast India: Matthew P. Watkinson, Malcolm B. Hart and Archana Joshi

A broad study of basin formation by continental rifting and the resulting patterns of basin infilling interpreted in the context of tectonic events, major sea level fluctuation and depositional episodes.

2) Sea level changes in the upper Aptian-lower/middle(?) Turonian sequence of Cauvery Basin, India  An ichnological perspective: Amruta R. Paranjape, Kantimati G. Kulkarni, Anand S. Kale.

Ichnology is the study of trace fossils. These are tracks, trails and burrows made by the movement of  creatures living on the basin floor. Traces differ depending upon the nature of sediment substrate and environmental conditions and can be used along with other sedimentological and fossil data to interpret patterns of sea level change.,

3) Siliciclastic-carbonate mixing modes in the river-mouth bar palaeogeography of the Upper Cretaceous Garudamangalam Sandstone (Ariyalur, India): Subir Sarkar, Nivedita Chakraborty, Anudeb Mandal, Santanu Banerjee, Pradip K. Bose.

The Garudamangalam Sandstone formed during a sea level highstand i.e. at the peak of a sea level change cycle, when the rate of sea level rise finally slows down and stops. Rivers bringing in sediment from the east began building a delta. The exposed Garudamangalam Sandstone is part of this delta complex. This is a very nice analysis of sedimentary processes and products. The various subenvironments in this delta complex are identified and the chemical changes in the sediment after their deposition are documented using various techniques like chemical staining and cathodoluminescence. I really enjoyed reading this one!

On a personal note, the Covid catastrophe unfolding in India is making reading and writing difficult. However, I did find that a few hours of geology time that I am managing to hold on to brings me some comfort. 

Photomicrograph: Authigenic Feldspars From The Neoproterozoic - South India

Feldspars (plagioclase and alkali feldspars) are the most common minerals in the earth's crust. The vast bulk of them crystallize out of magma and lava. Feldspar also forms during metamorphic reactions. In sedimentary rocks they are commonly seen as detrital grains in sandstones. What is less appreciated is that they can also grow de novo in sediments during diagenesis i.e. during chemical reactions that take place as loose sediment reacts with fluids and gets transformed into rock.

  Authigenic twinned euhedral feldspar cross cutting mud clast

I noticed some lovely examples of such diagenetic or authigenic feldspars from the Neoproterozoic Banganapalli Formation from the Cuddapah Basin in South India during my M.Sc. dissertation project work. I recently got a chance to photograph my old thin sections again and I am posting some more photomicrographs of these authigenic feldspars below.
.
The Banganapalli Formation also termed the Banganapalli Quartzite is made up mostly of conglomerates and sandstones. They rest with an unconformable contact on the Paleoproterozoic Tadpatri Shale. There is spatial variability in the composition of the Banganapalli sediments. In my study area south of the village of Gani in Andhra Pradesh, the conglomerates and sandstone interfinger with limestones. These limestones appear a light purple in outcrop and are made up of carbonate mud with intermittent conglomerate layers and lenses of quartzite and jasper pebbles and cobbles, thin bands and layers of quartz sand along with limestone and siliciclastic mud intraclasts often showing a chaotic fabric (left). Fine clay layers are dispersed through the succession.

Near the contact between the Banganapalli limestones and the overlying Narji Limestone is an  intraclast conglomerate layer (right) made up of carbonate and siliciclastic intraclasts indicating rapid lithification of the sea floor and the subsequent disruption of hardened sea floor crusts during storms and seismic events.

The authigenic feldspars are present in this Banganapalli limestone succession. Feldspars are euhedral (well formed facets) and show contact twinning (separate crystals grow symmetrically forming mirror images across a common plane). They contain inclusions of calcite and clay. Mineral composition studies have shown that authigenic feldspars are either albite (sodium alumimium silicate) or orthoclase (potassium aluminium silicate). At that time (in the late 1980's)  I did not have access to an electron microprobe to accurately ascertain the composition of these feldspars. The twinning exhibited by these feldspars suggests to me that these are albite.

The feldspars cross cut intraclasts (arrow)

and calcite veins (arrow)

They grow in the carbonate mud matrix (arrow)

When did they form?

Feldspar crystals cross cut fractures and veins filled with calcite. This implies that they formed after lithification of the sediment. This could have occurred very early in the sediment history. Sea floor cementation and lithification of carbonate is commonly observed from the Proterozoic, which had calcium carbonate supersaturated oceans. The presence of intraclast layers with chaotically oriented clasts and cracks filled with detrital silica pebbles and sand indicate early lithification and disruption of sea floor. Calcite veins may simply suggest hardening and breakage of lithified sediment layers.  Sea water percolating through cracks in this early lithified sediment would have supplied sodium to the growing feldspars.

Alternatively,  the feldspars formed later under burial conditions. The Banganapalli Formation is made up of immature sandstone bodies containing plagioclase and alkali feldspar detrital grains. They show signs of dissolution and corrosion during diagenesis (in image below arrows point to partially dissolved feldspar grains).

Alkali released from the dissolution of detrital feldspars was transported by groundwater flow and used up in the growth of authigenic feldspars in adjacent limestones.

I'll leave the question open.

There are other interesting diagenetic features too in these sediments. The siliciclastic mud intraclasts show alteration to chlorite and glauconite and there is extensive neomorphic recrystallization of carbonate mud.

Authigenic feldspar are reported from sandstones too. They usually occur as tiny overgrowths on detrital feldspar grains. They are less well known from limestones. So, these unusually large euhedral authigenic feldspars stole the show for me.

Finally, the satellite image below shows the location of the limestone layers (black arrow) containing these authigenic feldspars. They occur on the south dipping limb of the Gani-Kalava anticline near the town of Nandayal.

Photomicrograph: Treasure Inside A Brachiopod Shell

Couldn't help posting this picture. I am currently creating a catalog of carbonate textures and diagenetic fabrics for the geology department at Fergusson College, Pune, which I hope will be used as a teaching aid.

This photomicrograph captures the inside of a Mid Ordovician brachiopod shell. A complex cement sequence is present inside the pore space. The sequence represents passage of the sediment from depositional marine settings to later deep burial depths. During that long journey the sediment encountered fluids of different chemical make up resulting in the precipitation of different cement types.

Pure magic!

Photomicrograph: Botryoidal Silica And Dolomite Cement In Proterozoic Sandstone

This week, a gorgeous example of botryoidal and banded silica cement filling pore spaces in Proterozoic sandstones from Central India.

The sandstone has a complex history of cementation. Pore spaces are filled with dolomite or siderite, chalcedony and calcite.

Isolated dolomite rhombs (image above) were the first mineral to precipitate around quartz grains, growing inwards into pore spaces. Another strong possibility is that the rhombs are the mineral siderite which is the iron carbonate FeCO3. Siderite often alters into a mixture of hydrated iron oxides known as limonite which preserves the shape of the original siderite forming pseudomorphs.

Silica precipitation was either contemporaneous or succeeding the dolomite/siderite cements. Occasionally, silica cements cross cut the iron carbonate (image above, white arrow), indicating that at least some silica was introduced after the dolomite/siderite.

Finally, calcite cement filled the remaining open spaces.

 
It replaces the dolomite/siderite cement (top image, white arrows) but retains the iron oxide bands thus preserving the original shape of the dolomite/siderite crystals.

Calcite also cuts across (bottom image, white arrows) the silica geodes.

#ThinSectionThursday

Photomicrograph- Micro Fault Displacing Proterozoic Stromatolite Laminae

From the Paleoproterozoic Vempalle Dolomite near the village of Gani, Cuddapah Basin, South India,

This was my M.Sc dissertation area. Vempalle Dolomites got me fascinated with carbonate rock textures and diagenesis.

The image shows a micro fault displacing stromatolite laminae. Stromatolites are biosedimentary structures formed when sediment is either trapped within microbial sheets or when CaCO3 minerals like aragonite precipitate around the sheets that cover the sea floor. The microbial colonies grow in a variety of shapes and structures in response to the wave energy conditions. Flat sheet like structures like the one seen in outcrop from where I sampled this rock indicates a low energy regime.

Of interest here:

a) The presence of oolites associated with these lamellar stromatolites. Oolites form in high energy conditions where sediment grains are constantly rolled around and held in suspension for periods of time. This allows layers of calcium carbonate to precipitate around a nucleus resulting in a coated grain containing concentric rings of CaCO3. The presence of layers of oolites in a lamellar stromatolite rock suggests that oolites forming in high energy tidal channels and shoals were transported by storms onto adjacent lower energy settings such as these microbial covered tidal flats.

b) There is variation in the shape and size of dolomite crystals. This variation is not randomly distributed but is fabric selective. The fine grained stromatolite laminae has been replaced by fine grained dolomite. There is some patchy neomorphic (recrystallization) growth of this dolomitized mud into coarser irregular dolomite.  Pore spaces and sheet cracks and fractures are filled with coarser irregular shaped dolomite crystals.  Rhomb shaped dolomite crystals are associated with oolites. This suggests that the rock underwent multiple episodes of dolomitization. The fine grained stromatolite aragonite mud got replaced early by very fine grained dolomite crystals. Contemporaneously, sheet cracks and pores filled with a coarse irregular shaped dolomite crystals.  Both the saturation levels of the replacing fluid and the abundance of nucleation sites affect dolomite crystal shape and size. Finer grained substrates offer abundant nucleation sites resulting in finer grained dolomite. Crystals growing from supersaturated fluids form quickly and interfere with adjacent crystals resulting in irregular shaped interlocking textures.

Oolites made up of either aragonite or high Mg calcite crystals were replaced by rhomb shaped crystals. Rhombic shapes form when dolomite replaces coarser grained substrates or precipitates from fluids which are mildly saturated. In such instances there are fewer nucleation sites and individual crystals have a degree of freedom to grow crystal facets.


There is also chert (microcrystalline silica) in this rock. Its replaces oolites and is present in pores spaces and in fractures.

#ThinSectionThursday

Photomicrograph- Marine, Meteoric And Burial Carbonate Cements

JSR Paper Clips in their "A Look Back" series highlights an influential paper by J.A.D. Dickson on the use of staining of carbonate rocks to differentiate in a thin section the different mineral phases of calcium carbonate.

A staining procedure consisting of preliminary etching with dilute hydrochloric acid, treatment with a mixed solution of alizarin red-S and potassium ferricyanide, and a final treatment with alizarin red-S alone (Dickson, 1965) permits the distinction of orthorhombic carbonates and of calcite from other trigonal carbonates. The potassium ferricyanide stain reveals the distribution of iron in both calcite and dolomite. The use of the stains is illustrated by a discussion of the petrography of selected specimens and interpretations of the origin of various petrographic entities.

I am heartily thankful for this technique. I stained literally hundreds of thin sections of Ordovician carbonates for my PhD work. It helped me understand the changes in cement types and their chemical composition as the limestones passed from a marine setting to becoming a freshwater aquifer during sea level drops to their ultimate burial to depths of hundreds of feet where they encountered Mg rich brines from which precipitated the mineral dolomite.

Here is that sequence brought out so clearly by a mix of Alizarin Red S and Potassium Ferricyanide.

1) Bladed crystals of non ferroan marine calcite nucleated on a brachiopod shell (stained pink)
2) Equant crystals of ferroan calcite precipitated in a confined fresh water aquifer that formed during a late Ordovician sea level drop (stained purple)
3) Rhombic crystals of a non-ferroan dolomite precipitated during deep burial (not stained). This dolomite cuts across the early marine and later ferroan calcite cements.

... my series on photomicrographs of carbonates will continue...

Photomicrograph- Late Ordovician Calcite Cement Stratigraphy In Cathodoluminescence

Cathodoluminescence (CL) brings out beautifully the hidden growth history of calcite crystals. This photomicrograph is of a Late Ordovician pore space from the Fernvale Limestone, Georgia, Southern Appalachians. It is showing calcite cement grown syntaxially over echinoid fragments. Echinoid skeletons are monocrystalline. A syntaxial overgrowth means that pore filling precipitated calcite has maintained the same crystallographic orientation over this monocrystalline substrate. As a result, successive crystal masses even if precipitated at different times under different conditions appear to be one continuous block under polarized light and under crossed nicols. It takes CL to reveal these different growth phases.

The black growth zones were precipitated in oxidizing conditions by fresh water in the vadose zone (above the groundwater table). The black zones are pendant, hanging on the underside of skeletal grains. They are in essence micro-stalactites.

This was followed by another growth phase in suboxic conditions with the incorporation of divalent Mn(+2) in the calcite lattice. Divalent Mn is an activator of CL, hence the bright yellow growth bands interspersed with a thin black bands indicating periodic return to Mn poor oxidizing conditions.

The last phase is a pore filling phreatic ferroan calcite cement precipitated by reducing meteoric fluids in deeper burial conditions. Fe+2 is a quencher of CL. The cement appears dull brown.

The pore space is a couple of millimeters across.

#ThinSectionThursday

Simon Conway Morris On The Burgess Shale

Don't miss listening to Prof. Simon Conway Morris on the Burgess Shale fauna on Paleocast hosted by Dave Marshall. The Burgess Shale is an important Middle Cambrian deposit in the British Columbia Rocky Mountains. It is a Lagerstatte, i.e. it contains exceptionally well preserved fossils and therefore gives us rich details about the animal life and biodiversity of the early Paleozoic oceans and some insights into the geologically rapid diversification of early metazoans. (the Cambrian "explosion").

Why is preservation so exquisite in the Burgess Shale? Reconstructions of the sedimentary basin indicate that the mud that became the Burgess Shale was deposited at the base of a high relief limestone reef which essentially formed a sort of an underwater sea cliff.  Periodic turbidity currents swept in fauna living in shallower  areas and buried them rapidly in the deeper  water at the base of the cliff. These currents form deposits a few cm thick, encasing animal remains a few mm in dimensions. The waters were oxygen starved, thus there was less aerobic bacterial degradation of soft tissue. Add to that were some peculiar geochemical conditions of the Cambrian ocean. One was a paucity of sulphate which retarded degradation by sulphate reducing bacteria. The other, as some recent work by Robert Gaines and colleagues suggest, was the high calcium carbonate saturation levels of the ocean, which lead to rapid cementation of the sea floor in between episodes of turbidity flows.

The image on the left shows CaCO3 cement rich layers in the Burgess shale (source: Gaines et al 2012). These cemented crusts on the sea floor formed an impermeable barrier and reduced the influx of sulphate and oxygen bearing sea water in to the sediment, further slowing down microbial activity. How do we know there was less activity of sulphate reducing bacteria? The researchers analyzed the patterns of sulphur isotopes in the fossil rich turbidity layers and the background sediment.  Sulphate reducing bacteria preferentially take up the lighter isotope of sulphur from sea water. Thus, background deposits with normal or enhanced microbial activity have a lighter isotope signature relative to the Cambrian sea water standard. On the other  hand, less microbial  activity means  less fractionation of the lighter isotope into bacteria and ultimately into the sediment matrix. In Burgess Shale type deposits the fossil rich turbidity layers capped by CaCO3 cements show an enriched or heavier sulphur isotope signal indicating less microbial activity. Finally, since the bottom  waters were anoxic, there was little benthic fauna living there. This meant that the cement crusts were not disturbed and broken by bioturbation and remained effective seals throughout the crucial first few weeks of burial when degradation is at its peak. Soft tissue does break down due to slowed microbial activity and fermentation and methanogenesis. The three dimensional carcass collapses into a nearly two dimensional carbon rich film. The final result is that recalcitrant extracellular organic material like cuticles, chaetae, and jaws are preserved as compressed thin carbonaceous films often just a few microns thick, the soft fine grained mud encasing the carcass helping preserve fine morphological details. This preservation style also meant that some animals lacking recalcitrant tissues like flatworms, mesozoans, nemerteans and unshelled molluscs are less well represented in the Burgess Shale style deposits ( Butterfield 2003). Peculiar preservational styles by their very exceptional and localized nature impose a bias on the fossil record that palaeontologists must recognize to understand true evolutionary patterns.

The examples on the left shows Burgess Shale style preservation of Arthropod (B), Polychaetae worm (C) and Arthropod (D) [source: Gaines 2014]. This style of preservation actually appears first in the early Neo-Proterozoic  and then disappears for about 150 million years until the earliest Cambrian. It again declines by late Cambrian with the earliest Ordovician being the last recorded example of this taphonomic style. A unique combination of geological conditions and early diagenesis of sediment prevailing in the latest Proteozoic and earliest Cambrian resulted in these fossil deposits. This time period also has other forms of detailed preservation of soft tissues, the two most important being the Edicaran style preservation wherein the remains of macroscopic plants and animals deposited in sandy and silty sediment were draped by microbial mats and compressed to form impressions (death masks) on the sediment surface. The other important style is the Doushantuo style preservation (named after the Doushantuo fossil beds of late NeoProterozoic age, China, containing preserved algae and putative embryos and larval stages of early animals ) where phosphate minerals are attracted to and precipitate around organic tissue preserving delicate cell outlines and internal organs. Very occasionally, the same fossil will show two different preservational styles, for example, the extracellular tissue preserved in the Burgess Shale style while  internal organs preserved in the Doushantuo style. These taphonomic "windows", as they are referred to, appear and disappear through the Neo-Proterozoic to Cambrian period. For example, the Edicaran style preservation first appears in the late NeoProterozic around 580 million years ago or so. Considering that microbial mats which play an important role in this style of  preservation are pervasive through the late Archaean and the Proterozoic, the first appearance of the Edicaran remains is then likely an evolutionary signal of the first appearance of  macroscopic multicellular  eucaryotes on earth. The disappearance of Edicaran style by the earliest Cambrian also suggests a biological feedback. The evolution of macroscopic benthic animals burrowing and grazing on bacterial mats may have destroyed the cover protecting the faunal remains. Preservational styles are controlled not just by geological conditions but due to contemporaneous evolutionary innovations too.

Coming back to the talk! Prof Simon Conway Morris describes the history of research on the Burgess Shale,  how he got into researching it, details of some of the animals found in it including the famous Pikaia. This has been interpreted as an early representative of  the chordates from which the vertebrates evolved. Overall, Conway Morris gives a masterly authoritative talk.

I would have loved to hear him talk a little more about the broader questions that arise from this deposit. Are the origins of the Burgess animals to be found in the earlier Edicaran fauna? Does the Cambrian have greater morphological disparity than later periods in earth history? Has life followed a contingent unique pathway or does examples of convergence tell us something deeper about the general principles of evolution? ..or an intelligence which frames the ultimate laws and guides evolutionary processes. Simon Conway Morris has indicated elsewhere his thoughts that the Universe is the product of a rational mind and that evolution is but a search engine and I wish Dave Marshall had pressed him on his theist beliefs. But I guess the topic was the fauna of the Burgess Shale and in particular that iconic quarry in British Colombia.

He did mention in passing something which I think  is an important aspect of this story. Just as he had finished his Master's degree from Bristol University in 1972, a project headed by Prof. Harry Whittington on the Burgess Shale was starting at Cambridge. Simon Conway Morris saw this as a good opportunity. At around the same time, the Chicago school of palaeontologists lead by David Raup (who died last week), Jack Sepkoski and Tom Schopf had started a program to broaden the scope of palaeontology to include rigorous quantitative methods on large sample sets to understand biodiversity and patterns of evolution, bringing the field of palaeontology, as John Maynard Smith famously said, to the "high table of evolutionary theory". These events underscore the important point that for all your brilliance in something, circumstances and timing matter. Simon Conway Morris was present at the right time at the right place. And he did the Burgess Shale fauna justice.

How Are Diagenetic Studies Useful In Understanding Sedimentary Basin History

I dusted of my PhD dissertation last week for two reasons. A friend insisted that she wanted to see my research.. and then this paper in the Journal of Sedimentary Research (behind paywall):

Diagenetic Evolution of Selected Parasequences Across A Carbonate Platform: Late Paleozoic, Tengiz Reservoir, Kazakhstan by J. A. D. Dickson and J. A. M. Kenter

The work is eerily similar to what I did for my PhD which was carrying out a detailed study of cementation patterns in Middle and Late Ordovician carbonate parasequences from the southern Appalachians.

Dickson and Kenter use petrographic techniques along with cathodoluminescence to tease apart the cementation sequence and pore space modification of the carbonate rocks. Hydrocarbon reservoir quality depends in part on how reaction of sediment with water either dissolves material to create pore space or precipitates cements to modify pore space. So, understanding the timing of these events in the context of the burial history of the sediment pile on a basin wide scale can help geologists predict reservoir quality.

Ok, so what are Parasequences?

Dating The Indian Proterozoic Sediments Using Diagenetic Glauconite

Geological radioactive clocks which rely on the known rate of decay of radioactive elements start ticking when these radioactive elements gets trapped in growing minerals and the mineral stops exchanging elements with its surroundings. This commonly occurs in minerals crystallizing out of magma or lava  or when a mineral recrystallizes during metamorphism. A radioactive clock from a mineral in an igneous rock tells us the date of the solidification of the magma or lava while a radioactive clock from a metamorphic rock may time the thermal event during which the host igneous or metamorphic rock recrystallized.

What about directly dating sedimentary rocks using radioactive clocks? Rocks like sandstones for example are made up of pieces of eroded igneous or metamorphic or sedimentary rocks. They may contain minerals like zircon that contain uranium or feldspars that contain radiogenic potassium, but dating these sedimentary particles  means finding out  the age of the source rocks and not neccessarily the timing of deposition of sediment.

One can indirectly date sedimentary rocks using radioactive clocks based on their geological relationship with associated igneous rocks. For example; 1) a sandstone sequence may unconformably overlie a granite. The age of the granite tells us that the sandstone sequence is younger than a particular date. 2) a sedimentary sequence may be intruded by an igneous body. The age of that igneous body if established tells us that the sequence is older than a particular date and 3) a sedimentary sequence may be bounded by igneous bodies, example volcanic eruptions many deposit lava or ash at various intervals synchronous with sediment deposition. In this case, the dates if established tell us that sedimentation occurred between two particular dates.

Often though a fortuitous association with igneous rocks is absent or saying "younger than", or "older than" or "between" leaves tens of millions or even hundreds of millions of years unaccounted for. In a study published a few years ago in GSA Bulletin, James E. Conrad and colleagues use another approach to directly date when sedimentation occurred. Although sedimentary rocks like sandstones are mostly made up of eroded particles they also occasionally contain minerals that grow on the sea floor or a few cms below  the sediment water interface. Such new minerals that form during or just after sedimentation are called diagenetic or authigenic minerals. Glauconite is one such diagenetic mineral which grows on the sea floor. It is a iron potassium (K) silicate and the radiogenic K40 decays into Argon40. Using the ratio of Ar40 to non radiogenic Ar39, the age of glauconite formation and thus sediment deposition was established.

Indian Proterozoic basins are just beginning to get their chronology established more robustly and diagenetic glauconite which is present in many different Indian basins offers another tool besides associated igneous rocks to more firmly establish the timing of basin formation, sedimentation, sequence evolution and changes in ocean geochemical conditions (glauconite forms under reducing conditions on the sea floor) during Proterozoic times.

Abstract:

Ages of some key stratigraphic sequences in central Indian Proterozoic basins are based predominantly on lithostratigraphic relationships that have been constrained by only a few radioisotopic dates. To help improve age constraints, single grains of glauconitic minerals taken from sandstone and limestone in two Proterozoic sequences in the Pranhita-Godavari Valley and the Chattisgarh basin were analyzed by the 40Ar/39Ar incremental heating method. Analysis of the age spectra distinguishes between ages that are interpreted to reflect the time of glauconite formation, and anomalous ages that result from inherited argon or postcrystallization heating. The analyses indicate an age of 1686 ± 6 Ma for the Pandikunta Limestone and 1566 ± 6 Ma for the Ramgundam Sandstone, two units in the western belt of Proterozoic sequences in Pranhita-Godavari Valley. Glauconite from the Chanda Limestone, in the upper part of this sequence, contains inherited 40Ar but is interpreted to reflect an age of ca. 1200 Ma. Glauconite from the Somanpalli Group in the eastern belt of the Pranhita-Godavari Valley gives an age of 1620 ± 6 Ma. In the Chattisgarh basin, glauconite from two units gives disturbed ages that suggest a period of regional heating in the Chattisgarh basin at ca. 960–1000 Ma. These new ages indicate that these sequences are 200–400 m.y. older than previously recognized, which has important implications for geochemical studies of Mesoproterozoic ocean redox conditions in addition to providing important constraints on regional tectonics and lithostratigraphy.

Periods Of Non Deposition of Sediments Are Transformative And Useful Recorders Of History Too

Came across this interesting article published a few months ago in Sedimentology:

Deciphering condensed sequences: A case study from the Oxfordian (Upper Jurassic) Dhosa Oolite member of the Kachchh Basin, western India -Mathhias Alberti, Franz Fursichi, Dhirendra Pandey

Properties of sediments and sedimentary rocks tell us a lot about the geological history of the depositional basin as well as for clastic sedimentary rocks the history of the source terrains from which the sediment was weathered, eroded and transported. But why study phases of basin evolution when no sediment or very little sediment is deposited. What imprint do such events leave on sedimentary basins? What can such episodes tell us about sea level change, tectonic movements and climates? The authors address this issue by examining one of the most prominent marker beds from the Jurassic rift basins of western India, the Dhosa Oolite.

In large parts of the Kachchh Basin, a Mesozoic rift basin situated in western India, the Oxfordian succession is characterized by strong condensation and several depositional gaps. The top layer of the Early to Middle Oxfordian Dhosa Oolite member, for which the term ‘Dhosa Conglomerate Bed’ is proposed, is an excellent marker horizon. Despite being mostly less than 1 m thick, this unit can be followed for more than 100 km throughout the Kachchh Mainland. A detailed sedimentological analysis has led to a complex model for its formation. Signs of subaerial weathering, including palaeokarst features, suggest at least two phases of emersion of the area. Metre-sized concretionary slabs floating in a fine-grained matrix, together with signs of synsedimentary tectonics, point to a highly active fault system causing recurrent earthquakes in the basin. The model takes into account information from outcrops outside the Kachchh Mainland and thereby considerably refines the current understanding of the basin history during the Late Jurassic. Large fault systems and possibly the so-called Median High uplift separated the basin into several sub-basins. The main reason for condensation in the Oxfordian succession is an inversion that affected large parts of the basin by cutting them off from the sediment supply. The Dhosa Conglomerate Bed is an excellent example, demonstrating the potential of condensed units in reconstructing depositional environments and events that took place during phases of non-deposition. Although condensed sequences occur frequently throughout the sedimentary record, they are particularly common around the Callovian to Oxfordian transition. A series of models has been proposed to explain these almost worldwide occurrences, ranging from eustatic sea-level highstands to glacial phases connected with regressions. The succession of the Kachchh Basin shows almost stable conditions across this boundary with only a slight fall in relative sea-level, reaching its minimum not before the late Early Oxfordian.

So even a thin layer of sediment (compared to the time it represents) can be an important recorder of history.

Here, the Dhosa oolite, a complex bed of detrital particles, diagenetic coated grains and intraclasts cemented together to form a distinctive horizon represents very slow accumulation of material in a basin.

My interest is in the even more extreme situation when absolutely no sediment accumulates in a basin. In fact, my entire PhD research was on such events of non-deposition and what effect they have on earlier deposited sedimentary sequences.

Lessons In Carbon Storage From Geological Analogues - Open Access Geology

Mike Bickle and Niko Kampman in an open access article in the April issue of Geology summarize the findings of two papers (1 , 2)  published in the same issue on naturally occurring CO2 accumulations in sedimentary reservoirs and inferences drawn on their long term fate.

One potential strategy to manage growing atmospheric CO2 is to inject and store it in deep sedimentary aquifers with a retention time of at least ten thousand years.

The article identifies the key questions:

(1) how quickly will the buoyant CO2 dissolve in formation brines (good), (2) how quickly will the CO2 brines react with silicate minerals and precipitate solid carbonate phases (good), (3) will CO2 or CO2-charged brines corrode cap-rocks and escape upward (bad), and (4) will CO2 penetrate up fault zones (bad)?

It is dense but rewarding reading for those into mineralogy, phase equilibria and fluid-rock interaction. 

This is something that should be of enormous interest to Indian sedimentary basin specialists and climate change mitigation planners. In a recent post I mentioned about India's plans to build 400 odd more coal power plants in the next few decades. Many will be located near sources of coal in the continental rift basins of eastern India. Is carbon dioxide sequestration in natural reservoirs economically viable? Do these basins have favorable conditions deep underground for long term storage of CO2? It is research well worth funding given that our dependance on coal will last several more decades.
 

Gorgeous Paper On Carbonate Diagenesis In Journal Sedimentary Research

The January 2013 issue of the Journal of Sedimentary Research is open access. There is a long and beautifully illustrated study on the diagenesis of Permian carbonates from the Guadalupe mountains of New Mexico U.S.A. by David A.Budd and colleagues.

These carbonates were fractured very early during their depositional history, in fact the fractures are syndepositional i.e. they formed as the sediments were accumulating. Sometimes calcium carbonate sediments undergo cementation and hardening by sea water just a few centimeters to meters below the sediment water interface and they then are rigid enough to fracture. Budd and co-workers painstakingly analysed the materials filling these fractures using a variety of sedimentary petrology and geochemical techniques and found out that these fractures acted as conduits for the movements of fluids not just early on but through the entire geologic history of these rocks. So the geometry of fluid flow networks may be established very early in the rock history with implications for the distribution of porosity and localization of economic deposits.

I love such detailed petrologic studies. I was consumed by this kind of research during my PhD days, thinking about marine and meteoric cements and porosity formation and the movement of fluids and its interaction with rock from its deposition to deep burial. It was a real pleasure to read this long and exhaustive work.

I said beautifully illustrated so let me post below an example from the paper. The image shows the cementation and interpreted geologic history of a fracture using plain light and cathodoluminescent microscopy.

Explanation from the paper:

A) Paired plane light and B) cathodoluminescent photomosaics through fracture fill B. The wall of the large fracture is lined with bladed dolomite cement (black arrows). Overlying the dolomite is a first generation of luminescently zoned (non to dull to bright orange) calcite cement (CC1). With subsequent refracturing, those first generation cements were mechanically rotated, brecciated, and lightly etched (white box), and then the combined fracture opening was encased in a bright orange luminescent calcite (CC2).

Just to show off, let me put up a similar kind of image from my PhD work in the Upper Ordovician strata of the southern Appalachians U.S.A.  Again a pore space illuminated by cathodoluminescence shows different cement generations.

Long live Sedimentary Geology!

Reservoir Rock In World's Biggest Oil Field Is Made Of Shit

From Ken Deffeyes book “Hubbert’s Peak”, via the Oil Drum:

Most massive and nonporous limestones contain textures made by invertebrate animals that ingest sediment and turn out fecal pellets. Usually, the pellets get squished into the mud. Rarely do the fecal pellets themselves form a porous sedimentary rock. In the 1970s, the first native-born Saudi to earn a doctorate in petroleum geology arrived for a year of work at Princeton. I used the occasion to twist Aramco’s collective arm for samples from the super-giant Ghawar field. As soon as the samples were ready, I made an appointment with our Saudi visitor to examine together the samples using petrographic microscopes. That morning, I was really excited. Examining the reservoir rock of the world’s biggest oil field was for me a thrill bigger than climbing Mount Everest. A small part of the reservoir was dolomite, but most of it turned out to be a fecal-pellet limestone. I had to go home that evening and explain to my family that the reservoir rock in the world’s biggest oil field was made of shit.

A bewildering variety of particle types get bound together to form limestones. Post Cambrian times, the calcium carbonate shells of marine organisms have been the most common particle type, the primary building blocks of limestones. But other particle types like fecal pellets are also common.

For carbonate sedimentologists involved in oil exploration, the most important task , is understanding the origin and distribution of porosity and permeability i.e. the open spaces in which oil migrates and is naturally stored. Sedimentologists recognize two broad categories of porosity. Primary porosity and secondary porosity.  Primary porosity is the open space between the grains and forms as grains settle down during deposition into different packing configurations depending on their shape and size.  Sediments that are deposited in environments where wave and tidal movements are vigorous will have high primary porosity because in such settings finer mud that can clog up interstices between coarser grains is winnowed away, leaving behind a lag of clean sand.

The image below is a photomicrograph of a fecal pellet sand from the Jurassic of England. The shining white material between the dark pellets is calcium carbonate cement which has filled up the primary porosity. Occasionally, there may be no precipitation of cement as the sand gets buried. In such situations the primary porosity is preserved and the deposit may become a reservoir rock.

 

Source:  SEPM Strata

And here is a picture of the Bahama Banks of the coast of Florida.

Arrows and labels show environments facing open ocean where currents and waves are vigorous and where primary porosity in sediment will be high. In the interior of the Bahamas, wave energy is much lower, resulting in sediment with less primary porosity. Fecal pellets may originate in the interior of platforms, in low energy settings. They often harden in these settings due to precipitation of cement in micro-pores within the grains. Often due to storms, these hardened pellets are then transported to high energy settings. Due to this early hardening, pellets resist getting squished against each other as the sediment is buried. Open spaces are thus preserved in such early hardened fecal pellet deposits.

Although the paleo-geographic setting would have been different than the Bahamas, the sediments of the Jurassic Ghawar reservoir limestone would have been deposited in high energy settings resulting in substantial primary porosity.

 Another category of porosity is secondary porosity that forms due to the reaction of the sediment with water during burial. It results in open spaces being created by the dissolution and leaching away of mud and grains and also due to volume changes as calcite gets replaced by the denser dolomite. This type of porosity is also present in the Ghawar limestone.

Since 1950's the Ghawar field has produced over 65 billion barrels of oil. Daily production is about 5 million barrels, about 6% of global production.

And.. what do  you know?  Glenn Morton has found another use for those famous fecal pellets..refuting young earth creationism:

One of the interesting things about Ghawar is the nature of its reservoir which provides an argument against an ideology I fight all the time, Young-earth Creationism. Ghawar is largely made of dung, which would be hard pressed to be concentrated during a global flood and thus contradicts the young-earth creationist claims.

A chaotic flood would have dispersed and broken up fecal pellets in to mud. Only long periods of  wave action and winnowing and early cementation on a sea floor would have produce the well sorted fecal pellet sands of the Ghawar reservoir deposit.

Accretionary Wedge: Geo-Images ..Calcite Cements

Entry for the Accretionary Wedge carnival hosted by Chris and Anne at Highly Allochthonous.

When I viewed these crystals under a microscope for the first time... I felt more relief than elation. I finally had a story to tell my PhD committee!!

These are photomicrographs of pendant calcite crystals which precipitated within meteoric aquifers that developed during late Ordovician sea-level falls....location... Appalachians...northern Georgia.

On left is a view of the crystal stained with potassium ferricyanide. Non ferroan early cements are not stained. Burial Fe rich calcite stains blue. On right is a view of the same crystal in cathodoluminescence (crystals are bombarded with cathode rays in a vacuum chamber).

Cathodoluminescence helps understand pore fluid fluctuations between oxidizing and reducing conditions. In oxidizing pore-fluids, neither Mn+4 or Fe+3 is incorporated into growing calcite crystals, and thus cements are black (non-luminescent). In pore fluids with progressively lower Eh , reduction of Mn first and then Fe leads to their incorporation into the growing cements, giving the crystals a bright to dull luminescence. The image on left shows the changing geochemistry of pore fluids from oxidizing to mildly reducing to reducing as the sequence got buried.
 
All this geochemistry makes more sense when placed in a stratigraphic context. The image below shows a cyclic late Ordovician sequence and the position of calcite cements within it. Cathodoluminescent signatures help constraint the lower limits of the fresh water aquifer that developed during successive sea-level drops.

During the first sea-level drop both vadose and phreatic meteoric conditions are recognized and groundwater fluctuated between oxic to mildly reducing. During the next sea-level drop (Ordovician-Silurian unconformity) the meteoric aquifer was reducing and only bright luminescent phreatic cements precipitated. The vadose zone is not preserved in the younger sequence.

This kind of cement stratigraphy helps understand the geochemistry and the lateral and vertical extent of groundwater systems and fluid flow patterns that develop as sea-level drops and basins are exposed to fresh water infiltration. Fluid - sediment interaction create and destroy porosity and permeability,  properties which in turn influence the hydrocarbon and mineral potential of the sedimentary sequence.

Diagenetic History Of The Great Barrier Reef Of Australia

That giant organism is slowly giving up its deepest secrets.

From the November 2009 issue of Sedimentology : The Great Barrier Reef: a 700 000 year diagenetic history - Colin J. R. Braithwaite and Lucien F. Montaggioni

Most people rightly tend to think of the Great Barrier Reef as a living wonder. But the current living ecosystem, the coral communities and associated faunal and floral assemblages have been built on a foundation of a community of dead corals. And that ancient community when it was living grew on a foundation on an earlier community and so on.. for hundreds of thousands of years through many episodes of community growth and decline and growth again.

A borehole recovered sediments from Ribbon Reef 5 northwestern Australia. Analysis shows ten such coral reef growth episodes, ten sedimentary depositional units beginning around seven hundred thousand years ago. These episodic events correspond to major sea level changes driven by Pleistocene glacial growth (sea level fall) and melting (sea level rise). Coral reef building stopped when sea level fell and resumed when sea-level rose again.

This study delves into the mineralogical and geochemical transformations (Diagenesis) that occur in these reefs when sea-level fell and exposed the sea bed and the coral communities to fresh water.

As is the case with diagenetic research a lot of the base data is collected from micro scale analysis and this study does a detailed job. Cement mineralogy, (chemical precipitates) morphology, habits and associations are carefully documented. Textural features such as solution fabrics and pore space types are noted. The overall conclusion is that during sea-level fall meteoric (fresh water) aquifers developed in the exposed layers and there was considerable amounts of reaction of the coral skeleton with fresh water leading to destruction of minerals like aragonite and high Mg calcite and precipitation of low Mg calcite.

Based on these diagenetic textures distinct vadose zone (soil profile above the water table) and phreatic zone (below water table) demarcations are recognized in these ancient aquifers and some units show evidence of the water table being mobile i.e. migration up and down likely due to changing rainfall conditions.

That's the micro-level stuff and its a well characterized in this study.

My interest in diagenesis tends to make me look at the larger basin scale picture and I found much to think about in this paper. Diagenesis occurs during the establishment of large scale hydrologic systems in sediments and rocks. The scale and geometry of these hydrologic systems controls the extent and 3-D shape of alteration of the sediment.

Such thinking in terms of diagenetic reaction fronts, shapes and volumes is very important because these altered volumes of sediment may due to their properties of porosity and permeability act later in the sediment history as pathways and reservoirs for hydrocarbons and other economically important minerals like copper, lead and zinc.

Although not looked at from such an economic angle this study does recognize the larger controls on the development of hydrologic systems during sea-level falls.

One important diagenetic pattern revealed was that the fresh water reactions that took place cannot be tied to any particular emergent event although aquifer formation likely affected only the youngest deposition unit. This conclusion is based on the observation that there is no superimposition of diagenetic textures and cements.

This is a bit tricky. Say during a sea-level fall a particular depositional unit experiences meteoric diagenesis and then sea-level rises and drowns that unit. A new coral community will grow on that surface and form a younger depositional unit. Later sea-level falls again. Another fresh water aquifer develops. If it is thick enough it will affect not only the youngest sediment unit but the ones underlying it. So the older depositional units might then contain evidence of more than one generation of diagenetic products, a superimposition of events.

That kind of evidence of superimposition is lacking in these reefal units, leading the authors to conclude that the meteoric aquifers that formed were thin and did not penetrate deeper into the sequence.

The major reason for this is the lay of the land and the location of the reefal bodies in the context of the Australian shelf.  The Barrier Reef is located some distance away from the Australian mainland and away from any continental relief. During sea-level falls the undulating topography of the reef, made up of mounds and depressions would have meant that the reefs would have been exposed as a chain of islands.

The depth to which groundwater circulates in these exposed islands i.e. the thickness of fresh water lenses under these islands is governed by the width of the islands, the hydrologic conductivity of the sediment and the amount of recharge. These relationships observed in several Holocene islands suggest a meteoric lens thickness of just 1% of island width.

This contrasts with carbonates sequences which accumulate as bodies fringing continental land masses with coastal relief. Here during sea level falls, the hinterland relief provides a stronger hydraulic head for fresh water to flow towards and into the exposed sea bed and circulate deeper into the sediment layers, generating diagenetic reaction zones which cut across depositional units. 

One consequence of the thin meteoric lenses that episodically developed during the Barrier Reef history is that much of the sediment remained only partially altered. This is indicated not just by the sporadic presence of mineral aragonite which usually dissolves in fresh water, but by a cross plot of the oxygen and carbon isotopes of coral skeletal material in which the overall pattern of fresh water sediment interaction is brought out beautifully.

Here is the theory. Meteoric water is enriched in the lighter isotopes compared with sea water and compared with marine carbonate sediment. Imagine a marine carbonate sediment body reacting with meteoric (fresh) water. Marine carbonate is the dominant reservoir of carbon relative to meteoric water (means there is much more dissolved carbon in marine water than in fresh water) and thus the carbon composition of pore fluids quickly reaches a state of equilibrium with dissolving marine carbonate during water-rock interaction. So any product precipitated from this pore fluid will have the signature of marine carbon. In contrast, meteoric water is the main oxygen reservoir, and thus the oxygen composition of pore fluids with change towards a marine signature only after prolonged interaction with the isotopically heavier marine carbonate.

A pattern of relatively invariant oxygen and variable carbon is thus indicative of less water rock interaction during diagenesis.

That's what the bulk of data points of the key units in the above graph is pointing to. Here units 1, 9 and 2 are mostly marine and are not altered much by meteoric diagenesis. In the  rest of the units the bulk of the samples show a constrained oxygen isotope signal (compared to total range of 0 - 9.5 parts per thousand) and a very variable carbon isotope signal. The depleted but constrained oxygen isotope values suggest that the pore fluid composition were not impacted much by marine oxygen implying limited reaction with rock as does the variable carbon which would have been quickly acquired a uniform marine signature if water rock reactions had been prolonged.

All this is because of thin discontinuous meteoric lenses and thick vadose profiles where diagenesis is concentrated in pockets and along very thin reaction fronts leaving other pockets of the sediment body unaltered.

Enjoyed the paper. It's a good mix of micro-scale detailed work on cements and geochemistry but done in a way that allows you to draw a larger picture of the controls that landscape, platform geometry and hydrologic systems exert on diagenesis.

BRAITHWAITE, C., & MONTAGGIONI, L. (2009). The Great Barrier Reef: a 700 000 year diagenetic history Sedimentology, 56 (6), 1591-1622 DOI: 10.1111/j.1365-3091.2008.00982.x