Thursday, July 18, 2013

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.

I study the accumulation and transformation of calcium carbonate sediments into the rock limestone. Most carbonate sediments either are the shells of marine organisms or precipitate as oolites and other diagenetic particles from sea-water. When sea level falls because the basin floor is uplifted due to tectonic movements or more commonly due to locking up of sea water in growing polar ice sheets, the sea bed is exposed to the atmosphere and this carbonate factory shuts down.

These non-depositional episodes can last between few thousand years to hundred of thousands of years to millions of years depending on what caused the sea level to drop. Sea level changes due to ice sheet growth and decay can last tens to hundreds of thousand of years. Major tectonic movements may cause basins to emerge and be above sea level for millions of years.

When the sea bed emerges as exposed land, rainwater infiltrates these calcium carbonate sediments forming extensive aquifers and groundwater circulation systems. They corrode and dissolve the sediment and rock creating porosity and permeability. In places, such waters become saturated with the calcium carbonate dissolved elsewhere and precipitate that material as calcite crystals that cement the sediments into hard rock. This destroys porosity and permeability.

At some point sea level rises again drowning this exposure surface and deposition begins anew. Sea water start filling up spaces earlier occupied by fresh water. Over time a vast thickness of sediment may accumulate burying this once exposed surface and older sediments and destroying the groundwater system that existed before. But this fresh water fluid circulation system has left an important imprint, which is, the network of open spaces and cracks and solution fissures. Geologists want to know what the 3 dimensional geometry of this network is because in deeply buried sediment, new fluid circulation systems, this time made up of trapped sea water which has been reacting with the sediment itself, come into existence. These deep basinal brines exploit and move along the earlier formed permeability pathways. They often contain all the stuff that makes this world turn; oil, gas, lead, zinc and copper. This mineral wealth ultimately gets deposited in the open spaces formed long back when the sea level dropped and there was no deposition.

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