Monday, September 22, 2008

A Brief Return To Those Ancient Carbonates

I got an email from a faculty friend who teaches sedimentary geology and stratigraphy in the deep American south. Yes, they have Universities there too and daaawwg gon good ones I might add!

Can you summarize your thesis work and send it to me by this week? He's teaching a course in sedimentary petrology over this semester and needed some material for the carbonate part of it. I've spent this past weekend writing up a summary of my research on calcite cements of Middle and Late Ordovician basins of the southern Appalachians. Cements are chemical and biochemical precipitates that form between pore spaces of sediments and help bind loose sediment into rock. Their morphology and chemistry can tell a lot about ambient conditions of temperature, pore fluid chemistry and basin history. It's been several years since I thought and wrote about carbonates and diving into my thesis was fun. Here's a snippet:

Both the Middle Ordovician Chickamuga Group and the Upper Ordovician Shellmound Formation were affected by meteoric diagenesis relatively early in their individual burial histories. Detailed examination of the diagenetic products using petrography (light and cathodoluminescence ) and geochemistry (trace and stable isotopes) has shown that the precipitation environments were very different. Late Ordovician carbonates of southern Tennessee and northeast Georgia were exposed to subaerial diagenesis during Richmondian sea-level falls associated with the formation of the Taconic unconformity, and show substantial vadose and phreatic zone cements and alteration.

The underlying Middle Ordovician limestones also contain abundant phreatic calcite cement, but cementation is not related to synsedimentary emergence, or to direct vertical infiltration of meteoric water sourced from the overlying Upper Ordovician unconformities. Instead, recharging meteoric water in basin-margin highlands to the southeast, entered Middle Ordovician limestones through confined aquifers during Late Ordovician to Early Silurian times. This meteoric cementation history suggests that patterns of groundwater flow in the basin were strongly influenced by regional shale and lime-mud facies that occur between these two units. These low-permeability strata compartmentalized the Mid-Late Ordovician basin fill into contemporaneous, but hydrologically isolated surficial and deeper aquifers.

I wrote incredibly dense and cryptic passages such as these, not too confuse anyone. That's just the way formal communication in science sometimes reads like. My work was a comparative study of early diagenetic patterns in Middle and Late Ordovician carbonates. The climate changed from a Middle Ordovician greenhouse to a Late Ordovician ice-house and I wanted to find out if this left any impact on the diagenetic patterns.

In many ways I regard the 5 years of graduate research as the most creative period of my life so far. I struck a good rapport with my adviser early on, and a lot of positive outcomes flowed naturally from there. The informal graduate lab atmosphere made work enjoyable. And then there was the bonus of finding treasures like the one below.

I wrote in an earlier post on sea-level fall and the formation of erosional unconformities. The image shows a thin section of a limestone magnified under a microscope. This rock was sampled below a Late Ordovician unconformity. As sea-level fell and exposed the basin a fresh water aquifer developed in the sediments below the unconformity. Meteoric (fresh) water seeping down from the exposed erosional surface into the underlying sediment precipitated the early zoned cement in the pore spaces present between skeletal grains. These cements are tiny, rarely more than a millimeter along the long axis and can be seen only under a microscope. Much later in the Silurian the sea-level rose again and the Late Ordovician sediments were buried under a thick cover of Silurian sediments. The deep burial cement was precipitated then from fluids that were expelled from adjacent iron rich mud.

The image is taken in a cathodoluminesence microscope chamber. When you place a calcite sample in a vacuum chamber and bombard it with cathode rays, the sample will turn luminescent depending upon the presence of certain trace elements like Mn and Fe. The intensity of luminescence in calcite cements has been found to be related to the concentrations of Mn+2 and Fe+2, which act as activator and quencher of luminescence respectively. Reduction of Mn and Fe to a divalent state is necessary for these elements to enter the calcite lattice. 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.

That's what the black-golden-brown cement bands in the image show. An initial phase of oxidizing conditions in the aquifer that developed below the Late Ordovician unconformity. And then progressively reducing conditions as the Ordovician sediments got buried under Silurian age deposits.

I obsessed over such stuff for several years. I've moved on to other work which I enjoy as much. Well.... almost.

What do they say? The first cut is the deepest :)

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