Evaporites - Mesohaline Source rocks 

Video streaming

Content of this set of training videos ranges across aspects of mesohaline source rocks in evaporitic settings. It gives a feel for the content level in the various online training and in-house workshops we offer to commercial clients. 

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"A crucible of pink and crimson fire"

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1. Mesohaline source rocks - Feast or famine and the flamingo connection

One of the most visually impressive indications of periodic but very high levels of organic productivity, in a well-adapted biota, in a salinity-stressed layered water body, resides in the "flamingo connection." The bright-pink lesser flamingo (Aves, Phoenicopteridae) flourishes in the alkaline lakes of the African Rift Valley and represents an ancient lineage of long-legged, microphagous, colonial wading birds.  

Flamingoes are filter feeders that thrive on the dense halotolerant cyanobacterial blooms (particularly of Arthrospira fusifomis) in the mesohaline shallows of saline lakes around the world. This creates a connection between flamingoes, mesohaline planktonic blooms, density-stratified waters and pulses of organic matter settling into a perennial anoxic layer at the base of the brine column.

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Salty water blooms can be green, bright pink, or purple

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2a. Mesohaline source rocks -The Halobiota (I of II): What lives in salty water? - Primary producers

This video focuses on the halobiota and what produces high volumes of organic matter in saline waters - The Primary Producers, especially in the mesohaline salinity range. For many geologists, saline environments are seen as biotal deserts, mostly devoid of biological activity and little source rock potential.

But at the microbial and phytoplankton level in the saline ecosystem, we now know that this is not the case. Instead, many modern saline water bodies are sites of periodic but intense organic activity. Numerous highly specialised algal, bacterial and archaeal species infrequently flourish and dieback (cycles of feast or famine).
Occasionally, haloadaptive photosynthesisers can leave behind substantial volumes of oil-prone organic residues in laminated bottom sediments, especially in mesohaline carbonates, which on burial can evolve into prolific source rocks.

2b. Organic Matter in Saline Geosystems - The Halobiota (II of II): What lives in salty water? - The consumers

This video focuses on the halobiota and what consumes and degrades the high volumes of organic matter periodically produced in saline waters - The consumers. They thrive in salinities ranging from brackish to halite-saturated. For many geologists, saline environments are seen as biotal deserts, mostly devoid of biological activity and little source rock potential. But at the microbial level in the saline ecosystem, we now know that this is not the case. Instead, many modern saline water bodies are sites of periodic but intense microbial activity ranging across the bacteria and the archaea, fungi and protists. When salinities are appropriate numerous highly specialised algal, bacterial and archaeal species can flourish (followed by dieback in cycles of feast or famine). This periodic enlargement of the halobital biomass is also seen in some multicellular eukaryotic species such as particularly well-adapted species of brine shrimp, ostracods, gastropods, molluscs and fish, as well as vast flocks of birds visiting saline settings when the plankton are blooming across the upper freshened water mass in a density-stratified brine column.

3. Organic Matter in Saline Geosystems - Mesohaline source rock maturation

This video discusses the mechanics of maturation of mesohaline source rocks and the various techniques and approaches needed to quantify the different maturation stages. A hydrocarbon source rock is generally considered a fine-grained rock that generates and releases enough fluids to form commercial accumulations of oil or gas during its burial and heating. m

Mesohaline source rocks are typified by:
• Dominantly halotolerant phytoplanktonic organics that were deposited at the base of a density-stratified brine column in a hypersaline anoxic lower water mass
• The organics tend to be Type 1 or Type 1-2 source rocks (alginites and liptinites) with only minor input from higher plant material -hence lesser Type 3 inputs and only minor vitrinite
• Accordingly, mesohaline source rock maturation is early, and generation indications are likely to reflect vitrinite suppression
• Nearby evaporites make excellent seals, and if the salt seal is halokinetic, it can create stacked high-efficiency structural traps (as discussed in the salt tectonics modules)
• The widespread influence of dissolving salt edges drives "salting-out" in the carrier fluids, enhancing hydrocarbon volumes in many evaporite-related accumulations.

4. Organic Matter in Saline Geosystems - Biomarkers and their significance in saline geosystems

At its simplest, much of the utility of biomarkers in saline environments comes from salinity-related differences in contributions to organic matter of the general categories of primary producers (autotrophs), namely prokaryotes (cyanobacteria and bacteria) and eukaryotes (higher plants, algae) and archaea. Most triterpanes are associated with prokaryotic sources, whereas steranes tend to be produced by eukaryotes. Thus, the triterpane/sterane ratio can be a rough measure of the prokaryote/eukaryote contribution to the organic material. As salinity increases, the less salinity-tolerant eukaryotic organisms (mostly green algae) give way to more halotolerant bacteria and cyanobacteria (tricyclic and hopane producers) with a corresponding increase in the triterpane/sterane ratio. Thus high alkalinity/salinity settings are characterised by tricyclics (C20-C24;m/z 191), β-carotene (C40H56 compound; m/z 125) and gammacerane (C30 triterpane; m/z 191), all with prokaryote sources. Hence, high levels of gammacerane and β-carotene in ancient organic signatures are typically associated with highly saline and density-stratified, often lacustrine environments.

5a. Source rocks in Saline Geosystems - Enrichment Present and Past: Modern with same-scale ancient counterparts

The video focuses on the worlds mesohaline source rocks focuses on hypersaline settings where organic enrichment occurs today in modern and ancient saline lacustrine and supra-salt allochthon brine sump (DHAL) settings. It emphasises the various physical and biological properties and associations that facilitate the organic enrichment and deposition of source rocks. Much of the organic matter preserved in evaporitic carbonates, and the resulting source rocks, originated as planktonic blooms (pelagic “rain from heaven”) or from the benthic biomass (“in situ” accumulations). Such organics typically settled out as seriate pulses of organic matter (often pelleted) that sank to the bottom of a layered brine column. Each pulse was tied to a short period when surface brines were diluted and halotolerant producers (mostly cyanobacteria and algae) flourished in the freshened lit zone. That is to say, the laminated mesohaline mudstones that constitute evaporitic source rocks reflect biological responses to conditions of “feast or famine” in variably layered bodies of brine. 

5b. Source rocks in Saline Geosystems -  Enrichment Present and Past: Ancient mesohaline settings without modern same scale  

Worldwide, studies of ancient evaporitic basins have shown that organic-rich mesohaline sediments can accumulate beneath ephemeral to perennial, typically schizohaline, surface brines in lakes, salterns, or basin and slope settings, in both marine and continental settings. However, the most prolific accumulations of hydrocarbon-prone organic-enriched sediments tend to have been deposited as laminated micritic carbonates beneath density-stratified moderately-saline (mesohaline) anoxic water columns of varying brine depth.

In this first part of the final lecture in the source rock training module, we look at the nature of ancient large-scale evaporite deposition and how mesohaline settings are a predictable part of the evolution of platform and basin-wide successions.

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