Evaporitic Source Rocks: A Geological Response to Biological Cycles of “Feast or Famine” in Layered Brines

John Warren
University Brunei Darussalam, Bander Seri Begawan, Brunei

Modern organic-rich evaporitic mudstones accumulate in lacustrine depressions beneath layered hydrologies subject to oscillations in salinity and brine level and anoxia. Saline-associated organic matter is not produced at a constant rate, rather, it is produced in pulses by a halotolerant community in relatively short times when less stressful conditions occur in the layered hydrology. This happens whenever an upper less saline water mass forms atop nutrient-rich brines or in wet mudflats when waters in and atop the uppermost few millimeters of any microbial mat freshen. The resulting bloom (a time of “feasting”) by halotolerant algal, bacterial and archaeal photosynthesizers is a time characterized by very high levels of organic productivity. In mesohaline waters where light penetrates to the bottom, the organic-producing layer is typically the upper algal and bacterial portion of a benthic laminated microbialites (typically characterized by elevated numbers of cyanobacteria.

Same-scale lacustrine source rocks are common throughout the rock record. But, ancient large-scale mesohaline marine evaporite depressions where widespread source rocks accumulated have no Holocene analogues. Creating huge subsealevel depressions filling with mesohaline evaporitic carbonates and later salt seals requires varying combinations of greenhouse eustacy (epeiric seaways) and tectonics-climate (marine-seepage rifts, soft collision belts and far-field intracratonic sags) with no modern same-scale counterpart. Yet it was in the resulting salinity-layered intraplatform epeiric depressions and restricted basin centers that schizohaline waters and stable longterm brine plumes formed across time scales that allowed substantial volumes ofmarine evaporite source rocks to accumulate. As for most ancient marine evaporite styles, the present is not a suitable time for studying scales of development, but process and texture can be seen in smaller-scale Holocene lacustrine deposits.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005