Microbial Mats as an Indicator for Pauses during ‘Shale' Deposition - Kimmeridge Clay Formation (Upper Jurassic), Offshore UK
Egenhoff, Sven; Fishman, Neil; Hill, Ronald
Siliciclastic mudstones commonly consist of a bioturbated matrix made up of silt-size carbonate and quartz, clay minerals, and organic matter. Intense burrowing typically observed in shale successions, however, often homogenizes these fine-grained rocks and destroys sedimentary structures that would help to identify original depositional processes. It therefore remains unclear whether thick shales were deposited by a constant rain of material in suspension or how much episodic ‘events' influenced sedimentation. Small microbial mats present in three cores of the Kimmeridge Clay Formation, UK, a world-class source rock, are used to address this problem.
Microbial mats in Kimmeridge siliciclastic mudstones occur as sub-millimeter long discontinuous laminae of organic matter. They vary in thickness laterally and characteristically contain minor amounts of clay-size quartz, feldspar, and illite grains equivalent in composition to surrounding detritus. The mats occur spaced just a few tens to hundreds of micrometers apart at irregular intervals within the formation and locally drape over agglutinated foraminifera.
The abundance of microbial mats in the Kimmeridge suggests that mat growth on the seafloor (distal shelf environment) was a common phenomenon. As mat-forming organisms require oxygen, and mat-forming bacteria live at redox boundaries (i.e., sulfate-sulfide interface), their mere presence argues for at least dysoxic conditions on the seafloor during deposition. Their abundance indicates that 1) oxygen levels might have been too low for grazers to thrive; 2) their discontinuous nature reflects some grazing activity with preserved mats representing remains of a larger, more continuous mat; and 3) photic zone water depths were common during deposition.
In order to grow on the seafloor, it is most likely that the microbial mats reflect ‘quiet' depositional times, with only minor very fine-grained detrital input from suspension settling. During phases of enhanced runoff or storm events, clay and some coarse silt grains were transported even into the most distal parts of the Kimmeridge basin covering existing mats. Subsequently, another generation of organisms formed a new mat, the net affect being alternation of mats and sediment; quick burial of mats by the sediment probably aided in their preservation. Microbial mats therefore indicate pauses in deposition of shales, likely in a dysoxic environment, alternating with sedimentation pulses during events.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013