Not Anoxia! How Do You Preserve Organic Matter Then?
There is a growing body of evidence (e.g. presence of small burrows, in-situ communities of benthic foraminifer) that indicates the requirement for bottom water anoxia, as a pre-requisite for enhanced organic matter preservation in fine-grained sediments (e.g.“Black Shales”), has been overstated. Recent research suggests that primary production, combined with optimal sediment accumulation rates (that balance dilution with preservation and destruction) are the crucial factors controlling organic matter enrichment. At sequence / parasequence scales these processes are well-understood. At bed / stacked-bed scales, however, the specific mechanisms responsible for sediment delivery and organic matter production/destruction are poorly known.
Here we investigate (optical, electron optical and geochemical analyses) sedimentary rocks (e.g. Kimmeridge Clay Formation, Pebble Shale Unit, Jet Rock) unusually enriched in total organic carbon to determine how physical, chemical, and biological processes interacted to enhance organic matter burial.
These analyses reveal that the studied lithologies contain a variety of sedimentary structures including: thin beds, rip-up clasts, burrow mottles, ripple lamina, organo-mineralic aggregates (comprising faecal pellets, organic matter and clay minerals), agglutinated foraminifer, in addition to up to 44% TOC.
These data indicate that these organic-rich mudstones were not deposited under low energy conditions in bottom waters that were persistently anoxic. Instead they indicate that a) the sea floor was being occasionally reworked by physical processes, b) sediment was transported both by waning, advective currents and by suspension settling, and c) sediment was not being delivered as a continuous rain of fine grained particles but rather as large, organo-mineralic aggregates (marine snow). Once deposited, the tops of individual genetic beds were commonly colonised by a diminutive infauna that homogenised the surficial sediment layers.
This study indicates that enhanced organic matter preservation was likely caused by high concentrations of primary organic matter and sediment being fluxed to the sea floor, as aggregate grains. Inevitably, some of this organic matter was destroyed when the surficial sediment layer was colonized. Episodic sedimentation and rapid delivery, however, ensured that at least some was preserved. There is little sedimentological evidence for persistent exunia during deposition of these units.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009