Sedimentary Processes and Myths About the Optimal Depositional Environments of Source Rocks

Joe H. S. Macquaker¹ and Duncan McIlroy^2
1 Manchester University, Manchester, United Kingdom
² Memorial University, St. John's, NF

Organic-rich fissile mudstones ("black shales") deposited on marine shelf environments are commonly interpreted as having been deposited in low energy environments under conditions of bottom water anoxia with the clastic sediment having been delivered to the sediment water interface as hemipelagic settling. Recent advances in microscopy (both optical and electron optical) suggest this view is rather simplistic even in the most organic-rich successions.

In order to investigate the fundamental controls on sediment delivery and the conditions at and close to the sediment water interface, organic-rich siliciclastic mudstones (e.g. Jet Rock, Kimmeridge Clay, Hue Shale, Cleveland Ironstone and Westbury Formations) have been investigated utilizing optical and electron optical methods.

These analyses reveal that these units are commonly pelleted, may show evidence of ripple lamination, contain intraclasts, upward-fining laminae, shell pavements, and are mostly bioturbated. Bioturbation comprises a variety of diminutive trace fossil taxa including Chondrites, Paleophycus, Planolites, Phycosiphon and a diverse assemblage of burrows of uncertain origin.

These data indicate that conditions at the sediment water interface during deposition of these rocks were predominantly oxic, that the sediment was regularly reworked by currents (mainly storms) and that there was commonly sufficient energy to form intraclasts and concentrate the shell debris into pavements. These data also imply that in such successions the significance of long term bottom water anoxia have been overestimated, thin bedding has been commonly misinterpreted as laminae, and the benthic fauna was restricted because of the existence of a liquid mud substrate rather than bottom water anoxia.

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