Wednesday, 8 October 2008: 9:05 AM
George R. Brown Convention Center, 332BE
The Cambrian Eau Claire Formation of Indiana is a heterolithic shallow marine succession, dominated by interbedded shales and siltstones. Shale-rich intervals from cores collected in NW Indiana were examined for sedimentary features and stratigraphic stacking patterns. Within these intervals, silt beds range in thickness from a few mm's to more than 10 cm's, and are characterized by low angle cross-laminae and internal low-angle truncation surfaces. In places they show bundle-wise up-building and alternating dips of foresets, suggestive of wave reworking or combined flow deposition. Sharp erosive bases, sole marks, graded rhythmites, and graded tops of these beds indicates that they are event beds, most likely resulting from storm deposition. Bioturbation is low to moderate and rarely affects more than 10% of the rock volume. Escape traces and burrowed tops of event beds are most common. Shale–dominated intervals range in thickness from some mm's to several dm's. They contain thin, mm-scale silt laminae with load casts, scoured bases, and low-angle laminae, may be graded at the top, and show disruption by escape traces. Pure shale layers are mm's to cm's in thickness and come in two types: (1) as homogenous drapes atop of graded silt beds, or (2) as beds with subtle low-angle lamination that is suggestive of deposition from continuous currents. The silt as well as the shale-rich portions are suggestive of an offshore depositional regime dominated by intermittent storm deposition. The shale beds with subtle low angle laminae probably represent compacted ripples that originally consisted of flocculated clays. The latter may have been deposited from sand-silt deficient storm suspensions that were carried into the basin by off-shore flowing gradient currents. The low level of bioturbation suggests frequent storm reworking that precluded establishment of stable benthic ecosystems.