Transport Mechanisms and Genesis of Limestone Clast Conglomerates with Examples from Cambrian of East Tennessee

Michael G. Kozar, Lawrence J. Weber, Kenneth R. Walker

Limestone clast conglomerates are common sedimentary features of most Cambrian strata. Cambrian researchers have considered many of these conglomeratic deposits to have formed from erosion and redeposition of partially lithified sediments by storm currents in shallow subtidal and intertidal environments. Analysis of conglomeratic beds from the Middle and Late Cambrian (Maryville Limestone and Nolichucky Shale) in east Tennessee suggests that other, more dominant, processes were responsible for their genesis and transport. The proposed processes may serve as an alternative explanation for limestone clast conglomerates deposited elsewhere in Cambrian sequences.

Limestone clast conglomerates in the Tennessee Cambrian sequence were deposited within ramp, slope, and basinal settings representing a complex array of depositional processes. In addition to storm transport, many beds have resulted from mass-gravity transport mechanisms. Based upon sedimentary fabric, clast to matrix ratios, matrix type, and bed geometry and thickness, two dominant varieties of conglomerates can be perceived. (1) Mud-matrix monomictic conglomerates typically occur in slope environments. These beds underwent minor transport resulting from slumping and debris flow mechanisms. (2) Coarse-matrix polymictic conglomerates have a more complex history. These conglomerates were transported from shallow ramp and slope environments to the basin by debris and modified grain-flow mechanisms. Moving as nearly frictionless masses, these deposits required very low slopes to initiate and sustain movement.

As we gain a greater understanding of transport mechanisms and their resultant sedimentary features, many of the conglomeratic beds that were once thought to be the result of storms may be reinterpreted as mass-gravity flows. Differentiating between these various types of transport mechanisms may be crucial to paleo-environmental interpretation.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.