Rhythms in Deep-Marine Turbiditic Shales and Fine-Grained Debris Flow: ABSTRACT

Abstract

Rhythms in fine-grained sediments have not been studied previously because of their complex depositional cycles and diagenetic changes. Examination of nearly 9,000 layers in outcrops, polished slab samples, and thin sections has led to recognition of rhythms in the Middle Ordovician Whitesburg, Blockhouse, and Sevier formations in eastern Tennessee. Rhythms have been described in four orders, based on their magnitude. First-order cycles (basin-fill sequences, hundreds of meters thick) are composed of thinning-upward debris-flow sequences and thickening-upward turbiditic shale sequences. Second-order cycles (multiple-sedimentation units, tens of centimeters thick) constitute six types: (1) thinning-upward cycles, (2) symmetrical cycles, (3) thickening-upward cycles, (4) minor multiple cycles, (5) uniform cycles, and (6) dubious cycles. Third-order cycles (single-sedimentation unit, tens of millimeters thick) have thinning-upward and asymmetric types. Fourth-order cycles show the grain-size variations within a single silt layer of the third order. First-order rhythms were controlled by tectonism, progradation of a deep-sea-fan system, and debris flow. Second- and third-order rhythms were controlled by depositional processes, bottom topography, and sediment source. In the fourth-order rhythms, depositional processes, sediment source, and bioturbation were the dominant controlling factors. Rhythm analyses of fine-grained sediments are significant in understanding (1) major events related to tectonism, End_Page 525------------------------------ (2) deep-sea environments, and (3) frequency and dynamics of depositional events. End_of_Article - Last_Page 526------------