Rhythms in deep sea, fine‐grained turbidite and debris‐flow sequences, Middle Ordovician, eastern Tennessee

Abstract

ABSTRACTRhythms in fine‐grained sediments have not previously been studied because of their complex depositional cycles and diagenetic changes. Examination of nearly 9000 layers in outcrops, polished slab samples, and thin sections has led to recognition of rhythms in the Middle Ordovician Whitesburg, Blockhouse, and Sevier formations of east Tennessee. Rhythms have been described in four orders based on relative magnitude. First order cycles (basin‐fill sequences, hundreds of metres thick)are composed of thinning upward debris‐flow sequences and thickening upward turbiditic shale sequences. Second order cycles (multiple sedimentation units, tens of centimetres thick) consist of: (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 millimetres thick) have thinning upward and asymmetrical cycles. Fourth order portrays the grain‐size variations within a single layer of the third order. First order rhythms were controlled by tectonism and progradation of a deep‐sea fan system. Second and third order rhythms were controlled by depositional processes, bottom topography, and sediment source. Depositional processes, sediment source, and bioturbation were the dominant controlling factors in the fourth order. A deep‐sea fan model is proposed for the fine‐grained turbidites in which channels and lobes coexist in the lower fan. The lower fan/mid fan boundary is marked by the presence of a channel cycle near the top of a lobe sequence. The basin plain/lower fan boundary is suggested by the appearance of minor lobe cycles over the non‐cyclic basin plain sequence.