Bioturbation, sedimentation rates, and preservation of flood events in deltas

摘要

We examine the nature and number of event beds preserved in heterolithic prodelta and delta-front strata in five proximal-accumulation-dominated (PAD) North American delta systems including the Quaternary Gulf of Mexico and the Late Cretaceous Western Interior Seaway. We also explore the continuum between PAD, more distal deposits of mud belts, and thicker subaqueous-delta-clinoforms (SDC). Sediment accumulation rates in modern PAD and SDC systems are commonly on the order of several to severaltens of centimeters per year. They also show that several event beds may be created annually, with preservation depending on supply and reworking. The PAD heterolithics commonly comprise interbedded sand, silt and clay. These show normal and inverse grading and Bouma sequences, indicating deposition from turbidity currents and hyperpycnal flows that are interpreted as the products of river plumes generated during flood episodes. Re-equilibrated Rosselia socialis in stacked delta front turbidite bedsets in the Cretaceous examples show that floods may occur as a cluster of events over several months. Shelf areas adjacent to river mouths may experience high rates of sediment delivery from rivers, but oceanographic processes are instrumental in moving much of the silt and clay farther along the shelf. Direct, hyperpycnal delivery from a river mouth on a low gradient shelf (i.e., typically<0.05 degrees) is rare, owing to the difficulty of maintaining a high suspended sediment concentration, but sedimentation rates are nevertheless high. Shelf facies show evidence for extensive and frequent remobilization during storms and other meteorological events. Storm-wave-enhanced sediment-gravity flows (WESGFS) may form under favorable conditions, such as strong waves, high suspended sediments, and convergent currents, and can be significant in building SDC and other shelf deposits. The proximal portions of PAD and SDC systems typically show low intensity and diversity of bioturbation (BI 0-1). The bioturbation styles encountered in these deposits record: 1) high sedimentation rates, which preclude wholesale biogenic reworking; 2) high-frequency river floods and storm reworking, expressed by reduced colonization windows; 3) soupy substrates, likely reflecting formation of a permanent fluid-mud lutocline; and 4) elevated turbidity near the sediment-water interface, which may cause further suppression of trace-making.