How Many Turbidity Currents Pass Through a Submarine Channel and What is Their Stratigraphic Expression?

Abstract

The ability to directly monitor sediment gravity flows that shape submarine channels on the modern seafloor remains largely elusive. As a result, the sedimentary record is perhaps the most accessible and suitable laboratory from which to consider the dynamic history of channelized sediment transfer and deposition within slope channels. Our analysis features the well-preserved fill of an outcropping submarine channel (“Unit -2”) in the Tres Pasos Fm (Cretaceous), Chile. A spectrum of processes results in fill patterns within channelform bodies 15–25 m thick and 200–400 m wide, including an axial sandstone-dominated zone bound by composite erosional surfaces that define lateral contacts with thin-bedded and finer- grained channel margin units. Channel margin deposits drape or lap onto the composite edge of the channel form. Seventy to eighty percent of the cross-sectional channel fill consists of sandstone-dominated (axis) strata and 20–30% mudstone-prone (margin) strata. A series of 17 stratigraphic sections measured at 0.1 cm resolution through the channel fill, from the channel axis through margin transition, document the number and spatial distribution of sedimentation units, or turbidity current events. Our analysis reveals that >500 individual, distinct sedimentation units are present in the single channel fill. However, <5% of the recorded sedimentation units are preserved in channel axis deposits. Insight into prolonged sediment transfer is preserved in cross-sectionally limited channel margin sedimentation units; many of these fine-grained units overlie erosion surfaces, recording deposition from the dilute tails or fringes of highly erosive, mainly bypassing turbidity currents. The total number of distinct sedimentation units represents the minimum number of turbidity currents that passed through the channel over its lifecycle. Stratigraphic correlation reveals that thinner-bedded, finer-grained margin facies track into thicker-bedded axis facies. At least ten discontinuities form the composite surface that defines the contact between channel axis and margin; a nuanced, time transgressive channel evolution interpretation is required to account for the observations made. A simple, two-step cut and fill model for submarine channels does not represent the prolonged and dynamic nature of evolutionary processes. Thick-bedded, sand-rich channel filling is highly episodic and rare against a background of channel maintenance by bypassing turbidity currents.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015