Short-term morphodynamics and depositional architecture of river-fed turbidite systems in modern and ancient settings.

Abstract

Deep-marine environments dominated by submarine avalanches, called turbidity currents, are difficult to study due to their inaccessibility and destructive nature. Linking turbidity current processes to their deposits is particularly challenging in short-time periods (months to centuries) because direct observations of such flows are rare and time resolution of most geological records is limited. Yet understanding short-term morphodynamics is key to improving predictive models. This thesis provides a multidisciplinary analysis using modern and ancient datasets to help bridge the gap between modern flow dynamics and short-term-to-ancient stratigraphic record.
The analysis uses repeat-bathymetry maps from months to decades in two modern river-fed systems: Bute Inlet, West Canada, and the Congo Fan, West Africa. Measurements of sediment accumulation, erosion and migration rates over channels, levees and lobe show persistent relative variabilities over different event magnitudes and system settings which quantified the system´s self-organisation (autocyclicity). However, extreme events substantially increase rates of degradation and knickpoint migration and deposition area. Time-lapse maps reveal several-km-long downstream alternations of erosion and deposition, forming Erosion-Deposition Zones (EDZs), where large turbidite macroforms evolve in backstepping and onlap the erosion zones. Extreme events produce EDZs of longer wavelengths and higher amplitudes. EDZs and macroforms are formed by cyclic-step instabilities at the dense-and-fast flow front, while mesoscale bedforms are formed synchronously by cyclic-step instabilities at the flow body. These EDZs short-term morphodynamics create large and thick isolated deposits.
An ancient-modern analysis tests the geologic relevance of these new short-term morphodynamics insights using the Miocene turbidites of the Tabernas basin, Spain. The study shows two bedset facies associations pointing to: (A) cyclic-step bedform deposits, and (B) decelerating-flows deposits. Different bedsets combine into turbidite storeys allowing analogies with the Bute Inlet´s macroforms. A sensitivity analysis of sedimentation rates and stratigraphic completeness quantifies the highly fragmented nature, and the high time-space heterogeneities of small, metre-scale deposits in the stratigraphic record.