Deep-sea sedimentary processes, diagenesis and paleoflow histories determined from high-resolution seismic stratigraphy in the Amazon Fan and Argentine Basin.

摘要

High-resolution seismic profiles were used to determine the nature of the upper few hundred meters of sedimentary bedforms within the Amazon Fan and Argentine Basin. A cyclic sedimentation pattern has been seismically determined for the upper and middle Amazon Fan. Two kinds of seismic units alternate to build the fan: large levee complexes and widespread mass transport deposits. Temporal development of a levee complex is on the order of 36,000 yr suggesting it can form during a single glacial sea-level lowstand. Mass transport deposits appear instantaneous in their emplacement. The cyclic pattern seems consistent with sea-level fluctuations; however, different depositional regimes may be occurring simultaneously on different parts of the fan which may give rise to the apparent cyclicity.; Three mudwave fields wee surveyed in the Argentine Basin to determine their past and present mudwave activity. Two sites located near the Zapiola Drift showed intense erosion on their upstream flank but only one site showed present-day migration. Using a lee-wave model and sedimentation rate ratios, a paleoflow history was determined from a mudwave located south of the Ewing Drift. The studied mudwave shows active migration from before 420 ka until 26 ka when migration ceased. The paleoflow history indicates that steady bottom water flows existed at this site throughout the last glacial period and abruptly decreased near the end of the last glacial stage. Paleocurrents were moderately high during the previous interglacial period; thus there is no simple relationship between flow variations and paleoclimate at this site.; A newly-recognized, 3.5 kHz reflection pattern appears to be caused by diagenesis of abundant organic material contained within mudwaves near the Zapiola Drift. This diagenesis effects the physical properties of the sediments and leads to a reflection pattern not directly correlatable to bedding planes. Two possible mechanisms for explaining this observation are (1) methane-derived authigenic carbonates in the sediment or, more probably, (2) a methane hydrate concentrated along pathways of methane migration at shallow depths. In either case, the diagnosis directly effects the physical properties of the sediments. This diagenesis appears not to be restricted to the Argentine Basin as similar reflection patterns are found in other ocean basins.