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ABSTRACT: Modern Sand-Rich and Mud-Rich Siliciclastic Aprons--Alternative Base-of-Slope Turbidite Systems to Submarine Fans

C. H. Nelson, A. Maldonado, J. H. Barber, Jr., B. Alsonso

Base-of-slope aprons that lack significant channel development and are fed by multiple sediment sources (i.e., numerous slope failures or debris chutes) represent an alternative facies organization to submarine fans that possess channels and are fed by canyon point sources of allochthonous sediment. Two styles of base-of-slope apron deposition are evident: (1) sand-rich aprons with rhythmic, gradational sand and gravel sheets that evolve to basin-plain turbidites as exemplified by Crater Lake aprons and (2) mud-rich aprons composed of chaotic sediment flow lobes without gradation as exemplified by Ebro margin aprons.

Sand-rich aprons typically occur at the base of steep slopes or fault scarps in volcanically or tectonically active basins (i.e., trench floor, rift, back arc, transform) in areas that lack large point sources of sand. These single or coalesced cones usually are small (several kilometers long), have their apices at the base of slope, decrease gradually in thickness toward the basin center, and lie conformably on the basin floor without erosional truncation of underlying beds. Proximal aprons exhibit numerous discontinuous high-amplitude and wedging seismic reflections together with high sand-shale ratios from thick, unsorted sand and gravel beds. The proximal aprons evolve gradually to basin-plain environments that exhibit low amplitude, parallel and continuous seismic reflections and low sand-shale ratios from thin and fine-grained turbidite sand layers. The sedimentary processes also evolve gradually from slides on the slope to a variety of sediment-gravity sheet flows over the aprons and then to

turbidity current sheet flows over the basin-plain areas.

Mud-rich aprons may occur at the base of any mud-rich basin slope with multiple retrograde failures but are most common on mud-draped passive margins associated with prograding river deltas. Compared with sand-rich aprons, mud-rich aprons are usually larger (tens of kilometers long), more lenticular, more elongate and irregular in thickness and shape, thinner at the apex (which may be detached from the slope base), and without gradual gradation of facies at distal margins. They exhibit erosional truncation with underlying beds, possess transparent or chaotic seismic facies throughout, contain chaotic mixtures of deformed mud and sand beds everywhere, and thus appear to be deposited mainly by debris-flow processes without distal gradation to turbidity currents.

Aprons may occur coevally with or evolve into other channelized turbidite systems. Consequently, definition of modern apron facies should help the interpretation of complex associations of aprons and fans and assist the search for ancient sand-rich aprons that sometimes possess excellent reservoir beds.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990