Abstract

Fan-delta and braid-delta deposits are distinguished in the Pennsylvanian Sandia Formation, Taos Trough, New Mexico, according to guidelines recently defined by McPherson and others. Subaqueous sedimentation styles and depositional sequences also are used for distinguishing fan-delta and braid-delta deposits in the Taos Trough.

Fan deltas were deposited in close proximity to the active western boundary fault of the Taos Trough and occur as coarse-grained aggradational sequences. Alluvial processes in fan-delta complexes were characterized by unrestricted sheetflood and ephemeral streamflow deposition. Subaqueously deposited, fan-delta conglomerates and sandstones arranged in lenticular units encased by basinal mudstones were emplaced by a variety of high-density and dilute gravity-flow processes.

Braid-delta complexes were characterized by channelized alluvial deposition in which discharge was variable, but perennial. Perennial discharge in braid deltas was in response to expansion of the drainage area in the Taos Trough hinterland through time. This resulted in increased sediment discharge into the trough and enabled basinward extension of the alluvial plain by braid-delta progradation.

This study concludes that fan deltas are associated with steep basin margins in which overall subsidence rates exceed sedimentation rates. Braid deltas are related to low-gradient basin margins along which sedimentation rates are greater than subsidence rates. Distribution of fan-delta and braid-delta complexes in the Sandia Formation is dictated by tectonic load-induced subsidence in the Taos Trough and may be used as a guide for resolving evolutionary history of other types of active sedimentary basins.

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