Architectural Elements of Fan-Delta Complex in Pennsylvanian Taos Trough, New Mexico

Kristian Soegaard

Identification of architectural elements within alluvial-fan and subaqueous fan-delta gravel units is fundamental to resolving depositional processes within fan-delta complexes of the Pennsylvanian Taos trough, New Mexico.

Architectural elements of alluvial-fan gravels are defined by fourth-order bounding surfaces and fall into two categories. Tabular bodies structured internally by concordant, horizontally stratified gravel beds represent unconfined sheet-flood deposits. The second category of architectural elements also consists of horizontally stratified gravel beds; however, these beds terminate against the lower fourth-order bounding surface. Discordant horizontally stratified gravel beds occupy channel complexes and exhibit a diverse assemblage of lateral accretion-set geometries. Accretion-set geometries are related to alluvial-fan channel patterns and channel-fill mechanisms.

Subaqueous fan-delta deposits consist of lenticular gravel-body complexes encased by black, basinal shales. Gravel-body complexes are composed of a series of stacked gravel lenses, each of which is enveloped by fifth-order bounding surfaces. The central portion of individual gravel lenses contains a channel complex. Channels are outlined by third- and fourth-order bounding surfaces and are infilled by high-density gravity flow deposits. The fringe of submarine gravel lenses consists of stacked, laterally continuous Bouma sequences separated by second-order bounding surfaces. Bouma sequences were deposited by dilute turbidity flows during evacuation of submarine channels. Subaqueous channel complexes within gravel lenses represent midfan channels, whereas the fringe of lenticular grave lenses represent outer-fan lobes.

Recognition of depositional processes and architectural elements of fan deltas in the Sandia Formation enables distinction between these and other types of coarse-grained deltas in the Taos trough. This, in turn, has implications for resolving evolution of the trough.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.