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Coastal Record--Preservation Potential, and Distribution of Facies Along Upper Cretaceous Point Lookout Shoreline, Northwest New Mexico

Robyn Wright, Robert S. Zech

Distinct assemblages of coastal depositional facies and progradational cycles document straight (wave-dominated) and embayed (fluvial- or tide-dominated) strandline segments in exposures of the regressive Point Lookout Sandstone in the southern San Juan basin, New Mexico. Wave-dominated strata include well-developed upper shoreface, foreshore, beach-ridge, washover, lower coastal-plain channel and marsh, and lagoonal deposits. Fluvial- or tide-dominated strata include transition from upper shoreface to delta front, foreshore, and lower delta-plain distributary and tidal-creek deposits. A varied coastal energy regime influenced the rate of shoreline migration and produced distinct patterns of small (< 9 m) progradational cycles.

Amount of cycle overlap and, to a lesser degree, distribution of erosional processes control the preservation of coastal depositional facies. Wave-dominated strata in the southwestern part of the basin show that progradation was relatively slow and cycle overlap was great, reflecting a stable, vertically accreting shoreline near its landward pinch-out. Tidal-channel and fluvial-channel erosion was minor, and wave-dominant facies are completely represented, including preservation of linear beach-ridge and back-beach paleotopography. Fluvial- or tide-dominated strata in the southeastern part of the basin show that progradation was abrupt, and vertical stacking of cycles was minimal. Here, in contrast, fluvial and tidal erosion effectively removed subaerial and shallow coastal deposition l facies and paleotopography.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.