LOOMIS, JENNIFER L., and LAURA J. CROSSEY, Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM

ABSTRACT: Diagenesis in a Cyclic, Siliciclastic Regressive Sequence: The Point Lookout Sandstone, San Juan Basin, New Mexico and Colorado

The Point Lookout Sandstone is a regressive, marine shoreline sequence deposited along the western margin of the Cretaceous epicontinental seaway. Other workers have identified parasequences and parasequence sets within exposures of the Point Lookout Sandstone in the San Juan Basin, CO and NM. Sandstone and mudstone samples collected from outcrops and two U. S. Geological Survey cores drilled in the northwest corner of the San Juan Basin, Colorado, have been characterized in terms of diagenesis and clay mineralogy.

Variations in detrital mineralogy correspond with depositional facies. Diagenetic reactions in sandstones include dissolution of feldspars and lithics, quartz overgrowth cementation, chlorite and kaolinite precipitation in pores, calcite cementation, dolomitization of carbonate grains and chert, occasional gypsum cementation (associated with altered pyrite), and authigenic Fe-rich dolomite rims on detrital dolomite grains. Early cements include poikilotopic calcite, quartz overgrowths, and Fe-dolomite rims on dolomite grains. The extent of quartz overgrowth cementation and porosity reduction by precipitation of Fe-dolomite rims are related to variations in initial sandstone composition (i.e., depositional environment). Quartz overgrowth cementation is most significant in subarkosic sa dstones deposited in the foreshore environment, and bar crest deposits in deltaically-influenced sections. Textural observations argue for a detrital origin for dolomite grains. Petrographic analysis shows an increase in detrital dolomite (up to 15%) with increasing depth in each of the cores, corresponding to deeper marine facies. An intrabasinal source is interpreted for the dolomite grains, based on texture and their absence in fluvial sands. The Fe-dolomite cement occurs as rims: its abundance is linked to the variations in intrabasinal detrital dolomite.

Poikilotopic calcite cement is a major control on present porosity within Point Lookout Sandstone. The extent of calcite cementation ranges from 0 to 25%, and is not correlated with specific depositional environments. The calcite-cemented sandstones do appear to be related to stratigraphic boundaries, and are interpreted to record early variations in pore fluid chemistry associated with cyclic changes in shoreline position.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.