Title

Diagenesis in the Point Lookout Sandstone, San Juan Basin, New Mexico and Colorado: Influence of depositional conditions, cyclic stratigraphy, and changing hydrologic regime

Abstract

The Cretaceous Point Lookout Sandstone in the San Juan Basin, Colorado and New Mexico, was deposited along the Western Interior Seaway in a marine strandline depositional environment that locally was supplanted by deltaic environments. Previous studies of outcrop samples documented precipitation of early diagenetic cements that was in part controlled by cyclic interactions between relative sea level in a meteoric water-seawater mixing zone. In this study, we investigated the Point Lookout Sandstone in 6 cores that transect the San Juan Basin to determine if similar relationships can be documented in the subsurface Early diagenetic events include precipitation of Fe-dolomite, Fe-chlorite, quartz, and Fe-calcite cements with minor pyrite and siderite, minor compaction, and probable dissolution of feldspar and lithic grains. Burial diagenetic events include continued precipitation of Fe-chlorite and quartz with minor illite, kaolinite, Fe-calcite, feldspar overgrowths, and barite and minor compaction. Alteration of lithic fragments to Fe-chlorite and feldspar to illitic clays, albite, and Fe-calcite is extensive. Late diagenetic events in shallow and basin-margin cores include dissolution of feldspars, lithic fragments, and earlier cements and precipitation of pore-filling kaolinite and minor Fe-calcite. Depositional environment, redox conditions, mixing of fresh water and seawater, and cyclic stratigraphy were important controls on the distribution of the early cements and porosity evolution. Burial diagenesis is interpreted to have initially been largely a closed-system process with some up-dip migration of compactional fluids. Late diagenesis is interpreted to reflect middle Cenozoic uplift, exhumation, and flushing of the Point Lookout Sandstone by meteoric waters around the periphery of the basin. Similar paragenetic sequences of diagenetic cements are documented in deltaic and shoreline sandstones in other Cretaceous Laramide-style foreland basins in western North America. These similarities suggest that diagenesis in these shoreline-deltaic depositional systems was initially controlled by depositional conditions and cycle stratigraphy, but was subsequently strongly affected by events related to tectonic evolution of the basins and associated changes in hydrologic regime.

Publication Title

Mountain Geologist

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