--> Abstract: Changes In Diagenesis And Hydrogeologic Regime In The Point Lookout Sandstone (San Juan Basin, Usa) During A Complex Burial And Thermal History, by D. Larsen, J. L. Loomis, and L. J. Crossey; #90928 (1999).

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LARSEN, DANIEL1, JENNIFER L. LOOMIS2, and LAURA J. CROSSEY2
1Univ. of Memphis, Dept. of Geological Sciences, Memphis, TN
2Univ. of New Mexico, Dept. of Earth and Planetary Sciences, Albuquerque, NM

Abstract: Changes in Diagenesis and Hydrogeologic Regime in the Point Lookout Sandstone (San Juan Basin, USA) During a Complex Burial and Thermal History

Samples from 5 cores of the Point Lookout Sandstone in the San Juan Basin, Colorado and New Mexico, were investigated using petrography and clay mineralogy to understand the effects of increased burial on early diagenetic events. The paragenetic sequence in the pro-delta, shoreface, and inner-shelf sandstones is chlorite, quartz, Fe-calcite, and Fe-dolomite, kaolinite and/or illite, and late-stage Fe-calcite. In the deltaic channel sandstones, quartz cementation preceded chlorite and carbonates are less abundant. Lithic grain and feldspar dissolution probably occurred throughout much of the diagenetic history. Correlation and factor analysis of the data suggest that 4 factors are most important in dictating the types and distributions of cements and porosity: marine vs. fluvialinfluenced facies, proximity to sequence-stratigraphic boundaries, lithic and siderite abundance, and burial depth. The diagenetic history can be interpreted in terms of three distinct time intervals, each with its own diagenetic events and hydrologic regime. (1) Western-interior-basin subsidence produced the early diagenetic phases within a freshwater-marine mixing zone in a dominantly topog raphy-d riven flow regime and subordinate compactional flow regime. (2) Laramide-foreland-basin subsidence produced additional chlorite, kaolinite, illite, and possibly Fe-calcite in a sluggish compactional and possibly thermobaric flow regime at depth. (3) Mid-Tertiary uplift resulted in meteoric flushing and associated dissolution and precipitation of kaolinite. Convective flow from volcanism may also have affected diagenesis in the northern part of the basin during this time.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas