Paleohydrology and Early Diagenesis within the Late Cretaceous Point Lookout Sandstone, San Juan basin, New Mexico and Colorado

J. L. Loomis, L. J. Crossey, and C. J. Yapp

A diagenetic study of the regressive marine Point Lookout Sandstone suggests that the paleohydrology associated with cyclical deposition and fluctuations in relative sea-level has a significant impact on the early lithification of a sandstone. Early cementation within the Point Lookout Sandstone is attributed to lateral geochemical variations within a coastal plain aquifer, where meteoric waters extended out into shelf sands during regressive progradation. Precipitation of the earliest cements during regressive deposition, the subsequent influx of seawater during relative rises in sea level, slow sedimentation rates during transgressive deposition, and the deposition of organic-rich transgressive sediments collectively influenced both the pore fluid chemistry and spatial ementation patterns within the cyclical stratigraphy.

Earliest pore fluids are interpreted to have been mixed marine and meteoric waters, as suggested by the presence of early authigenic chlorite, kaolinite and depleted ¹⁸O values in the laterally-continuous shoreface units. An increased meteoric component in landward porewaters, marked by kaolinite distribution, reflects the oceanward shift of a meteoric hydrologic regime during regressive progradation of the shoreline. The succession of Fe-calcite cement after Fe-dolomite overgrowths indicates an increase in sulfate concentrations (an increased flux of seawater during a relative rise in sea level, or decreased levels of sulfate reduction). Much of the Fe-calcite formed as a concretion zone beneath a major transgressive flooding surface within a decameter-scale transgressive- egressive cycle. Stable isotope analyses indicate that the concretionary Fe-calcite precipitated within a mixing zone (^dgr¹⁸O_(PDB) =-11 to -20 ± 2 ^pmil); a component of the carbon was likely derived organically (^dgr¹³C_(PDB)= -3 to -13 ± 1.5 ^pmil). Increasingly depleted ^dgr¹⁸O values in the Fe-calcite oceanward (within the decameter-scale cycle) coincide with an increasing abundance of early authigenic chlorite. Because meteoric waters would not produce this oceanward trend in ^dgr¹⁸O values, chemical modification of porewaters through diagenesis is called upon. Decreasing ^dgr¹³C values oceanward correspond to increasing Mn/Fe ratios within the Fe-calcite, reflecting an increasing organic componen in a shoreface underlying an increasingly thicker wedge of transgressive inner-shelf deposits. This implies that transgressive deposition also impacted porewater chemistry in the underlying shoreface units.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California