New Mexico Geological Society Annual Spring Meeting — Abstracts

Sedimentary processes active within a depositional environment to a large degree determine the texture and mineralogy of the deposited sediment and influence the type and degree of subsequent diagenesis. The upper Cretaceous Point Lookout Sandstone is a regressive coastal sandstone forming the basal member of the Mesaverde Group. Overlying the Point Lookout Sandstone is the continental Menefee Formation and the transgressive marine Cliff House Sandstone (uppermost member of the Mesaverde Group)" Recent outcrop studies of the Point Lookout Sandstone in the San Juan Basin have identified three major environments; deltaic, shoreface and foreshore.
Depositional environments impart distinct textural and mineralogical characteristics to deposited sediment which in turn influence the type and extent of early diagenesis. Petrographic examination yields the following observations.

Deltaic deposits, including bar crest to distal deltaic subenvironments exhibit a wide range of textures, but may be characterized as poor to moderately sorted, subangular, and very fine-to medium-grained sandstones with a mean composition of Q₆₅F₂₂L₁₃. Mineral paragenesis varies with position within the delta. Early cementation of bar crest deposits by quartz overgrowths prevents later stages of diagenesis. Bar front to distal deltaic deposits exhibit extensive alteration and replacement of quartz and feldspar by calcite with quartz overgrowths less commonly observed.

Shoreface deposits are moderatley sorted, subangular, and very fine-to fine-grained sandstones with a mean composition of Q₅₇F₂₂L₁₁. Replacement of quartz and feldspar by calcite, authigenic clay growth, and feldspar alteration are common in shoreface deposits. In both shoreface and bar front to distal deltaic deposits the percentage of primary carbonate grains increases with distance from paleo shorelines.

Foreshore deposits are well rounded, moderately sorted, and medium-grained sandstones with a mean composition of Q₇₇F₁₆L₇. As in bar crest deposits quartz overgrowth cementation is common, and primary carbonates are noticeably absent. Porosity throughout the Point Lookout Sandstone is typically very low, averaging 1.5%.

Burial history reconstructions of the Point Lookout Sandstone indicate maximum burial (12,000 feet) during the early Miocene. Paragenetic sequence as determined by thin section study indicates the following sequence of events; quartz overgrowth cementation, replacement of quartz and feldspar by calcite and growth of pore-filling kaolinite.

Diagenesis of the Point Lookout Sandstone can be correlated with specific depositional environments and may be understood in terms of the control depositional environment has on clastic mineralogy, textures, and early pore fluid chemistry.