--> ABSTRACT: Early Carbonate Cementation in the Cretaceous Point Lookout Sandstone: Relationship to Cyclical Deposition, San Juan Basin, Colorado, by Jennifer L. Loomis, Laura J. Crossey; #91020 (1995).

Datapages, Inc.Print this page

Early Carbonate Cementation in the Cretaceous Point Lookout Sandstone: Relationship to Cyclical Deposition, San Juan Basin, Colorado

Jennifer L. Loomis, Laura J. Crossey

The Point Lookout Sandstone, comprised of coastal marine siliciclastics, was deposited along the western margin of the North American Cretaceous western interior seaway. The Point Lookout is a 3rd-order regressive sequence containing 4th- and 5th-order parasequences. This study utilizes samples obtained from outcrop in Mangos Canyon, located along the northwest perimeter of the San Juan basin. Detailed sampling was conducted within one 4th-order parasequence (comprising four 5th-order parasequences), across laterally extensive three-dimensional outcrop exposure. In the 4th-order parasequence sampled, the transgressive sequences are dominantly inner-shelf sandstones and shales, while the regressive sequences are shoreface sandstones.

Early cements include kaolinite, quartz overgrowths, calcite and Fe-dolomite. Calcite cements are volumetrically the most important. Three types of calcite cement formed in the following order: poikilotopic Fe-calcite, poikilotopic calcite and sparry calcite. Poikilotopic calcite occurs both as large concretions (up to 3m in diameter) and as disseminated patches (<0.5 mm). The 4th-order transgressive event is always marked by concretions; the concretions are localized within the uppermost regressive shoreface units beneath this transgressive sequence. Except for a few rare exceptions, concretions do not occur in any transgressive units. The small patches of poikilotopic Fe-calcite are found throughout the Point Lookout and are closely associated with the poikilotopic calcite. The s arry calcite is present in between the poikilotopic calcite patches. The disseminated patches and sparry calcite cement occur in regressive and transgressive sandstones adjacent to transgressive shale units.

The spatial distribution of the calcite cements indicates that their precipitation was indirectly controlled by cyclical fluctuations in relative sea level in two different ways: 1) variations in pore fluid chemistry during early burial, as controlled by burial rates and the lateral shifting of hydrologic regimes in response to relative changes in sea level and 2) the geometry of sandstone and shale layers, reflective of cycle hierarchy and the relative magnitude (spatial and temporal) of the sea level fluctuations.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995