Strandplain and δic depositional models for the Point Lookout Sandstone, San Juan Basin and Four Corners Platform, New Mexico and Colorado
Robyn Wright-Dunbar, Robert S. Zech, Gregg A. Crandall, and Danny Katzman
The Point Lookout Sandstone of Late Cretaceous (Campanian) age is a regressive marine coastal sandbody that records variable influences of storm, wave and fluvial processes in the San Juan Basin area. Widespread linear strandplains and associated estuarine and inner-shelf deposits developed along storm- and, to a lesser extent, wave-dominated shorelines. The strandplain record is well preserved within high-frequency transgressive-regressive depositional cycles (parasequences and progradational parasequence sets) that have composite thicknesses of as much as 120 m. Fluvially derived sediment supplied to strandplain coastlines was either trapped within the intertidal estuarine system or was effectively reworked along depositional strike into marine shoreface deposits. Although linear strandplain deposits dominate Point Lookout facies on a basinwide scale, delta mouthbar sandstones are present locally and are especially well developed in the Durango area (Durango delta) of southwestern Colorado. The change from strandplain to deltaic deposits is characterized by a variable sandbody geometry, decrease in bed thickness, decrease in sorting, increase in organic matter, and a different suite of sedimentary structures. Eastward transition from strandplain to fluvial deltaic deposition in southwestern Colorado was accompanied by an abrupt basinward (northeast) shift in coastal-plain facies, a coincident thinning (to less than 30 m) of the Point Lookout, and a decrease in stratigraphic rise. Associated with these changes is a subtle, previously undocumented unconformity within the upper Point Lookout west of Durango. Whether this unconformity underlies or overlies the bulk of Durango delta deposits in the seaward direction is a subject of continuing study.
- Wright-Dunbar, Robyn; Zech, Robert S.; Crandall, Gregg A.; Katzman, Danny, 1992, Strandplain and δic depositional models for the Point Lookout Sandstone, San Juan Basin and Four Corners Platform, New Mexico and Colorado, in: San Juan Basin IV, Lucas, Spencer, G.; Kues, Barry S.; Williamson, Thomas E.; Hunt, Adrian P., New Mexico Geological Society, Guidebook, 43rd Field Conference, pp. 199-206.