ABSTRACT: Cretaceous Stratigraphy of the San Juan Basin, New Mexico and Colorado

James E. Fassett, Cornelius M. Molenaar, William M. Aubrey, Jennie L. Ridgley, Robert S. Zech, Mark R. Leckie, Robyn Wright Dunbar

Reference sections of Cretaceous strata in the San Juan basin, northwestern New Mexico and southwestern Colorado, were compiled for the Western Interior Cretaceous project of the Global Sedimentary Geology Program. Mostly Upper Cretaceous strata as much as 2000 m thick are unconformably overlain by Paleocene rocks. Deposition of Upper Cretaceous sediments took place in middle Cenomanian through late Campanian time during two basinwide and three less widespread transgressive-regressive events. Changes in the rate of sediment delivery to the subsiding seaway (the rate of subsidence increased in Coniacian time) produced these transgressions and regressions. (Eustatic sea-level changes were relatively minor compared to the rate of subsidence during Coniacian-Campanian time an thus played an insignificant role in these transgressions and regressions of the shoreline.) Northeast-trending cross sections (normal to paleoshoreline trends) show that the rock units consist of nested, marine and continental, wedges. The marine-rock wedges are thickest to the northeast and intertongue with continental-rock wedges that are thickest to the southwest. The lower of the two basinwide marine-rock wedges consists of 200-670 m of Mancos Shale and its members, overlain and underlain by shoreface sandstones and shales. The upper basinwide marine wedge consists of 0-700 m of the Lewis Shale and is also overlain and underlain by shoreface sandstones and shales. The lower continental wedge consists of the Crevasse Canyon and Menefee formations; the upper continental wedge consist of the Fruitland Formation and Kirtland Shale. Significant hiatuses occur in the section; the lowest (1 m.y.) is an incised paleodrainage surface that separates the Burro Canyon and Cedar Mountain Formations (Albian) from the overlying Encinal Member of the Dakota Sandstone (Cenomanian). This surface may correlate with the 98 Ma sea-level fall of Haq and others. Another hiatus (< 1 m.y.), in the lower part of the Mancos Shale, truncates the Gallup Sandstone, which rises stratigraphically to the northeast. Differential tectonism, possibly in conjunction with the large (125 m) late Turonian (90 Ma) sea-level fall, produced this unconformity. Smaller scale erosion or nondeposition surfaces (< 0.1 m.y.) have been described in shoreface sandstones, but none has been traced basinwide.

AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990