Sequence Stratigraphy of the Dakota Sandstone in the San Juan Basin, New Mexico, Usa

Peter Johannessen¹ and Dag Nummedal^2
1Geological Survey of Denmark and Greenland, Copenhagen, Denmark
²CO Energy Research Institute, Colorado School of Mines, Golden, CO

This paper reports on a comprehensive outcrop-to-subsurface study of the Dakota Sandstone in the San Juan basin and is based on 25 measured sections along the basin margin, 14 gamma ray logs from uranium boreholes along the southern basin edge, and 26 wireline logs from oil and gas wells in the deep basin interior.

The marine sandstone tongues often consists of one to three upward-coarsening successions, from 4 to12 m thick, and are interpreted to be delta front or shoreface deposits. They commonly overlie the offshore shales abruptly, but have gradational bases in the distal parts consistent with the interpretation that the marine sandstones were deposited during the final stages of relative sea level fall and the incipient phase of renewed rise. The proximal shoreface sandstones are commonly erosionally overlain by fluvial- or tide-dominated sandstones. The valleys were cut concurrent with the progradation of the shorefaces during the falling stage of sea level and the backfilling occurred during early lowstand and late transgressive phases of relative sea level rise. Dakota Sandstone deposits associated with the highstand systems tract are not positively identified.

Paleogeographically, the Dakota depositional systems of the San Juan Basin evolved significantly during their approximately 3 million years of existence (97 - 94 Ma). The first marine transgression in the early Middle Cenomanian (Upper Oak Canyon shale tongue at 95.5 Ma) established a fairly straight north-south shoreline. Subsequent Middle Cenomanian shorelines (Cubero and Clay Mesa tongues) remained regionally fairly straight but assumed more of a north northwest to south-southeast orientation. At the time of the Paguate and Twowells regression, in the latest Middle Cenomanian and Late Cenomanian, the shoreline had become distinctly lobate.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas