Dakota Sequence Stratigraphy, San Juan and Chama Basins, New Mexico

By

Peter J. Varney

Metropolitan State College of Denver, Denver, CO

Dakota outcrops in the eastern San Juan Basin and Chama Basin, New Mexico, show that this interval contains three depositional sequences within an overall third-order transgressive systems tract. Sequence 1 lies on the regional K2 unconformity above the Early Cretaceous Burro Canyon Formation and extends upward to Dakota Surface 2 (S2). S2 represents a combination lowstand subaerial erosion surface and a trangressive erosion surface. Sequence 2 extends from S2 upward to Surface 3 (S3), a regressive erosion surface that extends throughout the study area and perhaps much further. The interval between S2 and S3 comprises a transgressive systems tract in which there are two or more shorefaces that prograded basinward during fourth-order relative-sea-level fluctuations. Sequence 3 extends from S3 upward to the top of the Dakota interval. It contains fourth-order lowstand, transgressive and highstand systems tracts (HST). Dakota parasequence characteristics are continuous over great distances along paleostrike, allowing outcrop gamma ray profiles to tie to, and permit calibration of, well logs in the South Lindrith Field in the San Juan Basin, 44 miles to the southwest. Subsurface well-log cross sections show that the sequence framework established on outcrop applies to the subsurface. Within the S3, HST, northwest-trending shoreface deposits prograded northeastward. Low magnitude, syndepositional faults were detected on large-scale, small-contour-interval, contour maps that helped determine fault orientations. One Dakota reservoir produces gas from a high-porosity shoreface deposit within a fault-bounded, structurally enhanced, stratigraphic reservoir compartment. This example illustrates the importance of understanding structurally controlled depositional environments in the Rockies.

AAPG Search and Discovery Article #90004©2002 AAPG Rocky Mountain Section, Laramie, Wyoming