On-the-Rocks Field Trip – July 20-21, 2002

Title: On-The-Rocks Field Trips: On Again, Off Again: The Saga of Cenomanian Seas

Trip Leader: Peter J. Varney, PhD, Metropolitan State College of Denver

Dates: July 20 & 21, 2002

Publication: The Outcrop, July 2002, p. 10-11

This weekend trip will present the sequence stratigraphy of the Dakota near Chama, New Mexico, utilizing world-class outcrops. Subsurface well log cross sections show that the sequence framework established on outcrops applies to the subsurface. 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, 44 miles to the southwest.

Dakota outcrops in the eastern San Juan Basin and Chama Basin, New Mexico, show that the 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), a combination lowstand surface of erosion/trangressive surface of erosion that was subaerially exposed. Sequence 2 extends from S2 upward to Surface 3 (S3), a regressive surface of erosion 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).

Within the Sequence 3 HST, northwest to southeast trending shoreface deposits prograded northeastward. Syndepositional faults, detected through very close-spaced contour mapping, help determine their orientation. One of these shoreface deposits produces gas from abundant porosity in a fault-bounded, stratigraphic reservoir compartment. It helps confirm the importance of structurally enhanced reservoir compartmentalization in the Rocky Mountain area.

The field trip will visit outcrops in the vicinity of Chama, New Mexico. Driving time from Denver is approximately 6 hours. Participants will begin the field trip at the Chama Visitor’s Center, located at the intersection of NM17 and US64 on Saturday, July 20 at 1:00 p.m., and retire to lodging/camping at about 6 pm. The field trip will resume Sunday, July 21 at 8:00 a.m. at the Heron Lake Visitor’s Center (15 miles south of Chama) and conclude at about noon. All of the field trip stops are roadside with the exception of one which involves a moderately strenuous hike of 1 mile round trip.

According to the website, “All Roads Lead to Chama” and it is “a vacationers paradise;” visit http://www.chama.com for accommodations and attractions. Lodging suggestions include Gandy Dancer Bed & Breakfast (800.424.6702, 10% discount for RMAG) and Elkhorn Lodge (800.532.8874.) While in Chama, consider riding the Cumbres and Toltec Scenic Railroad (888.286.2737.) Nice camping facilities are available at Heron Lake State Park (www.icampnm.com or 887.664.7787.)

For field trip reservations contact Fred Mark, email (preferred): fmark@csd.net, or 303.674.1682.