New Mexico Geological Society Annual Spring Meeting — Abstracts

The oldest Cretaceous strata exposed in the Tijeras syncline (Cedar Crest Sandia Park Bernalillo County) belong to the intertongued Dakota
Formation and Mancos Shale of Cenomanian (early Late Cretaceous) age. Previous workers interpreted this stratigraphic interval as either: (1)
undifferentiated Dakota Formation directly overlain by undifferentiated Mancos Shale (Kelley & Northrop, 1975, NMBMMR Memoir 29) or (2) lower Dakota Formation overlain by a covered interval followed by a thin sandstone interval probably correlative with the Twowells Tongue of the Dakota Formation (Molenaar, C. M., 1983 in Mesozoic paleogeography of the west-central US: RMS-SEPM; Ferguson et al.,1996 NMBMMR Open File-Digital Map OF-DM-1). However, detailed stratigraphy of this interval and its correlation to nearby, better exposed sections in the Hagan basin and at Galisteo Dam, support a different interpretation of the Dakota-Mancos section in the Tijeras syncline.

The critical outcrop is along Gutierrez Canyon in the SE1/4 NW1/4 sec. 29, TIIN, R6E. Here, the section is: (1) about 10m of trough-crossbedded quartzarenitic sandstone, with a bioturbated uppermost 0.6 m, of basal Dakota Formation disconformably overlying Upper Jurassic Jackpile Member of Morrison Formation; (2) a 31-m-thick interval of gray shale that forms a mostly covered slope; (3) a 7-m-thick bioturbated sandstone cuesta; (4) a mostly covered 63-m-thick interval of dark gray shale with numerous thin bentonite beds; (5) the Bridge Creek Member of the Greenhorn Formation, a 3.7-m-thick limestone with the index bivalves Pycnodonte newberryi and Mytiloides myfiloides at NMMNH locality 4105 (UTM 379273E, 3890240N, zone 13, NAD27). By correlation to the Hagan and Galisteo sections, units 1-2 = Oak Canyon Member of Dakota, unit 3 = Cubero Member of Dakota and unit 4 = Graneros Shale. Stratigraphic position, lithology, relative thicknesses and position relative to the Greenhorn confirm that in the Tijeras syncline the upper Dakota sandstone interval is Cubero, not Twowells.