| OCR Text |
Show 30 sections measured by USGS stratigraphers (O'Sullivan, 1980a, 1980b, 1981a, 1981b, 1984b, 1991; O'Sullivan and Pierce, 1983; O'Sullivan and Pipiringos, 1983) correlate the USGS-defined units across the western Colorado Plateau from the study to area southwestern Colorado and northwestern New Mexico. of the Tidwell Member Age Radiometric (4oArP9Ar) dating of an ash bed that occurs 2.5 to 3 base of the Tidwell Member in southeastern Utah Ma (latest Oxfordian; Kowallis Member suggest appearances of an et or occur age of in the northern Western Interior basin forms occurring in this unit. Windy Hill (Peterson. 1994; Kowallis et and the Redwater Shale equivalent (Imlay, 1980). Thus, al., 1998). In the study strata are not (Figure 7). USGS Stump late and stratigraphers to 1 million years area, where the J-4 preserved, the gap unconformity represented by In southwestern Colorado the Tidwell Member overlies the Wanakah Formation, and O'Sullivan represented by the Early Member of the Morrison Formation suggest that the minimum duration of the J-5 unconformity is 0.5 the J-5 surface is greater than this 154.8 with first in the Redwater Shale Member of the Sundance Formations that underlies the or approximately early Kimmeridgian, diagnostic Kimmeridgian Oxfordian ammonite fossils an al., 1998). Fossil palynomorphs in the Tidwell age of latest Oxfordian some gives above the m (1992) suggests that the time J-5 surface includes all of the Oxfordian and part of the upper Callovian. Radiometric dates from Member (150.3 Ma; Kowallis initiated by Member deposition likely the middle to late et near the base of the upper part of the Brushy Basin al., 1998) suggest that deposition ofthis unit had Kimmeridgian. Therefore, occurred during the early Tidwell and Salt Wash and late Kimmeridgian. |
