New Mexico Geological Society Annual Spring Meeting — Abstracts

Anderson and lucas (New Mexico Geology, 1992, 14/4, 79-92) (A&l) recently criticized the present state of stratal relationships on the Colorado Plateau that we have been working on for the past 25 or so years. Here we respond to the most salient points of their arguments.

A major contention of A&l is that the Todilto ls. Mbr. and overlying largely red-bed strata of the Wanakah Fm. of NW New Mexico correlate with the Curtis Fm. and largely red-bed strata of the overlying Summerville Fm. in the San Rafael Swell, Utah. Part of their position is a statement (A&l, p. 88) erroneously attributed to Peterson (1988) that the type Cow Springs Mbr. of the Entrada Ss. in NE Arizona is a "pre-Todilto unit." No such statement was made in that report and the regional relationships by O'Sullivan (1978), Peterson (1988), and Condon (1989) strongly suggest the opposite--that the type Cow Springs is stratigraphically higher, and younger, than the Todilto. The same vertical sequence of units also occurs in the southern San Juan basin and is why we continue to apply the name Cow Springs there. The reports cited above show that the vertical and age sequence of units from NW New Mexico to Cow Springs, Arizona, to the San Rafael Swell, Utah, is Todilto (oldest), type Cow Springs, and Curtis (youngest), which makes it extremely difficult to correlate Todilto with Curtis.

A&L apparently eschew stratigraphic markers whereas our studies are based largely on identifying and tracing out such markers to establish the lateral and time relationships because so many of the beds are undatable by other means. Using these markers, O'Sullivan (1980a, 1980b, 1991, unpublished data) and Condon (1989) can now show that the Curtis correlates with the Moab Tongue of the Entrada and that most of the Wanakah of NW New Mexico correlates with the Moab/Curtis and underlying upper part of the Entrada; their work also shows that only uppermost Wanakah beds may correlate with lowermost Summerville beds. For this reason, but mainly because red beds in the Wanakah contain other lithologies than typical Summerville red beds, the name Wanakah is more appropriate than Summerville for upper San Rafael Group strata in the eastern Colorado Plateau, including NW New Mexico,.

A&L failed to consider the implications of the Redwater Shale Mbr. of the Sundance Fm., which only occurs as far south as NE Utah and NW Colorado, and of the J-4 and J-5 unconformities at its base and top. Contrary to A&L (p. 89), Pipiringos and O'Sullivan (1978) did not say or indicate that the J-4 unconformity "is present in the northern San Rafael Swell where it represents a break between the upper and lower Curtis or locally between the Summerville and the Redwater"--indeed, the J-4 and the Redwater are not even present there. Pipiringos and O'Sullivan also showed that the Redwater is cut out southward by the J-5 unconformity at the base of the Morrison. A&L (p. 87) criticized our lack of fossil control and isotopic dates, yet they present no such data either. Enough such data are now available, however, to shed light on an important aspect of this problem. An ⁴⁰Ar/³⁹Ar date of 154.9±1.5 Ma from a bentonite bed 8 ft above the base of the Tidwell Mbr. of the Morrison Fm. near Notom, Utah, determined by J.D. Obradovich (in Peterson, 1992) indicates an age essentially on the Oxfordian-Kimmeridgian boundary for lowermost Tidwell strata (time scale of Harland and others, 1990). Supporting this are charophytes from the Tidwell near Grand Junction (Peck, 1957; Ott, 1958) that suggest an age no older than Kimmeridgian according to biostratigraphic studies by Schudack (1990). A&L include Tidwell in their Summerville, which would make its top considerably younger than most workers currently accept. More importantly, by including Tidwell in Summerville, A&l gloss over significant erosional and depositional events (Redwater and bounding unconformities) that occurred in the post-Summerville pre-Tidwell time interval.

Working with marker beds and surfaces is difficult, frustrating, and time-consuming, but in our opinion it is a better methodology than lumping superficially similar lithologies together and neglecting the detailed interrelationships of the strata.