STRATIGRAPHY AND AGE

Stratigraphy

Strain (1976) divided the Anapra Sandstone into three sandstone members and one silty shale member (see Figure 2.1). Total thickness of the Anapra varies from 52.5 m to 64 m due to tectonic thinning of the shales around the pluton, Cerro de Cristo Rey. Footprints are preserved at the top of the second sandstone member (Figure 2.1), a tidal and littoral unit (LeMone and Kotlowski 1996) which consists of thin to massive beds of fine- to medium-grained, ferruginous sandstone and inter-bedded dark grey shales. Underlying cross beds are truncated by the track-bearing bed described here.

Marine shales (Mesilla Valley Formation below and Del Rio Clay above) bracket the Anapra (Strain 1976; figure 2b). Fossil oysters (Exogyra whitneyi ) occur in the top-most 1-2 m of the Anapra; worm castings and poorly preserved twig/stem-like land plant material occur on some sandstone bed tops. Lignite laminae occur (LeMone and Kotlowski 1996) in the siltstone that overlies the tracks. Sedimentary structures within the Anapra sandstones include extensive cross-beds, ripple marks, and channel-fill sequences indicating that deposition of the Anapra took place on a tidal shoal to fluvial-paludal delta complex along the northern margin of the Chihuahuan embayment (LeMone and Kotlowski 1996). For general stratigraphy of the Cerro de Cristo Rey uplift, see Lovejoy (1976).

Lithologically, the Anapra Sandstone resembles coeval clastic rocks in the region and has been renamed as the Sarten Member of the Mojado Formation by Lucas and Estep (1998). We support this correlation but will refer to the Sarten Member as the Anapra Sandstone here for clarity and simplicity. The Anapra is assigned to the Plesioturrilites brazoensis ammonite zone of latest Albian age (Lucas and Estep (1998)) based on stratigraphic correlation with the Sarten Member of the Mojado Formation (Figure 2.2). The Anapra correlates lithostratigraphically with part of the Dakota Sandstone, and so broadly correlates stratigraphically to the Dinosaur Freeway of Lockley et al. (1992). In the San Andres Mountains (60 miles to the north), east of Las Cruces, New Mexico, outcrops of the Fryingpan Spring Member of the Mojado Formation (Lucas and Estep 1998) and the overlying Dakota Sandstone are present (Lucas and Estep 1998). Historically there has been some disagreement about which unit of the Mojado Formation is present beneath the Dakota at this locality, and what the thickness is. We concur with Lucas and Estep (1998) as stated above, due to the petrologic similarity of the Fryingpan Spring Member to the Mesilla Valley Shale (Strain 1976), which underlies the Anapra Sandstone (Sarten Member) locally, and due to the petrologic similarity of the Dakota Sandstone to the Anapra.

In northern New Mexico, westernmost Oklahoma, and southern Colorado, dinosaur tracksites in the Dakota Sandstone constitute the "Dakota Megatracksite Complex" (Lockley and Hunt 1995). These tracksites occur in numerous outcrops of the Dakota, spread over the Colorado Front Range and southeastern Colorado, westernmost Oklahoma, and northeastern New Mexico. Age of the sites varies but most are thought to be Late Albian-Early Cenomanian. Others (Lucas et al. 2000) have argued that age disparities between individual tracksites render the "megatracksite" concept problematic. Nonetheless, the Cristo Rey site on the New Mexico/Republic of Mexico border stratigraphically extends the geographic distribution of Dakota Megatracksite 500-600 km to the south from northeastern New Mexico.

In Texas, approximately 400 km to the east, slightly older (Early Albian) strata contain the westernmost dinosaur tracks yet reported in the state.

Age

The Albian/Cenomanian boundary was placed (Strain 1976) at the contact between the underlying Mesilla Valley Shale and the Anapra, and Strain (1976) correlated the Anapra with the Main Street Formation (Cenomanian) of the Washita Group in central Texas. We have found that lens-like accumulations (<5 cm thick) of foraminiferal (Cribratina texana packstone occur sporadically throughout the Mesilla Valley Shale, but are not found in overlying strata. These foraminifera indicate that the Mesilla Valley is Albian in age (Loeblich and Tappan 1964). The presence of Exogyra whitneyi in the upper 1-2 m of the Anapra, and in the overlying Del Rio clay indicates that the Albian/Cenomanian boundary falls somewhere within the Anapra. Lucas and Estep (1998) placed the Albian/Cenomanian boundary at the top of the Anapra, at the Anapra/Del Rio contact. In this interpretation, the Anapra is chronologically equivalent to the Mesa Rica Sandstone, a dinosaur footprint-bearing unit in northeastern New Mexico. Scott (1970) correlated the Mesa Rica of New Mexico with the Dakota of southeastern Colorado.