GEOLOGIC SETTING AND AGE

The Red Hot Truck Stop section exposes the latest Paleocene and earliest Eocene upper Tuscahoma, and the early Eocene Bashi and Hatchetigbee formations; detailed stratigraphic sections were published previously (Ingram 1991; Harrington 2003a). The T4 sand unit of the uppermost Tuscahoma contains a vertebrate lag deposit with one of only two Wasatchian mammal faunas from the eastern USA (Beard and Dawson 2001; Dawson and Beard 2007; the other is less extensive and is from Virginia: Rose 1999). The Tuscahoma-Bashi contact is marked by a regional unconformity extending from western Alabama through easternmost Mississippi. The Bashi Formation locally has both a lower lowstand unit and an upper transgressive marine unit; the lowstand deposits, only known from the Red Hot Truck Stop section, are considered the oldest Bashi deposits on the Gulf Coast (Ingram 1991). The lowstand unit, 3 m thick, is composed of interbedded, white, fine to medium sands and brown to gray shales, containing the plant fossils in this study within the lower half of the unit. The transgressive marine unit of sand and glauconite, about 1.2 m thick, is unconformable upon the lowstand unit and contains a fossiliferous boulder layer that is easily recognized in outcrop to western Alabama (Ingram 1991). Abundant Ophiomorpha burrows extend downwards into the lowstand unit. The Bashi sediments are interpreted as remains of a range of coastal environments from near-shore marine to tidal channels and estuaries (Ingram 1991), the latter two being the most probable paleoenvironments for the leaf fossils due to small grain size and relatively dark color indicating organic content.

An unusual wealth of evidence constrains the Red Hot leaf flora to the early part of the early Eocene. Significantly, the mammalian fauna from the T4 sand immediately underlying the Red Hot leaf flora contains species restricted to earliest Wasatchian zone Wa0 (Gingerich 1989) in the Bighorn Basin of Wyoming (Beard and Dawson 2001). Zone Wa0 coincides with the PETM and associated carbon isotope excursion in Wyoming (e.g., Koch et al. 1992; Gingerich 2006), and ongoing work confirms an earliest Wasatchian age for the Red Hot local fauna (K.C. Beard, personal commun. 2007). The T4 sand locally contains a dinoflagellate assemblage that correlates to calcareous nannoplankton zone NP9 (discussed in Beard and Dawson 2001), which straddles the PETM (e.g., Gradstein et al. 2004).

Additional local chronostratigraphic data come from diagnostic early Eocene pollen, of Platycarya (Juglandaceae, genus currently endemic to East Asia) from the leaf-bearing layers of the basal Bashi at the Red Hot Truck Stop locality (Frederiksen 1998; Harrington 2003a), and from an omomyid primate, found in the Bashi above the leaf remains that further constrains the flora to the Wasatchian (Beard and Tabrum 1991).

The marine portion of the Bashi Formation, overlying the lowstand unit that bears the flora, has not produced calcareous nannoplankton locally, but these have been recovered from Bashi sections in Alabama that represent the same transgression. Siesser (1983) reported Discoaster mohleri from the type Bashi section in Clarke County, Alabama, whose last appearance is within NP9. However, Gibson and Bybell (1994) reported Tribrachiatus bramletti from the marine Bashi in Coffee County, Alabama, an indicator species for the lower half of NP10, and they disputed Siesser's identification of D. mohleri. Also in the Bashi of Alabama, foraminifera occur that are assigned to foraminiferal zone P6b (Oliver and Mancini 1980; Mancini 1981), which is consistent with NP10 but not with NP9 (e.g., Gradstein 2004).

One line of evidence suggests that the flora is basal Eocene in age, within the PETM. As discussed by Beard and Dawson (2001), Ingram's (1991) sequence stratigraphic assignment of the Bashi lowstand unit (lowstand systems tract of sequence TE1.1) is the same that Steurbaut (1998) assigned to the Belgian Tienen Formation strata that contain the well-studied, basal Eocene Dormaal fauna and which also contain the base of the carbon isotope excursion marking the onset of the PETM (Smith et al. 2006).

In summary, the maximum age of the Red Hot leaf flora is set by the underlying mammal fauna as earliest Eocene, or 55.8 Ma, and the minimum age is constrained by nannoplankton from the younger, marine Bashi that represent the first half of zone NP10, or 54.2 to 54.8 Ma (ages from Gradstein et al. 2004). If the sequence stratigraphic correlation discussed by Beard and Dawson (2001) is correct, the Red Hot leaf flora belongs to the early phase of the PETM.