PRELIMINARY XRF ANALYSIS OF LATE CRETACEOUS (CAMPANIAN) TURTLE BONE FROM A POND ON A VOLCANIC MAAR DEPOSIT IN THE AGUJA FORMATION, BREWSTER COUNTY, TEXAS

A pond/lag deposit at the highest level of a volcanic maar deposit, near the Aguja/Javelina contact, just north of Big Bend National Park (72.6 ± 1.3 ma) has yielded fragmentary, though abundant turtle, dinosaur, mollusk, gar and wood remains. The fossils occur as a surficial lag or are embedded in several centimeter thick strata only in the uppermost 0.25 meters of the 12 meter thick deposit. The host rock is thinly bedded pyroclastic debris containing poorly to moderately vesicular, angular olivine- and plagioclase basaltic ash and lapilli intermixed with sand- and mud-sized Aguja components. Pond sediment was cool when the fossils were buried and was likely derived from a more distant eruptive center. The co-occurrence of bone, wood, gar scales, bivalves and gastropods strongly suggests that a pond formed at the top of the ‘last’ eruptive sequence where skeletal material and wood was reworked into the upper-most beds. Over 95% of the plant material recovered is palm wood but dicot twigs are present. Fragments of dinosaur long bones (none larger than 12x5 cm) and a single ceratopsian(?) sacral centrum have been found, constituting < 5% of the recovered bone.

Chelonians are represented by carapace and plastron fragments, none larger than 7x5 cm and few smaller than 2x2 cm. Taxa identified (in order of abundance) include different species of Aspideretes, Baena, Bothremys, Basilemys and Adocus. Other taxa may be present but have not yet been identified.

The lapillistone host rock must have been an ion source during bone diagensis. A study has been initiated to determine which elements were enriched in the bone during diagensis and to determine if there were differences in element abundance between carapace and plastron and dense and vascular bone. A preliminary qualitative study, using a Brucker Trace III-SD XRF (X-ray refraction), is under way with planned petrographic studies to survey authegenic mineralization of the bone. Qualitative XRF spectra were taken of turtle bone (including external and internal bone surfaces both fresh and treated with weak HCL to remove caliche), ‘internal’ tissues of bone ground down approximately 1 mm, modern turtle bone (carapace and plastron) and host rock. Abundance differences are seen in Si, P, Ca, Mn, Fe and Sr peaks between modern and ancient bone and external and internal bone.