Carbon Isotope Stratigraphy of the Jaitique Formation: A Possible Albian-Cenomanian Boundary in Honduras

Robert D. Rogers and Aaron J. Cavosie
University of Puerto Rico, Mayaguez, PR

We present the first δ¹³C chemostratigraphy from Honduras - a 56 meter profile of shallow marine carbonates near the town of Illama along the Rio Ulua. This section was sampled to test the viability of using carbon isotopes to resolve stratigraphic uncertainties in Honduras. The interval consists of 224 non-faulted beds of mainly grey mudstone and wackestone carbonate strata with minor dark calcareous shale intervals continuously exposed along the Santa Barbara highway. This carbonate section occurs within a terrigenous clastic sequence of maroon siltstone, sandstone, and limestone-clast conglomerate. Mills et al. (1967) assigned the carbonate strata to the late Albian-age Atima Formation based on fauna content, while Finch (1981) preferred the Cenomanian-age Jaitique Formation designation for these rocks based on mapping correlations.

Samples taken at 0.5 meter intervals were analyzed at the Stable Isotope Laboratory at the University of Puerto Rico using continuous flow isotope ratio mass spectrometry, avoiding fossils and secondary calcite where present. Samples were standardized for accuracy and precision with multiple bracketing analyses of NBS19. The δ¹³C values vary from 0.52 to 4.28 (avg=2.77)(‰ VPDB, ±0.10, 2SD). Prominent positive δ¹³C excursions are noted at three intervals in the section, which bear resemblance to peaks B, C, D of the Mont Risou Albian-Cenomanian boundary interval of Kennedy et al. (2004). Initial interpretation of the isotopic data in the context of regional stratigraphy supports a tentative assignment of this interval to the Jaitique Formation but at the Albian-Cenomanian boundary rather than in the Cenomanian.