Isotopic composition of diagenetic carbonate concretions at the Cretaceous-Tertiary boundary in south-central Texas, USA: Implications for their growth

Abstract

Carbonate concretions 10–20 cm below and immediately above the Cretaceous/Tertiary boundary at the Littig Quarry, Elgin, Texas, differ significantly from each other morphologically, petrographically, and isotopically. The Cretaceous Kemp Formation concretions consist of oblate spheroidal nodules made up of microspar and pseudospar. The δ13C and δ18O values of the microspar and pseudospar are relatively invariant (mean ± standard deviation: δ13Cav = −19.4 ± 0.4‰; δ18Oav = −1.6 ± 0.2‰, n=16), indicating concretion grew in a restricted zone beneath the sediment/water interface. δ18Oav coincides with that of Cretaceous/Tertiary planktonic forams reported from North American DSDP foram data, suggesting that the concretion formed from marine-derived waters and was not altered by freshwater. The occurrence of pyrite framboids and the low δ13C values suggest that the concretions grew under sulfate reducing conditions. In contrast, concretions from the Paleocene Littig Member of the Kincaid Formation consist of scattered microspar/pseudospar nuclei cemented by variably compacted, fine to very fine sandy, glauconitic, phosphatic, starved-basin sediment. The Littig Member concretions range in isotope composition from δ18O of −2.0 to −4.8‰. and δ13C of −20.5 to −10.3‰. The trend from high δ18O and low δ13C, to low δ18O and high δ13C, corresponds to increasing compaction and suggests that the Littig Member concretions grew continuously during burial rather than at a single depth like the Kemp Formation concretions. The increase in the13C isotope with increasing compaction is interpreted to indicate that the concretions formed during the shift from sulfate reducing to methanogenic conditions and in the early stages of methanogenesis. *** DIRECT SUPPORT *** A00QA020 00006