LARGE ISOTOPIC DATABASE ON LACUSTRINE CARBONATES OF THE HORSE SPRING FORMATION CHARACTERIZES THE EXTREME VARIABILITY OF CONTINENTAL RIFT LAKES

The Oligo-Miocene Horse Spring Formation (HSF) in the Lake Mead region of southern Nevada comprises a thick and diverse suite of continental clastic and chemical sediments that formed in pre- and synextensional basins. Paleozoic marine limestones and dolomites encompassed these basins, providing a vast carbonate source. As a result, lacustrine carbonates form thin (1-5 m) to very thick (100-200 m) accumulations in the HSF that show microbially-influenced calcite precipitation and a lack of higher-order organisms. Thus, these carbonates formed in alkaline lakes that were inhospitable to multi-cellular life. We have lithologically and isotopically characterized four stratigraphic intervals in the HSF, yielding a database of >740 isotopic observations that highlight temporal and lateral variation in these significant lacustrine units. δ¹⁸0 of pre-extensional lakes range over 19‰, from -18-1‰, with δ¹³C ranging from -5-6‰, reflecting a complex range of groundwater and meteoric inputs and a patchwork of ephemeral, short-lived lakes and wetlands. Peak extension is recorded by the second stratigraphic interval, the Thumb Member. Lacustrine carbonates are limited in lateral extent and show less isotopic variability, but are enriched with respect to δ¹³C, suggesting either higher primary productivity and/or a spring-fed source. As extension evolved into transtension, a large lake formed, exemplified by the third interval the Bitter Ridge Limestone Member. This unit is isotopically similar to earlier lakes in terms of δ¹³C, but shows a strong (3-4‰) δ¹⁸0 depletion that we interpret as potentially climatically forced, with a shift toward more aridity. The final stratigraphic interval shows a return to smaller lakes that are isotopically similar to those of the Thumb Member with respect to δ¹⁸0, but have a very wide range and strong enrichment in δ¹³C (0 to 14‰). Interbedded tuffs are common in this unit and the River Mountain volcanic field, located nearby, was active during this time interval. We attribute the carbon enrichment to hydrothermal activity associated with this magmatism, although there is a lack of travertine facies that might be expected if hot springs were more prevalent. All told, these deposits and their chemistry illuminate a wide range of lake chemistries possible during continental rifting.