A NOVEL APPROACH TO ISOTOPE STRATIGRAPHY: USING LATERAL VARIATION IN STABLE δ13C ISOTOPES TO DETERMINE BASIN POSITION

Lacustrine carbonate sequences may be lithologically homogeneous over moderate lateral distances (~1-20 km), making it difficult to ascertain the relative basin position (marginal to basinal) based on lithofacies analysis alone. Miocene non-marine deposits from the Horse Spring Formation contain a number of these sequences and record extension in the Lake Mead area of the central Basin and Range. We set out to develop an innovative tool to determine relative basin position based on the assumption that biological productivity is higher in the photic zone than in more basinal, deeper lake positions. In comparing δ¹³C values at a known margin position (in the photic zone with higher primary productivity) with similar values sampled from an interbasinal, laterally equivalent unit, we expected an overall negative shift in δ¹³C values. Within the Lovell Wash Member of the Horse Spring Formation, a world-class exposure of lacustrine stromatolites interfingers with a conglomerate, providing a known basin margin. These units can be traced and sampled away from this margin for ~3 km. The average δ¹³C value for the nine carbonate samples near the margin is 7.58 ± 1.16‰ VPDB, whereas the average δ¹³C value for twenty-six samples taken from the same interval but from a more basinal position are 3.83 ± 0.53‰ VPDB. Using this new chemostratigraphic technique for relative basin position allows us to infer locations with respect to the basin margin on laterally equivalent units that offer little clues through traditional litho- or biostratigraphic means. For example, in the Bitter Ridge Limestone Member (upper Horse Spring Fm.) we used this method to find the position of two sections and their relative proximity to a major basin bounding fault. We are also applying this technique to determine the marginal proximity of several sections of the Rainbow Gardens Member. This work is part of an on-going study of basin reconstruction and tectonics in the Lake Mead area and may contribute an important basin analysis technique in other settings as well; it is also a logical extension of the foundational lacustrine isotopic work of Mike Talbot.