The Genesis of Late Quaternary Caliche Nodules in Mission Bay, Texas: Faithfulness of Stable Isotopic Compositions of Caliches for Paleoclimates

Jie Zhou and Henry S. Chafetz
Geosciences, University of Houston, Houston, TX

Two types of low Mg-calcite caliche nodules occur in 2 to 5m high cliffs of late Pleistocene bay-margin alluvial-deltaic sediments surrounding Mission Bay, Texas. They represent stages in formation from the soft incipient nodules (type I) in brown, tan, and gray soils to partially lithified chalky nodules (type II). The gray soils occur at the top of the deposits.

δ¹⁸O values range from 0.2 to -8.1‰ VPDB (48 analyses), mostly -2 to -4‰, whereas the δ¹³C values range from 0.7 to -10.6‰ VPDB and display a significantly greater standard deviation. Data from the incipient type I nodules in brown, tan, and gray soils plot in 4 distinctly different areas based on their δ¹³C values. Nodules in the tan (10) and the main group of brown (10) soils have δ¹³C values around -7.6 and -4.6‰, respectively. They most likely represent earlier Holocene climates when C₃ and C₄ plants were common in this area. Nodules in the presently active gray soils plot into two markedly different groups, one with δ¹³C values around -8.4‰ (11) and another with values around 0.1‰ (5). Although presently a C₄ plant-dominated environment, the ~-8.4‰ values in the gray soils can most readily be explained as due to the input of C₃-dominated plants and those with values around 0.1‰ are probably due to solution of locally adjacent shell hash in the soils.

These analyses demonstrate that within a 2 to 5m thick Holocene soil, stable isotopic values of caliche nodules can significantly vary and even within the modern gray soil markedly different signatures exist. Thus using isotopic data from caliche for paleo-reconstruction must be used with great caution.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas