T169 Chemostratigraphy: Physical and Temporal Applications

Tuesday, 24 September 2019
8:00 AM - 12:00 PM
Room 224B, North Building

METEORIC ALTERATION OF MARINE δ13CCARB BELOW SUBAERIAL UNCONFORMITIES: PALEOZOIC EXAMPLES ACROSS A RANGE OF SPATIAL AND TEMPORAL SCALES

Paper number: T169-9
Program: Technical Sessions
Time: 10:05 AM – 10:20 AM

Authors

Abstract

Despite decades of study, the interpretation of the carbon isotope record preserved in carbonate successions continues to be highly debated, particularly in regard to the effects of meteoric diagenesis on primary marine δ13Ccarb. Studies of Recent carbonates that show substantial modification of fabrics and chemical composition through processes related to subaerial exposure and shallow burial have introduced varying levels of uncertainty as to how δ13Ccarb values measured from ancient stratigraphic sequences should be interpreted. To better understand the signal origin of δ13Ccarb in ancient carbonates, we evaluate successions containing unconformities of varying scales, focusing on evidence for the modification of primary δ13Ccarb values by meteoric processes.

Subhorizontal, shallowly buried Paleozoic carbonates along structural arches in the U.S. Midcontinent contain infilled paleokarst cavities and locally thick paleosols preserved at unconformity surfaces, that range between a few hundred thousand to millions of years in duration. Here we present multiple high-resolution δ13Ccarb and δ18Ocarb profiles across several unconformities to evaluate the resetting of primary values. While we document distinct negative shifts in the range of -2 to -10‰, cements bearing these values are generally minor and highly localized. In an extreme example, δ13Ccarb negative shifts below the high-relief, 40 million-year-duration Walbridge Unconformity are relatively minor and generally restricted to rock clearly affected by paleo-weathering. Surprisingly, shorter duration subaerial surfaces associated with low-relief 3rd and 4th order depositional sequences show little evidence of δ13Ccarb modification. In fact, the largest δ13Ccarb shifts appear to be produced by marine diagenesis below unconformity surfaces through infilling of secondary porosity with younger marine cements and plugging of paleokarst features with transgressive sediments. Also noteworthy is the preservation of primary values and trends despite post-deposition dolomitization of some of these stratigraphic sections. Together, our extensive evaluation of subaerial unconformity surfaces testifies to the robust preservation of primary δ13Ccarb signatures in Paleozoic whole-rock carbonates.

Geological Society of America Abstracts with Programs. Vol. 51, No. 5

doi: 10.1130/abs/2019AM-340553

© Copyright 2019 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.