USING CORE-OUTCROP COMPARISONS TO UNDERSTAND THE EFFECTS OF RECENT SURFICIAL WEATHERING ON PALEOENVIRONMENTAL PROXIES IN CONTINENTAL SETTINGS (Invited Presentation)

Geoscientists interested in developing paleoenvironmental and paleoclimatic reconstructions using proxy records from continental settings have traditionally relied on sampling outcrop exposures because they are easy to access and have a wide lateral distribution. Recent efforts in applying continental scientific drilling to areas with adjacent outcrops allow for the direct comparison of proxy data recovered from core and outcrop. Such comparisons can identify the impact of surficial oxidative weathering on different proxy methods and thus help determine the reliability of these methods when applied to weathered outcrops.

The Bighorn Basin Coring Project (BBCP) recovered over 900 m of overlapping core from 3 different sites in late Paleocene to early Eocene fluvial deposits of northwestern Wyoming. These cores come from areas with extensive adjacent outcrop exposures allowing correlation, in some cases, of individual beds between core and outcrop. Visual inspection of the cores indicate that surficial weathering profiles extend at least 20-30 meters deep, which means normal outcrop sampling procedures do not eliminate the alteration effects of recent weathering in this field area.

We compared BBCP proxy data between correlative core and outcrop samples to better constrain the effects of modern oxidative weathering in these continental deposits. Proxy data included various geochemical, color, and rock magnetic measurements. Results show that some of the proxies are resistant to alteration during surface weathering (e.g. carbon isotopes of pedogenic carbonate and organic matter, percent total organic carbon, χ, ARM, IRM, B_c, B_cr) whereas others are not (oxygen isotopes of pedogenic carbonate, color, HIRM and M_r). We propose explanations for these patterns in the context of the chemical and physical changes these deposits experienced during surface exposure and highlight examples of previous studies that may need reinterpretation given these findings.