| ABSTRACT
Base level exerts a fundamental control on continental sedimentary architecture,
affecting the vertical and lateral deposition of sediments along the coastal plain. In the
Late Cretaceous Kaiparowits Formation of southern Utah, coastal and alluvial plain
deposition is recorded in a continuous ~2.1 myr section, with abundant and
well-preserved continental fossils, of which mollusks are the dominant fauna. This study
examines δ13C and δ18O stable isotopes in primary aragonite shell material from aquatic
gastropods in pond and river deposits to evaluate the impact of changes in fluvial
architecture, such as changes in pond stability and increased overbank flooding.
Analysis of 67 gastropod shells from three morphotaxa display similar carbon and
oxygen values, and show no significant variance attributed to vital effect differences
among species. Primary aragonite shell material is distinguished from diagenetically
altered calcite and dolomite bearing samples by X-ray diffraction methods. Samples
composed of more than 75% aragonite are inferred to record close to primary δ13C and
δ18O isotope signatures, while those containing less than 50% aragonite reflect a
significant increase in δ13C and δ18O shell isotopic values. Mixed carbonate shell δ18O
values are corrected to original aragonite isotopic composition using isotopic mixing
equations.
Pond gastropods record a distinct shift in the mean δ18O values between each unit.
The lower unit records a mean δ18O value of -12.7±4.3‰, while the marine influenced
portion of the middle unit has a heavier mean δ18O value of -9.8±4.2‰. The post marine
influenced middle unit records a similar mean δ18O value to the marine influenced
portion, with a value of -10.9±1.8‰, while the upper unit returns to δ18O values more
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similar to the lower unit with a mean of -12.3±4.4‰. This increase in δ18O values and
return to lighter δ18O values correspond to changes in the fluvial geometry between
meandering and anastomosing style rivers, and an incursion of marine waters up the river
channels.
Average values for pond unionoids (-9.5±1.6‰ δ18O) and pond gastropods
(-10.3±3.8‰ δ18O) are statistically similar. In most units, the range of values recorded by
the pond gastropods overlaps the range recorded by the pond unionoids. Occasionally, the
range of pond gastropods overlaps with the fluvial unionoids as well. The consistent
small offset of heavier δ18O and lighter δ13C values between gastropods and unionoids
suggests these two mollusks record a similar environmental signal, but may record
slightly different positions within the Late Cretaceous microenvironments, such as
shallow vegetated areas versus deeper non-vegetated areas. These relationships suggest
that aquatic gastropods promise to be an additional archive for geochemists, sensitive to
both local and regional environmental changes, with the potential to expand our
understanding of basin scale processes on the flora and fauna living in the Western
Interior Basin. |