Nonpedogenic calcrete is difficult to distinguish from pedogenic calcrete in the fossil record; both alpha and beta textures have been observed from fossil and modern examples. However, a calcrete from the New Haven Arkose (Hartford Basin, Connecticut) is shown here to be of a nonpedogenic origin through sedimentologic and petrographic evidence. An accumulation of thin sheets of displacive calcite layers found in a decimeter-thick horizon of anastomosing veins within the upper portion of a red mudstone is correlated to calcite cement found in the overlying sandstone. Based on petrography, we recognize six generations of calcite in the mudstone-sandstone hosts. The first five generations are associated with rhizoliths that can be related to deep taproots and are interpreted to have formed by precipitation from shallow groundwater. There are no vadose-type cement morphologies; the calcite has luminescent zones, indicating that Mn was soluble and thus oxygen levels were low. These cements clearly formed several meters below what would have been the surface of the channel sand body. We suggest that calcite cement stratigraphy combined with redox models for the behavior of Mn (as well as Fe and U) may aid in the identification of nonpedogenic versus pedogenic carbonates in the geologic record. Additionally, the calcite from this carbonate layer has been dated using the U-Pb method. Our results provide insight into the environmental and diagenetic fluid conditions favorable for providing a spread in U/Pb ratios that are suitable for precise dating of calcites in otherwise undateable sections.
Calcite cement stratigraphy of a nonpedogenic calcrete in the Triassic New Haven Arkose (Newark Supergroup)
Year of Publication:
Geological Society of America Special Paper