A novel approach to date continental sediment deposition and paleoclimate events using volcanogenic zircon in paleosols

Abstract

The Chronostratigraphy of continental deposits in the absence of discrete ash beds largely relies on maximum depositional ages (MDA) that are interpreted from detrital populations of heavy minerals from sandstones. Mudstones have not been conventional targets for the extraction of heavy minerals (e.g. zircon) due to the prevailing concept that their hydraulic size equivalence compared to quartz would be too small. However, based on the alternative idea that volcanogenic zircons may be deposited onto overbank depositional surfaces and worked into soil horizons directly from a volcanic ash cloud, hydraulic size equivalence is irrelevant. We propose that zircons extracted from mature paleosols will provide more depositional age-appropriate zircon populations than intercalated fluvial sandstones. This is based on the hypothesis that volcanogenic zircon grains are more abundant in sediments with long surface exposure times. This is investigated in the Cretaceous Cedar Mountain Formation (CMF), composed of lacustrine, palustrine, and fluvial sediments, and subdivided into the Yellow Cat Member (YCM), the Poison Strip Member (PSM), and the Ruby Ranch Member (RRM). The CMF was deposited when the Sierra Nevada magmatic arc was active, which provided a volcanogenic zircon source, and the mature stacked paleosols throughout the formation provide enough surface exposure time to capture said volcanogenic zircon populations. The purpose of this study is to refine 1) the chronostratigraphy of the CMF and 2) the stratigraphic position of the Aptian-Albian boundary within the CMF, and 3) the age of some important stable isotope excursions. Refining the chronostratigraphy of the CMF will improve future correlations of chemostratigraphic data with the marine record and contribute to understanding the global paleoclimate dynamics of the Cretaceous greenhouse world. Thirteen samples were collected for zircon U-Pb by LA-ICP-MS from two sections at Dinosaur National Monument and one section from Ruby Ranch Road, the type section for the RRM. Interpretation of maximum depositional ages interpreted in this study suggest that the studied part of the Cedar Mountain Formation (uppermost YCM to lowermost Naturita Formation) extends from the late Hauterivian-early Barremian to the late Albian (ca. 130 Ma to 106 Ma). Specifically, the results indicate that the Aptian-Albian boundary lies within the upper RRM. The results refine the ages and stratigraphic positions of carbon-isotope excursions (CIEs), in particular the placement of the Aptian C10 CIE to ca. 118 Ma, in agreement with studies on marine strata. The approximate age of the Aptian C10 CIE is based on the bracketing ages of overlying and underlying strata within the C10 interval of 117.4 ± 1.1 Ma and 118.6 ± 0.7 Ma at RRM and ages calculated from strata within the C10 interval at DNM of 118.1 ± 2.3 Ma and 119.7 ± 0.7 Ma.