Pliocene and Pleistocene sedimentation in south-central Colorado: Opportunity for climate studies

Abstract

A well-dated Plio-Pleistocene stratigraphic sequence, ranging in age from 2.6 to 0.68 Ma, has been recovered from the Alamosa Formation in the San Luis Valley, south-central Colorado. The sequence is dated by magnetostratigraphy tied into the Huckleberry Ridge (2.02 Ma) and Bishop ashes (0.74 Ma). Sediments, which were primarily deposited by meandering streams, exhibit intermittent modification by partial soil development. Aggradation was relatively continuous, with only minor variations in sediment accumulation rate. Samples of sediments deposited from 0.9 to 0.7 Ma are from surface outcrops. Classic paleontological techniques have been used to identify evidence of climatic cycles in these sediments. The climatic changes can be correlated with deep-sea core Oxygen Isotope Stages 22 through 18. Older sediments, obtained by coring, have been faunally analyzed for vertebrate fossils, ostracodes, and molluscs, florally analyzed through pollen, and geochemically analyzed through both inorganic and biogenic carbonate stable isotopes. The oldest sediments in the core may have been deposited during Oxygen Isotope Stage 110 (which occurs within the Gauss Chron, ∼2.6 Ma). Paleontological and isotopic evidence may eventually provide a long continental climatic record comparable to those from the deep-sea.