Use of Ar-Ar ages of individual mica grains to determine the provenance of loess
Abstract
Ar-Ar ages for individual biotite and muscovite grains have been used to evaluate the provenance of loess in two sections on Long Island. Ages were determined on mica from Cretaceous sands, glacial outwash and loess from Caumsett State Park and from loess on the Stony Brook campus. Most of the muscovite ages in loess range between 250 to 375 Ma and are similar to those found for the muscovite in the glacial and Cretaceous sands. However, one muscovite age is 1000 Ma. Biotite was not found in the Cretaceous sands, but was found in the overlying glacial sands and gravels and loess at Caumsett State Park. The reason for this is probably that biotite cannot withstand the extensive weathering or recycling that the sand in the Cretaceous sediments had undergone prior to deposition. The biotite in the glacial sediments is probably more dominantly derived from freshly glacially scoured local basement rocks. The ages for the biotite from the glacial sediments are mainly 250 to 400 Ma consistent with their derivation from the basement rocks underlying Long Island Sound or exposed in southern Connecticut. Ages for biotite from the loess range from 44 to 1287 Ma with the majority between 250 to 400 Ma. These mica ages suggest that the majority of the loess is of local origin and that the muscovite and biotite are derived mainly from sources similar to the underlying Cretaceous and glacial sediments. Some of the biotites have an ages of 700 to 900 Ma consistent with derivation from Grenville basement sources further to the west in the Hudson or New Jersey Highlands. The 44 Ma and 1287 Ma biotite ages may have a source much further to the west in the Rocky Mountains. These results suggest that Ar-Ar ages for individual mica grains may be a valuable tool for provenance analysis of loess. Biotite which is more likely to be a first cycle sedimentary mineral may have potential for evaluating more distant sources.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2001
- Bibcode:
- 2001AGUFM.V41C..02Z
- Keywords:
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- 1035 Geochronology;
- 1040 Isotopic composition/chemistry