56 Geomorphology (Posters)

Tuesday, 21 March 2017
8:00 AM - 12:00 PM
Grand Ballroom, 17th Floor

DISTRIBUTION OF MAJOR AND TRACE ELEMENT GEOCHEMISTRY OF TILL, MONTPELIER 1:100,000 SHEET, CENTRAL VERMONT

Paper number: 56-17
Program: Technical Sessions
Time: 8:00 AM – 12:00 PM

Authors

Abstract

A study of major and trace element geochemistry of glacial till has been undertaken in order to explore the connection between till chemistry, surface water and groundwater chemistry. Of specific interest is the distribution in till of arsenic, uranium, and phosphorus, and their spatial relationship to bedrock sources. The study area includes samples from multiple geologic domains from the Green Mountains to the Connecticut Valley Trough.

A total of 94 till samples were collected from 67 sites (e.g. stream banks, road-cuts, excavations), and a strong effort was made to access unweathered samples. Sites were sampled within a grid of 10 x 10 km squares that covered the Montpelier1osheet. Data includes clast %, color, grain size, reaction to HCl, and pH. Clast lithology counts were undertaken at a subset of sites. Samples were air-dried, split, sieved to < 2 mm, and split for chemistry by ICP-AES, ICP-MS and bulk mineralogy using quantitative XRD.

Till pH differs notably across the major lithotectonic boundary known as the Richardson Memorial contact (RMC), the boundary between Cambrian and Ordovician (CO) transitional margin and accreted rocks of the Rowe-Hawley zone to the west and post-Taconian Silurian and Devonian (SD) rocks of the Connecticut Valley Trough to the east. Tills derived from SD metasedimentary rocks often contain calcite which serves to buffer pH to 7-8, whereas tills derived from CO phyllitic rocks west of the RMC and Devonian granitoids east of the RMC lack calcite and have pH < 6. Preliminary analysis suggests that high-U till occurs downgradient of Devonian granitoids.

40 bedrock sites across Vermont were sampled in 2013 for a study of geothermal properties. The distribution of elements from these metasedimentary rocks in concert with mineralogy reported in the literature is a preliminary proxy for bedrock chemistry in the study area. This study confirms the strong link between bedrock lithology and the composition of till derived from it. Our results confirm the idea that the background geochemistry of soils, surface water, and groundwater in this area will vary as the composition of the till varies. This, in turn, has been extensively shaped by the lithologies of the bedrock and bedrock-derived sediments that have served as the source materials for the till.

Geological Society of America Abstracts with Programs. Vol. 49, No. 2

doi: 10.1130/abs/2017NE-291382

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