38 Undergraduate Research I (Posters)

Saturday, 21 March 2020
8:00 AM - 12:00 PM
Grand Ballroom A–D

AGE AND ENVIRONMENTAL CHANGE INFORMATION ENCODED IN EARLY-MIDDLE (?) PLEISTOCENE PALEOSOLS EXPOSED IN BURIED VALLEYS OF THE EASTERN PENNSYLVANIA ANTHRACITE FIELDS

Paper number: 38-11
Program: Theme Sessions
Time: 8:00 AM – 12:00 PM

Authors

Abstract

Pleistocene deposits of unclear origin and age are preserved in the floors of deep, narrow valleys among the anthracite coal fields of eastern Pennsylvania. We collect stratigraphic, soil texture, soil iron chemistry, paleomagnetic, and infrared stimulated luminescene (IRSL) data from a paleosol exposed in the former valley bottom of the middle branch of Shamokin Creek near Mount Carmel, PA. The deposit and its paleosol are consistent with middle Pleistocene glacio-fluvial deposition, and subsequent chemical alteration in excess of the Holocene weathering environment. Our 450 cm thick measured section characterizes the upper part of ~7 m of crudely stratified, locally imbricated, rounded and angular gravel and boulders embedded in a clayey-sand matrix, with few, discontinuous, thin interbeds of silt and clay interpreted as an ablation till. The paleosol in the till has a truncated loamy epipedon, and a 260 cm thick, 5 YR argillic B horizon. This facies unconformably overlies an irregular bedrock surface with 1-2 m of local relief, and is conformably capped by ~6 m of well stratified, boulder gravel sand interpreted as a head of outwash. Both the till and outwash facies are truncated by a high-relief unconformity which in turn is buried by ~5 m of brown, poorly-stratified colluvium with a distinct stone line at its base where it overlies the paleosol. A saturated IRSL age extracted from a sand lens at 280 cm in the section is > 320 ± 60 ka whereas oriented samples of clayey-sand from the same horizon are determined to be paleomagnetically normal (≤ 780 ka), constraining the age of the till to a pre-Illinoian glacial advance into eastern Pennsylvania, possibly during MIS 16. The oxalate to dithionite extractable iron ratio (FeO/FeD) of the paleosol, known to be a relative indicator of the duration pedogenesis, and the goethite to hematite ratio (G/H), shown to be a sensitive indicator of precipitation in modern soils, is compared to a growing database of both modern and ancient soils in the mid-Atlantic region. The location of theses deposits and their weathering characteristics will require reassessment of the mapped pre-Illinoian ice margins in eastern Pennsylvania and challenges traditional ideas about the paleoenvironmental conditions during the middle Pleistocene.

Geological Society of America Abstracts with Programs. Vol. 52, No. 2

doi: 10.1130/abs/2020SE-344628

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