A DETRITAL PETROGRAPHIC STUDY OF LATE GLACIAL TO EARLY POST-GLACIAL (HOLOCENE) FLUVIAL TO MARINE SYSTEMS IN THE KENNEBEC REGION, MAINE

A combined light-mineral (LM) and heavy-mineral (HM) petrographic study has been initiated on detrital sands in the Kennebec drainage system in Maine. The goal of the study is to elucidate patterns of LM and HM compositional variation and correlate these variations with depositional environments and their sediment-transport and sediment-sorting potential. ~80 samples from ~40 sites were collected from the sediment source area to the Kennebec’s mouth at the Gulf of Maine. Source rocks are the primarily plutonic and medium- and high-grade metamorphic rocks found between the Maine-Quebec border and the drainage divide south of Rangeley and Flagstaff lakes. Low-grade metasedimentary rocks to the south of the drainage divide are presumed to contribute minimally to HM diversity. Recycling of glacial till and moraines probably was pervasive in upland regions, but less common closer to the coast where they are generally covered by post-glacial marine strata.

Meter-scale sedimentologic analyses were carried out at sample sites in both transgressive and regressive marine sequences, nearshore, beach, prodelta, delta front and top, and fluvial systems. Sediment sorting within 1- to 4-m, vertically composite samples is quantified by mechanical sieving and weighing of size fractions. Sorting of grain sizes varies considerably within depositional environments, and as of yet, no systematic correlation is established with transport distance from upland source regions.

Chemical alteration of detrital grains is negligible; an iron-oxide coating on both HM and LM is common though not pervasive. Surficial/diagenetic alteration appears to have had minimal effect on grains, though some fraction of feldspars show incipient sericitization. No authigenic or diagenetic minerals have been observed to date. We tentatively conclude (based on the few samples point-counted thus far) that the ratio of (Quartz+Kspar+Plag)/total grains increases from proximal to more distal depositional sites. The population of HM is quite diverse, consistent with the diversity of source rocks and the minimal chemical alteration of detrital grains. Both LM and HM population statistics will be analyzed in relation to depositional environment and distance from upland source regions.