Hydrogeologic framework of stratified-drift aquifers in the glaciated Northeastern United States

Abstract

The Northeastern United States was probably glaciated several times. In most valleys, however, the last ice sheet eroded down to bedrock, and all of the stratified drift that now overlies bedrock can be ascribed to the last deglaciation. Only in northeastern Ohio, northwestern Pennsylvania, and parts of southwestern and central New York do valleys commonly contain complex stratigraphy that results from repeated glaciation: multiple till sheets interlayered with lacustrine silt and with sand or gravel aquifers. Most stratified drift was deposited in the major valleys or lowlands when they were inundated by rising sea level or, more commonly, by proglacial lakes. Coarse-grained sediment was deposited in these water bodies as deltas and subaquatic fans, commonly amid stagnant ice blocks near the margin of the active ice, and also in channels within the ice sheet. The sediment was derived partly from debris-laden basal ice and subglacial till, entrained by meltwater flowing through tunnels, and partly from fluvial erosion of recently deposited drift in the uplands that bordered the valley-bottom lakes. The stratified drift consists of three facies deposited successively: proximal coarse-grained heterogeneous ice-contact deposits, followed by distal fine-grained lake-bottom sediment, and finally by coarse-grained surficial sediment deposited in shallow lakes or stream channels. One or two facies may be absent at any given site, but all three can be identified in many places along every major valley or lowland. Coarse-grained ice-contact deposits commonly constitute the bulk of the stratified drift in narrow or shallow valleys, whereas in broad lowlands they are widely scattered and occupy only a small fraction of the valley floor. In valleys where depth to bedrock exceeds about 100 feet, the bulk of the stratified drift commonly is fine grained, and transmissivity is not generally proportional to saturated thickness. Coarse sand and gravel tend to be more abundant in the southern part of the glaciated Northeast than farther north. Several concepts or generalizations are widely applicable in interpreting the distribution of coarse-grained aquifers within glacial drift in the glaciated Northeast. In many localities, stratified drift can be divided into a series of morphosequences, which represent successive time intervals during deglaciation. Grain size decreases distally within each morphosequence; coarse, heterogeneous ice-contact sand and gravel predominates at the proximal end, whereas coarse sand commonly overlies lake-bottom fines at the distal end. Successive morphosequences can be difficult to distinguish, however, and coarse proximal deposits that are especially prominent and easily recognized commonly overlie relatively high bedrock and have small saturated thickness. Some investigators have inferred that water-yielding coarse sand and gravel are widely distributed at the base of the stratified drift, overlying till or bedrock, as a result of continuous deposition of subaquatic fans at the ice margin during retreat, even in broad lowlands where surficial stratified drift is predominantly fine grained. Other studies indicate, however, that subaquatic fans did not form in all valley reaches and that within broad lowlands they are restricted to relatively narrow zones that follow former subglacial channels.