Alice R. Kelley

VARIABILITY IN THE EVOLUTION OF TWO ADJACENT BEDROCK-FRAMED ESTUARIES IN MAINE

Casco and Saco Bays are large, adjacent, bedrock-framed embayments in southern Maine. Bottom samples, underwater vibracores, bridge borings, and 500 km of high resolution seismic reflection profiles were used to compare the Holocene evolution of the bays. Each has experienced glaciation, marine inundation, subaerial emergence, as well as contemporary drowning in the past 13,000 years, but they nonetheless differ profoundly in bedrock composition, shoreline environments, bottom sediment, and bathymetry. Casco Bay is divided by islands of metamorphic rock ridges which parallel the coast. Glacial mud, derived from erosion of the “soft” rocks, filled the bay 12,000 years ago, but was deeply gullied and eroded during later emergence. Recent drowning left ravines partly filled with organic-rich sediment (and gas) and interfluves of bare rock. Bluff erosion of marine clay provides most sediment for todays ubiquitous mudflats. Saco Bay is more exposed to waves than Casco Bay and is bordered by granitic rocks. Sand ultimately derived from glacial erosion of granite, formed barrier spits offshore during emergence. Recent sea level rise has resulted in a smooth sandy seafloor, and sand beaches with salt marshes are the predominant coastal environments.

Controls on Surficial Materials Distribution in a Rock-Framed, Glaciated, Tidally Dominated Estuary: Cobscook Bay, Maine

Abstract Surficial materials were mapped on the bottom of Cobscook Bay, ME, through aerial photography of intertidal habitats, side-scan sonar, and seismic reflection profiling of subtidal regions. Like many other estuaries in northern New England, this rocky, macrotidal estuary has only slight riverine input and contains an abundance of till and fine-grained glacial-marine sediment. Contrary to conceptual models of estuarine sediment and habitat distribution, grain size does not become finer and habitats lower in energy in a landward direction within the estuary. The irregular shoreline shape, imparted by bedrock, forms a series of narrow constrictions separating broad bays. More than 70% of the bottom of the estuary is floored by gravel and rock; mud deposits are located in shallow-water coves throughout the Bay and in two large deposits in the Central Bay. Here, circulation models predict two large gyres form because water cannot pass through a bedrock constriction quickly enough. Natural gas is present in sufficient quantities in the sediment column to facilitate sediment mass movements near the mud deposits. Almost 60% of the intertidal zone is composed of mudflats that are uniformly distributed within and along the outside margin of the Bay, with increasing abundance of bedrock in a landward direction. Small beaches occur wherever coarse-grained glacial sediment erodes from bluffs. These observations depart from existing conceptual models of estuarine sediment distribution based on coastal plain estuaries and suggest that better understanding of biotic habitat or contaminant distribution in rocky glaciated estuaries will require more localized models. These estuaries appear more complex than coastal plain estuaries because of the unique outcrop pattern of bedrock and glacial deposits in each bay.

Late glacial and Holocene history of the Penobscot River in the Penobscot Lowland, Maine

When the Laurentide ice sheet retreated rapidly (~150 m/a) across the Penobscot Lowland between ~16 and ~15 ka, the area was isostatically depressed and became inundated by the sea. Silt and clay were deposited, but no significant moraines or deltas were formed. The Penobscot River was reborn at ~14 ka when ice retreated onto land in the upper reaches of the river’s East Branch. As isostatic rebound exceeded sea level rise from melting ice, the river extended itself southward. Between ~13.4 and 12.8 ka, it established a course across marine clay and underlying glacial till in the Lowland. Its gradient was low as differential rebound had not begun. Discharge, however, was higher and the river transported and deposited outwash gravel. During the cold, dry Younger Dryas, ~11 ka, eolian sand began to accumulate in dunes in the Lowland. Some of this sand, along with fluvial sediment from the headwaters, was redistributed into terraces along gentler stretches of the river and into a paleodelta in Penobscot Bay. Eolian activity continued to ~8 ka and aggradation in terraces until ~6 ka. The climate became wetter and warmer after ~6 ka, the dunes were stabilized by vegetation, the river began to downcut, and braiding became less intense. Pauses in the downcutting are reflected in discontinuous strath terraces. In due course, the river re-encountered the old outwash gravels, marine clay, glacial till, and, in a few places, bedrock. Its profile is now stepped, with gentle, gravel-bedded reaches between bedrock ribs that form rapids.

Evidence for a Former Transgressive Dune Field: Shetland Islands, United Kingdom

ABSTRACT Kelley, J.T.; Kelley, A.R.; Sorrell, L.; Bigelow, G., and Bampton, M., 2018. Evidence for a former transgressive dune field: Shetland Islands, United Kingdom. Transgressive sand dunes result from a large disturbance of a significant coastal sand dune field. Sand blows landward, covering whatever it encounters, including agricultural fields, forests, or human habitations. This investigation is of a beach-dune system in the Shetland Islands of northern Scotland that is known from historic documents and archaeological excavations to have experienced a sand invasion during the Little Ice Age (LIA). Ground-penetrating radar observations suggest remnants of pre-LIA dunes and buried soils. Excavation of dunes and optically stimulated luminescence (OSL) dating of sand deposits confirm historic accounts but also document that the “event” lasted centuries. Geomorphological observations and OSL dates also indicate that earlier events occurred in this region, sometimes in association with known archaeological sites like Old Scatness and Jarlshof. Although the site studied is stable now, a sand invasion could occur again owing to increased storminess, removal of dune vegetation, or both. Mining of the dunes for aggregate places the contemporary beach in a more vulnerable position than earlier.

Holistic geoarchaeology in the Penobscot Valley, Maine, USA: context, scale, and interpretation

A holistic landscape approach to cultural resource analysis of a set of archaeological sites in the central Penobscot Valley led to inferences regarding the Holocene physical and biological environmental context. Targeted environmental studies include (1) examination of forest and wetland changes through time and (2) lake-level studies as a key to paleohydrology. These studies were combined with broad-scale geomorphic investigations and detailed stratigraphic analyses, and studies of archaeological sites and their artifact assemblages. Together, these studies provide a picture of dramatic changes to the physical and vegetational landscape. These included reestablishment of a major river following deglaciation, evolution of extensive lakes to uplands and peatlands, and a shifting mosaic of open and closed forest composed of a variety of hardwood and softwood species. Inferences based on buried soils exposed in archaeological excavations supported climatic interpretations based on vegetation and paleohydrology. As a result, this work allowed evaluation of (1) site formation and preservation processes and (2) occupational patterns. Site formation and preservation of Early Holocene sites can be linked to sedimentation by hydraulic damming upstream of rapids at the mouths of tributary streams. Shifting human land use reflected by changes in occupation patterns appears to correspond to changes in forest composition as well as wetland and stream evolution through time.