Carbonate Accumulation on the Inner Continental Shelf of Maine: A Modern Consequence of Late Quaternary Glaciation and Sea-level Change

Abstract

ABSTRACT Accumulations of skeletal carbonates are associated with late Quaternary glacigenic sediments on the inner continental shelf of Maine despite cold waters that are generally considered corrosive to calcium carbonate. These shelly lithosomes (up to 100% CaCO3) are confined to a small area in the vicinity of Mt. Desert Island, a granitic massif on the deeply embayed coast. Exposure to waves is a strong control on sediment accumulation and varies due to orientation of the coast, presence of numerous islands, and extremely irregular bathymetry. Postglacial regression and subsequent transgression have greatly influenced the nature and distribution of surficial sediments on shallow parts of the shelf (less than approximately 60 in depth). On the basis of bathymetry, bottom sedimen properties, and geophysical characteristics, five major physiographic zones are recognized: nearshore basins, shelf valleys, outer basins, rocky zones, and nearshore ramps. Thick deposits of marine mud, often charged with natural gas and pockmarked in places, exist inside sheltered embayments (nearshore basins) and offshore below the influence of sea-level changes (outer basins). Shallower areas are dominated by rock outcrops (rocky zones) that support a prolific fauna of calcareous organisms such as barnacles, echinoderms, mussels, and assorted bivalves and gastropods. There are no significant sources of terrigenous sediments nearby. In their absence, abundant sand- and gravel-size shell fragments accumulate in rocky zones and adjacent nearshore ramps, form a thin layer capping glacial marine sediments and till, and supply sediment for two carbonate sand beaches. These deposits provide a useful analog for deciphering the origins of some ancient limestone-diamictite successions.