Textural and compositional characterization of recent sediments along a paraglacial estuarine coastline, Maine, USA

Abstract

Textural and mineralogical characteristics of bottom sediments collected from the lower Kennebec River estuary, Maine, and adjacent nearshore region were used to differentiate subtidal sedimentary environments. Five depositional environments, defined on the basis of bottom morphology, bathymetry, and dominant sedimentary processes—estuarine channel, channel margin, outer bar, shoreface, and offshore—possess distinct sedimentological characteristics. Trends in textural characteristics of these environments and those along the adjacent barrier beach include: (1) systematic decrease in mean grain size from the estuarine channel to the offshore facies; (2) similarity between the outer bar and estuarine channel sands; (3) increase in sediment sorting from the channel/margin and offshore to shoreface facies; and (4) fining of the low-tide terrace, beachface, and berm samples away from the bar-welding region. The fine-grained nature of the offshore facies is the result of deposition of suspended sediment delivered to the coast during large flood events, whereas the coarse fraction is contributed through the attrition of carbonate tests of the local macrofauna. Results of compositional analyses reveal: (1) a decrease in shell fragments from the estuarine channel (>50%) to the offshore (<10%); (2) a three-fold increase in mineralogical maturity from channel and outer bar to offshore sands; and (3) increase in mineralogical maturity of the beachface and low-tide terrace samples away from the bar-welding region. The similarity between channel-derived bar sands and those sampled from the bar-welding region of the adjacent barrier beach suggests that the sedimentological signature of subtidal environments is temporarily preserved. The wide range of textural and compositional attributes of sediments in paraglacial settings makes these environments ideal places in which to study the variations in sediment transport mechanisms over relatively short distances. We demonstrate the usefulness of primary physical characteristics of bottom sediments in studying the patterns of fluvial–marine interaction along other physiographically similar coastal settings.