MANIFESTATION OF BLUFF EROSION IN THE TRANSGRESSIVE STRATIGRAPHY OF MAINE ESTUARIES

Northern New England coasts display varying proportions of bedrock, eroding glacial sediments, as well as sand and gravel spits, barriers and pocket beaches. Seismic profiles and vibracoring in Maine estuaries reveal a transgressive erosional unconformity created during erosion of glaciomarine bluffs. Distinguishing this basal unconformity (Ub) from a later tidal ravinement unconformity (Utr) or from passive onlap of marginal marine environments over the upland is crucial for deciphering the stratigraphic record and history of estuarine evolution. Presumpscot Fm. glaciomarine mud is widespread on the central Maine coast, draped over bedrock and earlier glacial sediments and infilling paleotopography. Bluffs up to 10 m high are eroded by waves and tides at the base, and are subject to slumps and slides on scales from a few square meters to historical events >80,000 square meters. The scale of the retrogradational, rotational slumps is likely related to total glaciomarine thickness both above sea level and in underlying deeper sections. We identified a cycle of bluff erosion in the 1980’s that includes slumping, colonization of the toe by marsh, leading to protection from toe erosion, to marsh erosional retreat, ultimately exposing the bluff to renewed undermining and slumping. We are researching the time scale of this cycle, which is likely proportional to size. We identify the unconformity surface in recent outcrops by a contorted laminated mud with no soil development, overlain by marsh or tidal-flat sediments. Vibracores also demonstrate contortions, blocks of more desiccated, brecciated materials in a mud matrix, and abrupt contact with overlying estuarine sediments. Seismic evidence for this surface is an angular unconformity truncating the draped, well-bedded glaciomarine mud, and sometimes a wedge or lens of penecontemporaneous bluff-toe lag deposits. Clarifying the identity and processes of formation of this unconformity has important implications for determination of preservation potential and sediment budgets.