DETRITAL ZIRCON AGES FROM SILURIAN STRATA IN THE CENTRAL MAINE BASIN: IMPLICATIONS FOR THE EXISTENCE OF THE MESSALONSKEE LAKE THRUST

The Central Maine basin was positioned between the eastern margin of Laurentia and the micro-continent Ganderia and is composed of metamorphosed marine sedimentary rocks deposited in Late Ordovician to Early Devonian time. Repeated deformation from the Acadian and subsequent orogenies obfuscates the stratigraphy and has led to varied interpretations of Maine’s tectonic history. The Messalonskee Lake thrust (MLT) has been proposed to divide the Central Maine basin into western and eastern sections, separating two stratigraphic sequences of similar age and lithologic character: the Sangerville Formation in the western lower plate and the Vassalboro Group in the eastern upper plate. The MLT was first proposed to explain inverted sequences and the juxtaposition of proximal and distal turbidite sequences; however, no direct field evidence for faulting exists, classifying the Messalonskee Lake thrust as a conceptual fault. The Maine Geological Survey is mapping areas of Central Maine in greater detail, and the existence of the MLT is called into question as it is portrayed as a major structure in south-central Maine and is in a key position for understanding the Acadian orogeny.

Detrital zircon U-Pb dates were determined by LA-ICP-MS to further constrain depositional ages and sediment provenance of two samples of each unit mapped on either side of the proposed location of the MLT. Detrital age populations were compared against signatures from Laurentia and Gondwana to determine sediment provenance. Maximum depositional ages were calculated using youngest single grain, youngest grain cluster within two sigma, and maximum likelihood methods. Both Sangerville (n=280) and Vassalboro Group (n=372) samples show strong Laurentian signatures with major peaks at about 455 Ma, 1050 Ma, and 2720 Ma. Maximum depositional ages in the Sangerville formation range from 404-451 Ma and in the Vassalboro Group range from 401-414 Ma. Similarities in sediment provenance and maximum depositional ages for samples collected on either side of the proposed MLT, combined with the lithologic similarities of these sequences, does not require the existence of a thrust fault at this location. This also impacts debates on the sediment provenance of the Central Maine basin as there is no evidence of peri-Gondwanan sediment in either formation.