1 New advances in geological and geophysical research on the Appalachian orogen.

We invite contributions that discuss new research on the Appalachian orogen. Submissions should include, but are not limited to, geological or geophysical studies or works comprising a combination of both methods.

Sunday, 22 March 2026
8:00 AM - 12:00 PM
CCC, Room 22/23

Inception of Acadian Arc Volcanism in the Northern Maine Appalachians: Evidence from the Reestablished Lochkovian Seboomook Group

Paper number: 1-5
Program: Technical Sessions
Time: 9:25 AM – 9:45 AM

Authors

Abstract

The Lower Devonian Lochkovian Seboomook Group is a widespread component of the Connecticut Valley–Gaspé Trough in northern and northwestern Maine. This syntectonic flysch sequence, dominated by turbiditic slate and sandstone, was deposited in a foreland–foredeep basin system related to advancing Acadian deformation. Recent mapping around Cambrian–Silurian Munsungun, Winterville, East Branch, Lunksoos Lake–Weeksboro, and Number Nine Mountain inliers yields several new key findings: (1) the Seboomook Group rests unconformably on these inliers, with this regional unconformity marking the end of the Salinic orogeny and the onset of Acadian basin development; (2) The lower Seboomook comprises a conformable “trinity” of basal conglomerate, fossiliferous limestone and calcareous sandstone, and maroon–red slate-siltstone units; the latter locally contains laminated manganese–iron formation intervals. The trinity is continuous and widespread in northern Maine – it had previously been mapped as separate or even Silurian formations but now is constrained by new conodont data to be Lochkovian and to be the lower succession of the Seboomook Group; (3) The trinity contains multiple basalt flows and minor mafic pyroclastic rocks (including the “West Branch Volcanics” redefined in our study) in the headwaters of the East Branch Penobscot and Aroostook rivers and near Ripogenus Lake – the basalt flows are dominated by characteristic vesicular/amygdaloidal and peperite basalts. The trinity records the earliest Acadian marine transgression, earliest Devonian volcanism, and an early Devonian manganese–iron precipitation event, which are recognized for the first time. Whole-rock analyses of 14 basalt samples reveal an exceptionally homogeneous calc-alkaline geochemical signature characterized by pronounced negative Nb-Ta anomalies, enrichment in LILE and LREE, depletion in HFSE, low Nb/Y ratios (average 0.22), and elevated La/Nb (average 2.58), Th/Nb (average 0.46), and Th/La (average 0.18) ratios, which are all typical for subduction-related continental-arc basalt. The basalt data record the onset of Acadian arc volcanism in earliest Devonian; this followed the final Salinic arc-like mafic submarine volcanism preserved in the 425–424 Ma Ludfordian Spider Lake and Churchill Ridge formations in the Munsungun inlier, the rapid uplifting and erosion of Salinic basement, and the onset of Acadian basin development, in northern Maine.

Geological Society of America Abstracts with Programs. Vol. 58, No. 2

doi: 10.1130/abs/2026NE-411332

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