FILLING GAPS IN SILURO-DEVONIAN PALEOGEOGRAPHY OF THE NORTHERN APPALACHIANS

Siluro-Devonian strata of northern New England and eastern Canada preserve the local record of a complex convergent margin. Though the geology of the region has been studied for decades, the USGS STATEMAP Program has provided an opportunity to update existing geologic maps using new tools and collaborations. Recent mapping in five 7.5’ quadrangles at the eastern edge of the Connecticut Valley Synclinorium (CVS) in northern New Hampshire has allowed us to refine models for CVS paleogeography.

The mapped stratigraphic section comprises several kms of metasedimentary, metavolcanic, and metavolcaniclastic units with maximum depositional ages of ~434-390 Ma. Depositional sequences comprise mudstones interbedded with silty-sandy turbidites, debrites, and volcaniclastics. Limited sedimentary structures suggest deposition in an outer shelf to slope environment. Rare flute casts record NE-SW sediment transport. Sediment composition varies from lithic to quartzo-feldspathic, consistent with variable source terranes to the southeast, and implying multiple sediment transport pathways.

This stratal architecture is predictable using a sequence-stratigraphic model for punctuated deposition along the shelf-basin transition of a tectonically active margin. Stratigraphic sequences record overall retrogradation, likely due to increased accommodation relative to sediment supply. Post-depositional deformation has obscured the original thickness of the stratigraphic section; we estimate thickness from facies architecture, structural position, and mechanical contrast between highly sheared mudstones, tightly folded interbedded silts/mudstones and less deformed sandstones and volcaniclastics.

Stacking patterns in co-eval sections of southern Quebec are comparable, whereas contemporaneous units of the Gaspe Peninsula are generally thinner and appear to record retrogradation followed by progradation in an overall shallower water environment. The combined observations suggest asymmetric subsidence for the CVS, akin to marginal basins of the modern western Pacific. Future work will refine this paleogeography via geochemistry of co-eval volcanics, additional provenance analysis of sands/silts, and reconstruction of burial history.