The Late Devonian–Early Carboniferous St. Marys basin in mainland Nova Scotia, Canada, consists of Horton Group intracontinental clastic rocks that were deposited along the Avalon-Meguma terrane boundary during the waning stages of the Acadian orogeny and prior to terminal collision in the Appalachian orogen. These rocks represent the earliest stages of deposition along the southern flank of the composite Magdalen basin (Late Devonian to Permian) that extends over much of Maritime Canada and oversteps terrane boundaries in the Canadian Appalachians. As such, the Horton Group represents deposition in an episutural basin.

The Horton Group in the St. Marys basin predominantly consists of 3000–4000 m of clastic sedimentary rocks deposited in fluviatile and lacustrine nearshore environments.

Geochemical analyses of clastic rocks reveal a range of SiO2 content (60–94 wt%), high total rare earth elements and initial 87Sr/86Sr, and strongly negative ϵNd(t) (t = 360 Ma). The signature of the passive margin to active continental margin implied by standard discrimination diagrams probably is inherited largely from rocks in the source region and is not an indication of the setting in which the basin formed. Although the data indicate that some lithologies contain a minor Avalonian basement component, the geochemical and isotopic signatures of the sedimentary rocks reveal mixed sources that can be attributed largely to uplift and erosion of Meguma terrane metasedimentary and granitoid rocks immediately to the south of the St. Marys basin. Regional syntheses indicate that this uplift occurred before and during Horton Group deposition and was coeval with dextral ramping of the Meguma terrane over the Avalon terrane along the southern flank of the Magdalen basin.

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