The upper Hadrynian Jeffers Group of the Cobequid Highlands (Avalon zone of northern Nova Scotia) consists of volcanic rocks of the Gilbert Hills Formation, interfingering deeper-water argillite of the Humming Brook Formation, and overlying turbidite lithic arkose and siltstone of the Cranberry Lake Formation. The Jeffers Group has a spatially inhomogeneous flat-lying cleavage, associated with thrust faults of late Hadrynian age that were in part reactivated during Carboniferous strike-slip motion on the Cobequid fault.

The basal rocks of the Gilbert Hills Formation consist of felsic pyroclastics and minor mafic and felsic flows of the Lakelands facies, locally passing up into sandstone and marble, overlain by the Humming Brook Formation. Thrust over this sequence is the Harrington River facies of the Gilbert Hills Formation, comprising flows, ranging in composition from subalkalic basalt to rhyodacite, and pyroclastic rocks.

Two groups of igneous rocks are distinguished geochemically. High-Ti basalts of the Lakelands facies and associated hypabyssal intrusions have tholeiitic character; enrichment in Ba, Rb, and K; flat light-REE spectra; little or no depletion in Nb and Ta; and high FeOt and Ti. They are associated with voluminous felsic rocks. The Harrington River facies has a calc-alkaline differentiation series from basalt to rhyodacite but with more enrichment in FeOt, P, Nb, and Zr than is found in typical calc-alkaline rocks. These two groups are also represented in hypabyssal rocks.

The Jeffers Group rocks are compared geochemically with other predominantly continental back-arc basins in the Aegean Sea and Bransfield Strait. All three basins show the presence of a rock phase derived from partial melting of mantle only slightly modified by subduction processes (the Lakelands facies tholeiites of the Jeffers Group). They all also contain a more differentiated series showing more subduction-related chemical characteristics (the Harrington River facies of the Jeffers Group). Geochemical differences between these rocks and typical calc-alkali series are related to the lesser role of hydrous fluids in the back-arc basins. As a result, titanomagnetite and amphibole are not important fractionating phases, and Nb and Ta are not significantly depleted.

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