TECTONIC DEVELOPMENT, FACIES ARCHITECTURE, AND PALEOENVIRONMENTAL ANALYSIS OF THE NORTHEASTERN NORFOLK BASIN, EASTERN MASSACHUSETTS

The Pennsylvanian Norfolk Basin is a northeasterly trending erosional outlier of the larger Narragansett Basin to the south. Detailed analyses of depositional facies distribution and facies architecture and cyclicity in the northeastern portion of the basin demonstrates a history of early crustal extension and uplift followed by a phase of regional subsidence. During early rifting canyons were incised into prerift basement of thermally bulged rift shoulders. One of these paleocanyon systems is filled by approximately 140 m of locally derived, rounded, boulder conglomerate of the Pondville Formation (mean clast diameters up to 0.6 m with some clasts up to 2 m). This paleocanyon boulder facies rests nonconformably on the Blue Hills Igneous Complex and extends laterally for nearly 1 km. This very coarse proximal facies is overlain by finer grained distal facies of the Wamsutta Formation. Wamsutta facies comprise arkose and litharenite sandstones, polymictic conglomerates and red and black mudstones preserved as flood and possible climactic and/or tectonic cycles. Patterns of facies and architectural elements suggest deposition in humid braided river and possible anastomosing river systems and limited preserved alluvial fan systems. Three stratigraphic sections were studied in detail: Canton RR, Rt. 93-28, and Rt. 93-24. Canton RR has architectural elements CH, GB, SB, LS, and OF; Rt. 93-28 has CH, GB, SB, and minor OF; Rt. 93-24 has CH, SB, OF, and minor GB. Canton RR's basal 58m represents two coarsening upward cycles, each starting with Fm followed by fine sandstone SB elements and capped by facies Gp. The upper 42m is comprised of four 5 to 8 m thick amalgamated conglomerate units, each capped by 0.2m to a 4.5m thick Fm. These four cycles may represent channel elements that were filled by Gp by increasingly coarser bed load initiated by increased tectonism in the basin. Canton RR locality thus records rapid changes between high and low energy depositional systems and tectonism increasing with time. Rt. 93-28 and Rt. 93-24 exhibit larger scale fining upward sequences due to episodic sediment transport within an overall regime of regional subsidence and source area reduction.