ABSTRACT

Pennsylvanian coarse-grained fluvial sedimentation in the Norfolk basin of southeastern Massachusetts occurred in a tectonically active environment, possible related to sinistral strike-slip motion on faults west of the basin. Detailed measured sections reveal an overall upward-fining succession, with humid alluvial fan facies and braided stream sub-facies with associated overbank/flood-plain deposits. The humid alluvial fan contains massive bedded, clast supported conglomerates with infiltrated sandstone and siltstone. Fluvial dominance during deposition is indicated by frequent upward-fining cycles, lack of evidence of debris flow, and clast rounding and imbrication.

The braided stream sub-facies is distinguished by a reduction in bed thickness and clast size (maximum clast size = 5 cm), and commonly contains upward-fining cycles. Although trough cross-bedding angles are low throughout the basin, low angle (< 10°) troughs upslope grade into distal very low angle (< 5°) troughs to plane beds, indicating lesser channel incision and greater sheetflood conditions downslope. The overbank/flood-plain deposits include plane laminated siltstone and claystone with common desiccation cracks.

Evidence for active faulting during deposition includes: mismatch of detrital grain mineralogy with adjacent source terrains; soft sediment boudinage; mixtures of fresh and altered feldspar; and rapid lateral and vertical facies changes.

Provenance studies suggest a source area for many of the sediments might be metabasalts and marbles of the Blackstone Series of northern Rhode Island. If correct, these results support paleomagnetic and other regional studies suggesting sinistral strike-slip motion may have formed the Carboniferous basins of Massachusetts and Rhode Island.

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