Tectonic implications of the Indian Run Formation— A newly recognized sedimentary mélange in the northern Virginia Piedmont

Abstract

Sedimentary melange in the northeastern part of Fairfax County, Virginia, contains both mesoscopic and mappable fragments of Accotink Schist, Lake Barcroft Metasandstone, metagabbro, and ultramafic rocks as well as smaller fragments of other rock types. This melange was originally mapped as the Sykesville Formation, a major precursory sedimentary melange in northern Virginia and Maryland. The fragments of Accotink Schist and Lake Barcroft Metasandstone within the Sykesville were considered to be rip-ups of these units over which the Sykesville slid when finally emplaced. More recent study has shown that fragments of Accotink and Lake Barcroft are restricted to a certain area of sedimentary melange originally defined as Sykesville, and this part of the melange is now considered to be a separate mappable unit, here named the Indian Run Formation. The Indian Run underlies the sequence Accotink Schist and Lake Barcroft Metasandstone which is here formally named the Annandale Group. The Indian Run is intruded by the Occoquan Granite of Cambrian age, so it is of Cambrian or Late Proterozoic age. The Sykesville Formation (restricted) is a much more extensive unit than the Indian Run Formation and is characterized by its contained olistoliths of the Peters Creek Schist, the unit that tectonically overlies it. The Sykesville and Peters Creek constitute a precursory melange-allochthon pair which is here termed a 'tectonic motif.' The Indian Run-Annandale pair then forms a tectonically lower motif, and the overlying pair, the Yorkshire Formation-Piney Branch Complex, forms a tectonically higher motif. The Chopawamsic Formation and underlying sedimentary melange in the area south of Fairfax County may form a tectonic motif beneath the Indian Run-Annandale tectonic motif. Thus, three and perhaps four repetitions of precursory melange-allochthon pairs occur in northern Virginia. Other percursory melanges and motifs may occur in the Maryland Piedmont to the north. The tectonic setting of the motif formation and assemblage is uncertain at this time. A model involving the obduction of several separate sheets onto the ancestral North American continental margin is appealing in that it involves the closing and destruction of a marginal basin, a relatively simple concept. This model fails, however, to supply a source for the sediment necessary to form the precursory melanges. A trench-slope origin would supply the vast amount of needed sediment by accretion. The precursory melangeallochthon motifs would then be stacked near the base of the trench slope. This model is appealing and requires a rather complicated assemblage of continental, arc, and oceanic fragments, such as those that occur on many modern continental margins.