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Tectonic Implications of the Indian Run Formation, a Newly Recognized Sedimentary Melange in the Northern Virginia Piedmont

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Tectonic Implications of the Indian Run Formation, a Newly Recognized Sedimentary Melange in the Northern Virginia Piedmont Tectonie Implications of the Indian Run Formation- A Newly Recognized Sedimentary Melange in the Northern Virginia Piedmont U.S. GEOLOGICAL SU&V.&T PROFESSIONAL PAPER 1324 Tectonic Implications of the Indian Run Formation A Newly Recognized Sedimentary Melange in the Northern Virginia Piedmont By Avery Ala Drake, Jr. U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1324 UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1985 DEPARTMENT OF THE INTERIOR DONALD PAUL HODEL, Secretary U.S. GEOLOGICAL SURVEY Dallas L. Peck, Director Library of Congress Cataloging-in-Publication Data Drake, Avery Ala, 1927- Tectonic implications of the Indian Run Formation, a newly recognized sedimentary melange in the northern Virginia Piedmont. (Geological Survey professional paper ; 1324) Bibliography: p. 11 Supt. of Docs, no.: I. 19.16:1324 1. Melanges (Petrology). 2. Petrology Virginia. I. Title. II. Series. QE471.15.M44D73 1984 552.09755'6 84-600103 For sale by the Distribution Branch, U.S. Geological Survey 604 South Pickett Street, Alexandria, VA 22304 CONTENTS Page Page Abstract ...................... 1 Regional geology continued Introduction .................... 1 Annandale Group (here named) . 5 Regional geology ................ 2 Accotink Schist ......... 6 Popes Head Formation ......... 2 Lake Barcroft Metasandstone 6 Piney Branch Allochthon ........ 2 Indian Run Formation (here named) . 6 Yorkshire Formation .......... 2 Tectonic implications ............ 9 Peters Creek Schist ........... 3 Conclusions .................. 11 Sykesville Formation .......... 4 References cited ............... 11 ILLUSTRATIONS Page PLATE 1. Greneralized geologic map of part of Fairfax County, Virginia, and geologic map and section of the type locality of the Indian Run Formation ............................. In pocket FIGURES 1-4. Photographs showing: 1. Migmatitic Peters Creek Schist.............. 2. An olistolith of serpentinite in the Sykesville Formation 3. Peters Creek migmatite olistoliths in the Sykesville Formation ....................... 4. Soft sediment fold in the Sykesville Formation ..... 5. Photomicrograph and photographs of ultramafic rock fragments in the Indian Run Formation ................. 6. Photographs of Accotink Schist fragments in the Indian Run Formation ........................... 7. Photomicrograph of fine-grained metagabbro olistolith in the Indian Run Formation ..................... TABLE TABLE 1. Modes of Sykesville and Indian Run Formations in TECTONIC IMPLICATIONS OF THE INDIAN RUN FORMATION A NEWLY RECOGNIZED SEDIMENTARY MELANGE IN THE NORTHERN VIRGINIA PIEDMONT By A VERY ALA DRAKE, JR. ABSTRACT INTRODUCTION Sedimentary melange in the northeastern part of Fairfax County, In recent years, Drake (1980) and Drake and Morgan Virginia, contains both mesoscopic and mappable fragments of Ac- (1981,1983) have recognized the importance of precur­ cotink Schist, Lake Barcroft Metasandstone, metagabbro, and ultra- sory sedimentary melanges (Elter and Trevisan, 1973) mafic rocks as well as smaller fragments of other rock types. This as tectonic indicators in the central Appalachian Pied­ melange was originally mapped as the Sykesville Formation, a major mont. These melanges contain small to very large frag­ precursory sedimentary melange in northern Virginia and Mary­ land. The fragments of Accotink Schist and Lake Barcroft Metasand­ ments of rock units that now lie tectonically above stone within the Sykesville were considered to be rip-ups of these them and thereby mark the boundaries of allochthons. units over which the Sykesville slid when finally emplaced. More The melange and its overlying allochthon constitute a recent study has shown that fragments of Accotink and Lake Bar­ pair termed a "tectonic motif." Two such motifs, the croft are restricted to a certain area of sedimentary melange origi­ Sykesville Formation-Peters Creek Schist and the nally defined as Sykesville, and this part of the melange is now Yorkshire Formation-Piney Branch Complex, have considered to be a separate mappable unit, here named the Indian been mapped and described in the northern Virginia Run Formation. The Indian Run underlies the sequence Accotink Schist and Lake Barcroft Metasandstone which is here formally Piedmont (Drake and Lyttle, 1981; Drake and Morgan, named the Annandale Group. The Indian Run is intruded by the 1981, 1983; Drake and others, 1979). In the eastern Occoquan Granite of Cambrian age, so it is of Cambrian or Late part of Fairfax County, Virginia, a problem has existed Proterozoic age. within the sedimentary melange originally mapped as The Sykesville Formation (restricted) is a much more extensive Sykesville Formation (Drake and others, 1979; Drake unit than the Indian Run Formation and is characterized by its and Lyttle, 1981). The Sykesville as mapped in that contained olistoliths of the Peters Creek Schist, the unit that tecton- area contains, in addition to exotic rock types, frag­ ically overlies it. The Sykesville and Peters Creek constitute a pre­ cursory melange-allochthon pair which is here termed a "tectonic ments of Accotink Schist and Lake Barcroft Metasand­ motif." The Indian Run-Annandale pair then forms a tectonically stone, rocks that tectonically underlie the Sykesville lower motif, and the overlying pair, the Yorkshire Formation-Piney sedimentary melange. This relation presents a struc­ Branch Complex, forms a tectonically higher motif. The Chopawam- tural dilemma in that a melange contains fragments of sic Formation and underlying sedimentary melange in the area a unit upon which it was supposed to have moved. This south of Fairfax County may form a tectonic motif beneath the In­ problem was rationalized (Drake and Lyttle, 1981) by dian Run-Annandale tectonic motif. Thus, three and perhaps four repetitions of precursory melange-allochthon pairs occur in northern considering the fragments of Accotink and Lake Bar­ Virginia. Other percursory melanges and motifs may occur in the croft to be rip-ups or possibly to be units that consti­ Maryland Piedmont to the north. tuted a giant olistolith within the Sykesville. More re­ The tectonic setting of the motif formation and assemblage is un­ cent reexamination of critical exposures in the field led certain at this time. A model involving the obduction of several to the recognition that the olistoliths of Accotink Schist separate sheets onto the ancestral North American continental mar­ and Lake Barcroft Metasandstone are not in the gin is appealing in that it involves the closing and destruction of a marginal basin, a relatively simple concept. This model fails, how­ Sykesville Formation at all but are in an entirely dif­ ever, to supply a source for the sediment necessary to form the pre­ ferent sedimentary melange that lies beneath rather cursory melanges. A trench-slope origin would supply the vast than above the Accotink and Lake Barcroft. This amount of needed sediment by accretion. The precursory melange- melange is thought to be a precursory melange to the allochthon motifs would then be stacked near the base of the trench allochthonous Accotink and Lake Barcroft as the York­ slope. This model is appealing and requires a rather complicated assemblage of continental, arc, and oceanic fragments, such as those shire Formation is to the Piney Branch Complex and as that occur on many modern continental margins. the Sykesville is to the Peters Creek Schist, thereby TECTONIC IMPLICATIONS OF THE INDIAN RUN FORMATION constituting a third tectonic motif in this part of north­ POPES HEAD FORMATION ern Virginia (pi. 1). The primary purpose of this paper The Popes Head Formation (Drake and Lyttle, 1981) is to describe this newly recognized melange and to unconformably overlies all the other lithotectonic units name it the Indian Run Formation. and consists of a lower Old Mill Branch Metasiltstone The Accotink Schist and Lake Barcroft Metasand- Member and an upper Station Hills Phyllite Member. stone lie between two melanges, and in a sense, then, Both of these members contain interbedded felsic and could be considered to constitute a megaolistolith mafic metatuff. These rocks are graded but lack within a mass of sedimentary melange. The Eastern current-produced flow marks. They have been inter­ Fairfax sequence that is composed of the Accotink preted to belong to the Bouma (1962) turbidite se­ Schist and Lake Barcroft Metasandstone (Drake and quence Tde and to Mutti's and Ricci Lucchi's (1978) Lyttle, 1981) is here formally named the Annandale turbidite facies D, suggesting deposition from weak Group. turbidity flows, and to have resulted from the bilateral The data for this paper were collected during detailed filling of a backarc basin (Drake and Lyttle, 1981). The geologic mapping of the crystalline rocks of Fairfax typical mineral assemblage of rocks of the Popes Head County, Virginia, at the scale of 1:24,000 that resulted Formation is quartz-muscovite-biotite-plagioclase, in a preliminary geologic map of Fairfax County and they are at biotite grade. The unit has a maxi­ (Drake and others, 1979) at the scale of 1:48,000. mum thickness of more than 1,000 m. The Popes Head is intruded by the Occoquan Granite so is older than that plutonic unit and is either of Cam­ brian or Late Proterozoic age (Drake and Lyttle, 1981). REGIONAL GEOLOGY It is unconformable upon the other metamorphosed rocks in this area (Drake and Lyttle, 1981; Drake and The metamorphic rocks in northernmost Virginia Morgan, 1981) and has had a much less severe tectonic occur in a stack of six lithotectonic
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