Sedimentology of the Unicoi Formation in Southern and Central Virginia: Evidence for Late Proterozoic to Early Cambrian Rift-To-Passive Margin Transition
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Sedimentology of the Unicoi Formation in southern and central Virginia: Evidence for late Proterozoic to Early Cambrian rift-to-passive margin transition KENNETH A ^RIKSSON I department of Geological Sciences, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061 ABSTRACT is indicated by outer-shelf black mudstones lents of the Chilhowee Group are distal-shelf containing Tabc, Tbc, and Tc beds and gravi- and slope-rise deposits of the Evington Group Few detailed-facies analyses of the rift-to- ty-flow deposits at the base of the Hampton and Alligator Back Formation, which crop out passive margin transition have been under- Formation. Deepening may have been en- on the eastern limb of the Blue Ridge anticlino- taken in exhumed orogenic belts. The hanced by continued movement along listric rium (Fig. 1; Wehr and Glover, 1985; Schwab, Unicoi and lower Hampton Formations in the faults throughout the incipient phase of 1986; Patterson, 1987). In southern and central central and southern Appalachians record the passive-margin development. Most of the Virginia and northeast Tennessee, the Chil- transition from late Proterozoic to Early Cam- Hampton Formation and the overlying Erwin howee Group is subdivided into the Unicoi, brian rifting to initial opening of the Iapetus Formation and Shady Dolomite record pro- Hampton, and Erwin Formations (Fig. 2). The Ocean. gradation of the passive margin which culmi- Unicoi Formation bridges the transition from The lower and middle Unicoi Formation nated in development of a carbonate-rimmed rift-to-passive margin sedimentation. Overlying consists of feldspathic sandstones, conglomer- shelf. The rift-to-passive margin phases of Hampton and Erwin Formations record the con- ates, and basalts. Sandstones and conglomer- sedimentation in the central Appalachians re- struction of an east-facing, wave- and storm- ates were deposited from hyperconcentrated flect a continuum from fault-influenced to dominated, siliciclastic ramp and represent the flows and tractional currents in distal alluvial- thermotectonic subsidence. basal portion of the passive-margin prism fan and proximal and medial braid-plain envi- (Simpson and Eriksson, 1986; Simpson, 1987). A carbonate ramp to rimmed-shelf sequence of ronments. Extrusion of basalts was associated INTRODUCTION with influx of coarse-grained siliciclastics. the Shady Dolomite conformably overlies the Erwin Formation (Barnaby and Simpson, 1987; The presence of thick alluvial-fan sediments, In the central and southern Appalachian oro- Read, in press). basalts, and abundant lithic clasts and the gen, rifting associated with the initial phase of feldspathic nature of the sandstones are sug- development of the Iapetus Ocean commenced In view of its critical stratigraphic position in gestive of a rift setting. Paleontological and at -690 Ma (Odom and Fullagar, 1984). The the rift-to-drift transition, an understanding of geochronological data indicate that rifting transition from rifting to the onset of passive- the Unicoi Formation is important to the devel- continued into Early Cambrian time. The up- margin development is constrained poorly both opment of tectonic models for the early evolu- per Unicoi Formation is composed predomi- stratigraphically and temporally; estimated dates tion of the Iapetus margin in the southern nantly of transgressive, quartzose sandstones for onset of ocean-floor spreading vary from 660 Appalachians. Past studies of the Unicoi Forma- which represent the incipient phase of passive to 570 Ma (Bond and others, 1984; Odom and tion and its lateral equivalents have concentrated margin sedimentation related to a second- Fullagar, 1984; Fichter and Diecchio, 1986; on petrology, paleocurrent analysis, stratigraphic order, sea-level rise. Differences in degree of Williams and Hiscott, 1987). complexities, and broad paleoenvironmental re- crustal attentuation probably controlled the In southern and central Virginia, the rift-to- construction (Schwab, 1972; Whisonant, 1970, distribution of sedimentary environments dur- passive margin transition is well developed. 1974; Mack, 1980; Nunan, 1980). Based on ing transgression. On most attenuated crust Thick volcanic, volcaniclastic, and sedimentary mineralogical and textural immaturity, the Uni- to the east, initial transgressive facies consist successions of late Proterozoic age, including the coi Formation has been assigned to alluvial and of tidal sand-wave and sand-ridge deposits Swift Run Formation, Mount Rogers Forma- transitional marine settings. Imprecise knowl- intercalated with proximal and medial braid- tion, Catoctin Formation, and Lynchburg Group edge of depositional environments and their plain deposits. As transgression progressed accumulated in rift basins related to the initial temporal and spatial distribution has led to con- cratonward, tidal sedimentation was supplant- extension (Fig. 1; Rankin, 1975; Miller, 1986; flicting assignment of the Unicoi Formation to ed by tide- and wave-influenced sedimenta- Schwab, 1986; Wehr, 1985, 1986). Each of aulacogen-rift (Rankin, 1976; Bond and others, tion characterized by sand-wave complexes, these rift-related sequences, with the exception 1984), passive margin (Wehr and Glover, tidal inlets, and longshore-directed bedforms. of the Lynchburg Group and Swift Run Forma- 1985), and youthful passive-margin settings Drowning at the top of the Unicoi Formation tion, is overlain by the late Proterozoic to Early (Fichter and Diecchio, 1986). Using back- Cambrian Chilhowee Group (Fig. 2; Schwab, stripping techniques, Bond and others (1984) 1972, 1976, 1986; Miller, 1986; Rast and *Present address: Department of Physical Sciences, proposed that ocean-floor spreading and ther- Kutztown University, Kutztown, Pennsylvania 19530. Kohles, 1986; Wehr, 1986). Temporal equiva- mal subsidence commenced at the base of the Geological Society of America Bulletin, v. 101, p. 42-54, 13 figs., 1 table, January 1989. 42 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/101/1/42/3380432/i0016-7606-101-1-42.pdf by guest on 02 October 2021 SEDIMENTOLOGY OF UNICOI FORMATION, VIRGINIA 43 Figure 1. Generalized geo- Mesozoic Formations Grenviiie Basement logic map of the Blue Ridge Province in southern and Chilhowee Group Measured Sections' central Virginia, showing lo- cation of stratigraphie sec- Thrust Fault Evington Group, Alligator tions through the Unicoi Back Formation Formation: (1) Virginia LYNCHBURG Creeper Trail, (2) Elk Creek, Catoctin Formation and (3) Balcony Falls. Late Proterozoic Sequences Including Mount Rogers Formation 50 100 KM canics and Crossnore plutons have been demon- strated to be consanguineous (Rankin, 1968, 1970, 1975, 1976; Rankin and others, 1969). Felsic volcanic rocks beneath the Chilhowee Hampton Formation, and that fault-controlled conformable (Stose and Stose, 1957; King and were dated at ca. 820 Ma (U-Pb zircon; Rankin subsidence influenced sedimentation during Uni- Ferguson, 1960); the contact may also be struc- and others, 1969). This age was questioned ini- coi time. Wehr and Glover (1985) inferred the tural. In central Virginia, the Unicoi Formation tially because it necessitated the presence of a presence of a break-up unconformity, generated overlies Grenviiie basement or Catoctin Forma- significant unconformity at the Mount Rogers by domal uplift at the base of the Unicoi Forma- tion volcanics. Formation Chilhowee Group contact (Rogers, tion. Fichter and Diecchio (1986) developed a A maximum age for the Unicoi Formation is 1972; Rankin and others, 1969; Odom and Ful- stratigraphie model that placed the Weverton provided by basement rocks. Mount Rogers vol- lagar, 1984), and because it was at variance with Formation, a northern equivalent of the Unicoi preliminary Rb-Sr dating (Odom, 1971; Odom Formation, into the thermally subsiding, youth- and Fullagar, 1971). More recently, Odom and ful stage of Bott (1979) and thermal subsidence Fullagar (1984) have demonstrated the exis- and submergence stage of Kinsman (1975). tence of different age populations of zircons, one NORTHEASTERN inherited from Grenviiie basement and the other This study documents facies and interprets TENNESSEE depositional environments within the Unicoi SOUTHWESTERN representing the crystallization age of the Cross- and lower Hampton Formations. The results are AND CENTRAL nore magma. The younger zircons yield ages of VIRGINIA used to demonstrate that active rifting influenced 690 ± 10 Ma (Odom and Fullagar, 1984). A deposition of alluvial sediments, and that tidal- SHADY second cluster of radiometric ages for Crossnore and wave-produced deposits resulted from the DOLOMITE plutons is centered near 640 Ma (Mose and initial transgression associated with high thermal Nagel, 1984; Mose and Kline, 1986). Glacial CL subsidence and probable mid-ocean ridge devel- ERWIN deposits are present in the upper sedimentary o FORMATION opment. Few detailed stratigraphie analyses Q: member of the Mount Rogers Formation (Ran- O kin, 1967; Blondeau and Lowe, 1972; Schwab, have been undertaken across this critical transi- LU HAMPTON FORMATION 1976, 1981; Miller, 1986) and in the Rockfish tion (compare Miller, 1987); this study thus o& provides insights into how a rift-to-incipient Conglomerate of the Lynchburg Group (Wehr, 1986). In many parts of the world, two late passive-margin transition will be manifested in X UNICOI u other exhumed orogenic belts. FORMATION Proterozoic glaciogenic episodes are recognized (Crittenden and