Sedimentology of the Unicoi Formation in Southern and Central Virginia: Evidence for Late Proterozoic to Early Cambrian Rift-To-Passive Margin Transition

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.

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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 location 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 demonstrated 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 others, 1983; Harland, 1983) and are considered to be 650 and 600 m.y. old, AGE CONSTRAINTS MOUNT ROGERS respectively (Harland, 1983). It is, however, not FORMATION, possible to assign the glacial deposits in the cen- In southern Virginia, the Unicoi Formation CATOCTIN FORMATION OR tral Appalachians to either of these events. either nonconformably overlies Grenviiie base- GRENVILLE Badger and Sinha (1988) have established the ment or overlies Mount Rogers Formation (Fig. BASEMENT age of the Catoctin volcanics as 565 + 36 by 2). The transition from the upper sedimentary Rb-Sr whole-rock analysis. The Catoctin For- member of the Mount Rogers Formation into mation provides a maximum age for the Unicoi the overlying Unicoi Formation has been inter- Figure 2. Correlation chart for the Chilho- Formation in central Virginia. Evidence will be preted as conformable (Rankin, 1967) and non- wee Group.

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presented later, however, for the Catoctin For- BALCONY FALLS VIRGINIA CREEPER TRAIL SECTION SECTION mation being correlative with volcanics within the middle Unicoi Formation in southern Virgin- ia, implying that Unicoi sedimentation in this area commenced in the late Proterozoic. HAMPTON FORMATION F Biostratigraphic control within the Chilhowee r, D Group is limited. Early Cambrian fossils are UNICOI known from the Erwin Formation and its FORMATION C correlatives (Walcott, 1891; Stose and Stose, B tmm: 1957; Laurence and Palmer, 1963; Wood and MfÎS; Clendening, 1982). Recently, a fragment of a hyolithid was recovered from the Hampton IV M LAPILLI TUFF Formation; Paleophycus and Rusophycus, a rest- FTvl BASALT IMäSi FACIES ASSOCIATION A ing trace of a trilobite, were discovered in marine FACIES ASSOCIATION B strata from the upper Unicoi Formation (Simp- feljl FACIES ASSOCIAT.ON C son and Sundberg, 1987; Fig. 3). The hyolithid Sgj FACIES ASSOCIATION D and Rusophycus fossils indicate a Tommotian Egg FACIES ASSOCIATION E or younger age, implying that the majority of the •• FACIES ASSOCIATION F I* »I DIAMICTITE Chilhowee Group is of Cambrian age (Simpson . . \ MOUNT ROGERS |!ii"l GRENVILLE BASEMENT x FORMATION and Sundberg, 1987). I DETAILED MEASURED SECTIONS * RUSOPHYCUS AND PALEOPHYCUS TRACE FOSSILS METHODS Figure 3. Generalized stratigraphy of the Unicoi and lower Hampton Formations for the three measured sections. See Figure 1 for locations of sections. In central and southern Virginia, the Unicoi and lower Hampton Formations are exposed on a number of thrust sheets within the Blue Ridge Province (Fig. 1). At three localities on different thrust sheets, complete stratigraphie sections position; quartz arenites form continuous spines, undulatory, and rarely channelized bases. Beds were measured on a 1-cm by 1-cm scale record- but feldspathic arenites are recessed. The out- are tabular and range in thickness from 0.4 to ing grain size, mineralogy, sedimentary struc- crop does not permit architectural element anal- 6.0 m. Beds are structureless but display distribu- tures, and bedding morphology; observations ysis necessary for detailed interpretation of tive normal grading which is best observed in from rock slabs and thin sections supplemented ancient fluvial systems (Miall, 1985). Vertical the coarser fraction, as well as inverse-to-normal field data. Photo mosaics were constructed of changes in facies and facies associations in the grading and nongraded base overlain by nor- critical exposures. At the facies level (see Table Unicoi and lower Hampton Formations, how- mally graded top (Fig. 4). Crude horizontal 1), depositional processes were reconstructed. ever, permit generalized depositional environ- stratification may be present within beds and is Facies could be grouped into facies associations ments to be reconstructed, from which pertinent manifest as weak normal grading. The massive at which level depositional environments were information can be gleaned concerning the tec- conglomerate beds resemble the Gms facies of interpreted. tonic history of the early Iapetus margin. Miall (1977, 1981) but differ in that the matrix Sections are -90% to 95% complete with lat- Six facies associations make up the Unicoi is predominantly sand sized rather than mud eral extent varying according to the rock com- and lower Hampton Formations in the meas- sized, and clasts are smaller. ured sections. The stratigraphic relationships of Massive conglomerate beds are capped by 5- the facies associations are illustrated in Figure 3. to 50-cm-thick sandstone intervals (Fig. 4)

TABLE 1. FACIES ABBREVIATIONS which are massive (Sms), horizontally stratified FACIES ASSOCIATION A: (Sh), or trough cross-bedded (St). The propor- Abbreviation Facies DISTAL ALLUVIAL FAN tion of stratified and massive sandstone increases upward in the facies association coupled with an Gms Massive conglomerate Description increase in the Sh and St facies (Fig. 4). Massive Sms Massive sandstone sandstone intervals typically display normal Sh Horizontally stratified sandstone Facies association A is confined to the Elk grading from very coarse to fine sand. Horizon- St Trough cross-bedded sandstone Creek and Virginia Creeper sections (Fig. 3). tal stratification consists of normally graded Gm Massive, clast-supported, pebble conglomerate Four facies are recognized in this association: (1) coarse to fine sand. Trough cross-bedded sand- Sp Tabular-planar, cross-bedded sandstone massive conglomerate (Gms); (2) massive sandstones are coarse to fine grained; cross beds are Sr Ripple cross-laminated sandstone stone (Sms); (3) horizontally stratified sandstone medium scale. Fl Graded, finely laminated silstone and mudstone (Sh); and trough cross-bedded sandstone (St). Fm Mudstone Massive conglomerates (Gms) are poorly sort- Interpretation Smc Channelized, massive sandstone ed and matrix supported. Clast size ranges from Stc Channelized, trough cross-bedded sandstone granule to pebble. Matrix ranges from very fine Beds present in this facies association resem- Smr Megarippled sandstone to very coarse sand; minor silt- and clay-sized ble hyperconcentrated-flow deposits described Swr Symmetrical-rippled sandstone detritus is also present. Massive conglomerates from Holocene and ancient alluvial sequences Sep Compound, cross-stratified sandstone are present in beds with sharp, flat to slightly (Hubert and Hyde, 1982; Nemec and Mu-

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szynski, 1982; Wells, 1984). Hyperconcentrated internal parallel stratification are considered to turbulent flow followed by tractional processes flows belong to a continuum based on concen- have developed when turbulent suspension proc- and minor pseudolaminar flows allow facies tration of sediment in the flow (Beverage and esses yielded to upper-flow regime bedload proc- in this association to be interpreted as sheet- Culbertson, 1964; Bull, 1972; Wasson, 1977). esses (Nemec and Muszynski, 1982) or by flood deposits (compare Nemec and Muszynski, Mudflows contain greater than 80% by weight variation in flow intensity in the turbulent sus- 1982). Sheet floods are low-viscosity, short- sediment whereas hyperconcentrated flows con- pension flow (compare Hampton, 1975). A non- duration flows that occur typically below the tain from 80% to -40% sediment. Low- graded, basal layer overlain by a graded, intersection point on the distal reaches of allu- concentration flows fall below the 40% range. unstratified top indicates at minimum that the vial fans (Bull, 1972; Wasson, 1977, 1979). In Rheology and flow mechanisms vary for differ- top was deposited from turbulent suspension contrast, proximal reaches of alluvial fans are ent water:sediment ratios, and hence distinctive and the nongraded, basal sequence was deposit- characterized by abundant mass-movement, bed types are produced that permit separation of ed by pseudolaminar flow (Nemec and Muszyn- debris-flow sedimentation with pseudolaminar flow mechanisms (Hampton, 1975; Nemec and ski, 1982). flow (Bull, 1972). Sheet floods are characterized Muszynski, 1982). Mudflows and hyperconcen- Beds which are normally graded with either a by thin, wide sheets of sediment-rich water that trated flows are pseudolaminar (Nemec and horizontally stratified or trough cross-bedded moves under upper-flow-regime conditions. Muszynski, 1982), whereas low-concentration cap indicate a transition within a single flow Sedimentation takes place in response to slope- flows are characterized by fractional processes from turbulent suspension to fractional processes angle changes, water infiltration into the basal that produce distinctive sedimentary structures. (Nemec and Muszynski, 1982). Expulsion of wa- surface, or fluid migration to the flow top (Bull, Turbulent suspension is the predominant process ter as sediment was being deposited may have 1972). within hyperconcentrated flows (Lowe, 1976; been responsible for the dilution of a hypercon- Before the Silurian Period, the absence of Lawson, 1982). centrated flow to a point where tractive flow land plants permitted greater amounts of sedi- Nonstratified, normally graded beds in this processes became dominant. Water expulsion ment flux, increased erosion rates and limited facies association are interpreted to have been during deposition has been observed within mod- climatic effects on the delivery of detritus to a deposited by turbulent suspension currents ern flows of coarse-grained sediment (Lowe, catchment area (Schumm, 1968). Hyperconcen- which permitted normal grading to develop 1976; Lawson, 1982). The traction-produced trated sediment flows may have been easier to (compare Middleton, 1970; Middleton and structures on top of the massive beds indicate generate under these conditions, and debris may Hampton, 1976; Cas, 1979; Nemec and Mu- relatively high current velocities. have been subject to more extensive reworking, szynski, 1982). Normally graded beds with rare The predominance of structures generated by thus destroying more proximal debris-flow deposits (Heward, 1978).

Figure 4. Measured section through facies association A at Elk Creek. Insets illustrate bed types present in this association. See Figure 3 for location of section and Table 1 for facies abbreviations. M = NORMALLY GRADED TOP mudstone, S = sandstone, C = conglomerate. (Gms)

NORMALLY GRADED BASE WITH HORIZONTALLY STRATIFIED TOP (Gms ^Sh)

NORMALLY GRADED TOP

(Gms)

NORMALLY GRADED BASE WITH TROUGH CROSS-BEDDED TOP (Gms •St)

NON-GRADED BASE Gms NORMALLY GRADED BED MASSIVE SANDSTONE TOP Gms »•Sms

INVERSELY GRADED BASE TO NORMALLY GRADED TOP (Gms)

NON-GRADED BASE TO NORMALLY

m GRADED TOP (Gms) c —

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FACIES ASSOCIATION B: cross-bedded facies (Sp) is composed of sets of 9.5 to 50.0 m. Euhedral, relict plagioclase micro- PROXIMAL TO MEDIAL planar stratification which are tabular to slightly lites displaying hyalopilitic texture are present BRAID PLAIN wedge shaped. Set thickness varies from 10 to throughout the flows. Amygdules are common 45 cm. A maximum of five sets are superim- near flow tops. Tops of flows are either corru- Description posed to form cosets. Basal surfaces of beds rest- gated or defined by overhangs filled with sedi- ing on mudstone (Fm) or siltstone (Fl) contain ment. Corrugated tops of flows may be ropy; This facies association is developed in each of flute casts. Trough cross-bedded sandstone (St) these were accentuated possibly by cleavage de- the measured sections (Fig. 3). Seven facies are consists of medium-scale troughs which are velopment. The uppermost flows are separated recognized: (1) massive, clast-supported, pebble present in cosets up to 65 cm thick. Ripple cross- by thin beds of St facies. Immature soils are conglomerate (Gm); (2) horizontally stratified laminated sandstones (Sr) range in grain size developed on some flow tops (Simpson, 1987). sandstone (Sh); (3) tabular-planar, cross-bedded from very fine to medium. Ripples typically are The facies in this association occur in four sandstone (Sp); (4) trough cross-bedded sand- preserved as isolated form sets. repetitive groupings: (1) Gm, Sh, Sp; (2) Gm, St; stone (St); (5) ripple cross-laminated sandstone Graded, finely laminated siltstone and mud- (3) Sh, Sp; and (4) subordinate Sr, Fl, Fm (Fig. (Sr); (6) graded, finely laminated siltstone and stone facies (Fl) consist of a basal layer of 5). Paleocurrent data from facies association B mudstone (Fl); and (7) mudstone (Fm). In addi- siltstone that grades upward into mudstone. in the Virginia Creeper Trail section below the tion, lapilli tuffs and basalt flows are intercalated Mudstone layers may be laminated or massive. basalts show a general southwesterly trend; within sediments of facies association B (Fig. 3). Couplet thicknesses range from 1 to 15 cm. above the basalt, paleocurrents are to the south Pebble conglomerates (Gm) possess erosional Mudstone facies (Fm) is laminated. Laminations (Figs. 6A and 6B). bases with a maximum relief of 30 cm. Pebbles consist of mm-scale alternations of siltstone and are clast supported with a coarse-sand matrix mudstone. Interpretation and are present either in tabular beds up to 2 m Lapilli tuffs are exposed poorly and possess a thick or as 5- to 15-cm-thick lags. Apart from a mixed population of clast types. The dominant The facies in this association were deposited crude clast imbrication, beds are internally clasts are felsic in composition and range up to 2 from low-density, mostly tractive flows. Groups massive. cm in diameter. of facies are comparable to those documented The horizontally stratified facies (Sh) is com- Basalt flows are present in the Elk Creek and from Holocene proximal to medial braid plains posed of normally graded, medium- to fine- Virginia Creeper Trail sections but not in the (compare Bradley and others, 1972; Boothroyd grained sandstone. The stratification may be Balcony Falls section (Fig. 3). Rankin (1976) and Ashley, 1975; Desloges and Church, defined by granules or pebbles. Bed thickness reported the Unicoi basalts to be tholeiitic in 1987) and reflect both channel and overbank varies from 5 to 40 cm. The tabular-planar, composition. Thicknesses of flows range from sedimentation.

MASSIVE CONGLOMERATE OVERLAIN BY TROUGH CROSS-BEDOED. PEBBLY SANDSTONE 30 cm (Gm •St) jg&msk V V V V V y JVVVVV V V V V V V V V V V V

V V V -.'N.-V- V V V'V V V V V V V V V 30 cm HORIZONTALLY STRATIFIED AND V V V V V V - 60 TABULAR-PLANAR CROSS-BEDDED SANDSTONE V V V V V V V V V V V I (Sh *-Sp)

>><, » ¿., «« >-., ' ... - »fc. • • ». • > • .»v.• 'iv. V' vv -v. «¿i. »«

40 MASSIVE CONGLOMERATE OVERLAIN BY HORIZONTALLY STRATIFIED SANDSTONE FOLLOWED BY TABULAR - PLANAR CROSS - BEDDED SANDSTONE (Gm •Sh *-Sp)

INTERBEDDED RIPPLED SANDSTONE, GRADED AND LAMINATED SILTSTONE AND MUDSTONE AND MASSIVE MUDSTONE

25 cm (Sr, Fl, Fm)

Figure 5. Measured section through facies association B at Elk Creek. Insets show the most common groupings of facies. See Figure 3 for location of section and Table 1 for facies abbreviations. M = mudstone, S = sandstone, C = conglomerate.

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tory. The ripple cross-laminated sandstone facies FACIES ASSOCIATION B FACIES ASSOCIATION D FACIES ASSOCIATION E (Sr) ranges in thickness from 1 to 15 cm and occurs as isolated beds or as caps on tabular- FORESETS MEGARIPPLES WAVE RIPPLES planar cross-bed sets. Graded, finely laminated BELOW BASALT HORIZON NW Mode N = 17 siltstone and mudstone (Fl) contains 0.1- to 2.0- N Vm = 128.7 N = 18 N = 14 cm-thick laminations. Laminations consist of a Vm = 222 Vm = 324 Sd = 14.8 Sd = 70.6 Sd = 9.8 poorly developed basal siltstone layer overlain by mudstone. Mudstone facies (Fm) are massive SE mode with rare siltstone to fine-grained sandstone N = 9 laminations. Vm = 165 Sd = 8.2 V The facies in this association occur in three repetitive groupings: (1) Sh; (2) Sh, St; and (3) FORESETS ATROUGH AXES FRO M St PLANAR FORESETS N Sp, Sr, Fl, Fm (Fig. 7). Each of these groupings *ABOVE THE BASALTS N = 28 N = 62 of facies is present in the Balcony Falls section; N N Vm = 136 N = 25 N Vm = 232 Sh predominates over St, whereas the Sp, Sr, Fl, Sd = 34.6 Sd = 30.1 Vm = 190 Fm grouping is subordinate. In the Elk Creek Sd = 60.4 section, Sp, Sr, Fl, Fm are absent, and Sh, St > predominate. Interpretation B D F< The predominance of horizontal stratification Figure 6. Paleocurrent rose diagram for fades association B, D, and E. N = number of indicates that sheet flooding was the major depo- observations, Vm = vector mean, Sd = standard deviation. sitional process responsible for facies association C. Sheet floods are characteristic of ephemeral- river systems and represent unconfined flows out of channels. At Bijou Creek, Colorado, a lenticu- Gm, Sh and Sp represent the most proximal Interlayered Sr, SI, and Fm facies represent lar sandstone body up to 4 m thick and consist- environment of deposition and reflect growth of overbank deposits. Cross-laminated sandstone ing mostly of horizontal stratification was longitudinal bars. Massive gravels (Gm) form lenses (Sr) resulted from migration of isolated deposited during a single flood (McKee and oth- the cores of bars which grow from a diffuse ripple trains. Graded siltstone and mudstone (Fl) ers, 1967); a similar mode of deposition is in- gravel sheet as the river loses competence and mudstone (Fm) laminations (Fig. 5) reflect ferred for the Sh facies. Associated trough cross- (Smith, 1973; Boothroyd and Ashley, 1975; separate overbank floods. bedded sandstones (St) probably developed at Hein and Walker, 1977). As flow velocities de- lower-flow stages in channels incised into the cline horizontally, stratified sandstones (Sh) ac- FACIES ASSOCIATION C: sheet-flood deposits; stream-channel deposits in cumulated on bar tops, whereas tabular-planar, DISTAL BRAID PLAIN ephemeral-river systems are typically trough cross-bedded sandstones (Sp) probably reflect cross-bedded (Williams, 1971; Sneh, 1983). The migration of megaripples at low stage in chan- Description Sp, Sr, Fl, Fm group of facies is considered to nels adjacent to bars (compare Boothroyd and reflect deposition on distal margins of sheet Ashley, 1975). This facies association is confined to the Elk floods. Tabular-planar, cross-bedded sandstones (Sp) probably were produced during peak flood Massive gravels (Gm) and trough cross- Creek and Balcony Falls sections (Fig. 3). Six stage at the same time that horizontal stratifica- bedded sandstones (St) occur in stacked 0.6- facies are developed: (1) horizontally stratified tion developed more proximal to the channel. to 3.0-m-thick, upward-fining intervals (Fig. 5). sandstone (Sh); (2) trough cross-bedded sand- The Sr, Fl, and Fm facies reflect progressively By analogy with Holocene rivers such as stone (St); (3) tabular-planar, cross-bedded lower flow velocities. the Donjek, South Saskatchewan, and Brahma- sandstone (Sp); (4) ripple cross-laminated sand- putra (Coleman, 1969; Williams and Rust, stone (Sr); (5) graded, finely laminated siltstone 1969; Hein and Walker, 1977), the gravels are and mudstone (Fl); and (6) mudstone (Fm). FACIES ASSOCIATION D: interpreted as channel lag and the cross-bedded Horizontally stratified sandstone (Sh) is com- UPPER SHOREFACE sandstones as mid-channel bar deposits. Each posed of 1- to 3-mm-thick laminations which upward-fining sequence reflects a single flood make up beds ranging in thickness from 3.0 cm Description cycle. to 3.2 m.-Successive beds are separated by ero- The coarse grain size of the horizontally strati- sional surfaces. Trough cross-bedded sandstones This facies association is confined to the fied sandstones in the Sh-Sp grouping suggests (St) consist of medium-scale troughs. Bedding- Balcony Falls section (Figs. 3 and 8). Four that it developed in the lower flow regime. This plane exposures are rare, and, as a result, pa- facies are developed: (1) channelized, massive grouping of facies reflects deposition on trans- leocurrent measurements are sparse. Cosets of sandstone (Smc); (2) channelized, trough cross- verse or linguoid bars. During flood stage in trough cross beds are erosional into horizontally bedded sandstone (Stc); (3) megarippled sand- Holocene rivers, bar growth is initiated by depo- stratified sandstone intervals. Tabular-planar stone (Smr); and (4) feldspathic, trough sition of Sh, whereas Sp reflects bar migration cross-bedded sandstones (Sp) contain 10- to 25- cross-bedded sandstone (St). The latter facies during the waning stage (Hein and Walker, cm-thick sets that typically occur as a solitary demarcates the transition from the Unicoi For- 1977). set. Bases of beds are sharp to slightly undula- mation into the Hampton Formation.

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HORIZONTALLY STRATIFIED Figure 7. Measured section through SANDSTONE facies association C at Balcony Falls. (Sh) Insets show the most common groupings of facies. See Figure 3 for location 50 cm of section and Table 1 for facies abbreviations. M = mudstone, S = sandstone.

INTERBEDDED TABULAR-PLANAR AND RIPPLE CROSS-LAMINATED SANDSTONE, GRADED SILTSTONE AND MUDSTONE, AND MASSIVE MUDSTONE (Sp, Sr, Fl, Fm) 15 cm

HORIZONTALLY STRATIFIED SANDSTONE OVERLAIN BY TROUGH CROSS-STRATIFIED SANDSTONE 50 cm (Sh »St)

Channelized, massive sandstone facies (Smc) the top. The channelized, trough cross-bedded axes are oriented in general to the southeast. is composed of medium- to coarse-grained sand. sandstone facies (Stc) is dominated by fine- to Megaripples (Smr) are developed in medium- The base of the facies is channelized and trun- coarse-grained sand. Trough cross beds are grained sandstone above the massive and trough cates underlying beds of facies association E medium scale. Grain size and trough cross-bed cross-bedded sandstone intervals (Fig. 8). The with a maximum relief of 0.80 m (Fig. 8; inset set thickness decrease vertically in 0.5- to megaripples capping the trough cross-bedded D). Grain size and trough cross-bed set thickness 3.8-m-thick cosets. Tabular-planar cross beds sandstones are exposed on bedding planes (Fig. decrease from the base to the top of the 0.5- to are interspersed with the troughs and display 9A). On a single bedding plane, bedforms con- 4.6-m-thick bed. Rare troughs are found toward reactivation surfaces (Fig. 8; inset C). Trough sist of both straight-crested megaripples with a northwest orientation and more sinuous-crested megaripples with a southeast orientation (Fig. 6C); megaripples decrease in amplitude and A wavelength to the southeast. Cross sections TROUGH CROSS-BEDDED SANDSTONE through straight-crested megaripples display WITH SKOUTHOS BURROWS tabular-planar cross beds with reactivation sur- I 5 cm (St) faces (Fig. 8; inset C). The Smc, Stc, and Smr B facies are stacked in a 13-m-thick vertical sequence (Fig. 8). TROUGH CROSS-BEDDED SANDSTONE CAPPED 30 cm BY SYMMETRICAL WAVE-RIPPLED SANDSTONE Feldspathic, trough cross-bedded sandstones (St »Swr) (St) contain concentrations of opaque heavy minerals. Grain size varies from granule to fine TROUGH CROSS-BEDDED SANDSTONES sand. Coarse sand and granules are restricted to OVERLAIN BY MEGARIPPLES WITH REACTIVATION SURFACES the lower portion of the facies which fines up- |1 m ward through 5.6- to 4.8-m intervals. Trough (Stc »Smr) cross-beds are mostly medium scale (Fig. 9B), D and both set and foreset thicknesses decrease TRUNCATION SURFACE OVERLAIN BY vertically. Skolithos burrows disrupt laminations MASSIVE SANDSTONE AND UNDERLAIN BY TROUGH CROSS-BEDDED SANDSTONE (Fig. 8; inset A). Trough axes give a southwester- |1 m (St »Smc) ly paleocurrent mode (Fig. 6D). E TABULAR-PLANAR OVERLAIN BY TROUGH Interpretation CROSS-BEDDED SANDSTONE 125 cm (Sp »St) The erosional base and the vertical sequence of structures exhibited by the channelized mas- Figure 8. Measured section through facies associations D and E at Balcony Falls. Insets sive to trough cross-bedded to megarippled illustrate details of facies. See Figure 3 for location of section and Table 1 for facies abbrevia- sandstone facies (Fig. 8) are comparable to that tions. M = mudstone, S = sandstone, C = conglomerate. documented from tidal inlets (compare Land,

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Figure 9. Photographs of fades assodation D. (A) Megaripple fades. Photograph shows train of straight-crested megaripples. Amplitude of megaripples decreases toward top of photograph. Note hard hat for scale. (B) Feldspathic, trough cross-bedded sandstone facies.

1972; Kumar and Sanders, 1974; Hayes, 1980). a belt landward of the breaker zone but lacking of medium- to coarse-grained sandstone. As inlets migrate, a basal scour surface results bars. Heavy minerals probably were concentrat- Medium-scale, trough cross-bedding is the pre- from erosion into underlying units. Truncation ed in a swash zone and incorporated into the dominant stratification type; a few isolated angles may be very slight over short distances zone of longshore currents. sets of tabular-planar cross beds are present. (Moslow, 1984) and would resemble that at the The recognition of tidal-inlet deposits and Beds are sharp based and tabular, and they base of facies association E (Fig. 8; inset A). deposits produced by longshore currents estab- vary from 0.75 to 2.2 m in thickness. Paleo- Holocene tidal-inlet deposits characteristically lishes that facies assocation D accumulated in an currents are highly variable. In the Virginia display an upward-fining trend commencing upper-shoreface setting influenced by tides and Creeper Trail section, Rusophycus and Paleophy- with a basal lag of coarse pebbly sand or shell longshore currents generated by waves. The cus are present on a basal bedding plane. Beds debris (Kumar and Sanders, 1974). The absence shoreface, including tidal-inlet sequences, typi- often are capped by symmetrical ripples (Swr) of a basal lag in this facies association is proba- cally is capped by swash deposits. Indirect evi- in medium-grained sandstone (Fig. 8; inset B bly due to the lack of coarse-grained material in dence for a swash zone is provided by the heavy and Fig. 10A). Ripples are straight crested and the system. Deeper parts of tidal inlets generally mineral concentrates, but no swash laminations form discordant to internal cross-stratification. are dominated by offshore flow, whereas shal- are present. Swash deposits have a low preserva- Crest orientations indicate southeast-northwest low realms frequently contain bidirectional cur- tion potential along transgressive coastlines oscillatory flow (Fig. 6E). Symmetrical ripples rents related to ebb and flood flow (van Beeck (Davis and Clifton, 1984), and evidence will be may be overlain by current-rippled sandstone and Koster, 1972). The relative position in verti- presented below for rapid drowning above facies and parallel-laminated mudstone. cal sequence, coupled with the paleocurrent association D. The tabular-planar cross-bedded facies (Sp) is data, is compatible with the trough and mega- composed of medium- to coarse-grained sand- rippled sandstone facies representing deep- and FACIES ASSOCIATION E: LOWER stone. Beds consist of solitary, 0.15- to 1.00-m- shallow-channel deposits, respectively. Asso- SHOREFACE TO INNER SHELF thick cross-bed sets which are tabular and may ciated reactivation surfaces reflect modification be traced laterally for more than 55 m at Bal- of bedforms by subordinate currents. Description cony Falls. Reactivation surfaces are present The depositional environment of the feldspath- within some sets (Fig. 10B). Paleocurrents are to ic, trough cross-bedded sandstone facies is prob- Facies association E is present in each of the the southeast (Fig. 6F). Tabular-planar cross- lematical. Trough cross-beds were produced by measured sections (Fig. 3). Four facies are rec- bed sets are often capped by small-scale, trough the migration of sinuous-crested megaripples ognized: (1) trough cross-bedded sandstone (St); cross beds oriented orthogonal to the underlying (Harms and others, 1982). Skolithos burrows (2) symmetrical-rippled sandstone (Swr); (3) set (Fig. 8; inset E). indicate a high-energy marine environment of tabular-planar, cross-bedded sandstone (Sp); The compound cross-stratified facies (Sep) deposition with mobile substrates (Seilacher, and (4) compound, cross-stratified sandstone consists of granule conglomerate to medium- 1969). Paleocurrent data from this facies (Fig. (Sep). The St, Sp, and Swr facies are inter- grained sandstone. Tabular-planar cross-bed sets 6D) indicate a shoreline-parallel transport direc- bedded in each of the measured sections; they are 2 to 35 cm thick and contain reactivation tion. The barred high-energy shoreline of the are erosionally overlain by facies association D surfaces. Cross-bed sets are stacked in 0.8- to Oregon coast is characterized by oblique and at Balcony Falls and reoccur higher in the sec- 1.5-m-thick cosets which are defined by bound- shore-parallel bars (Hunter and others, 1979). tion (Fig. 8). Each of the facies is present in the ing surfaces inclined at 3 to 4 degrees to bedding Longshore channels behind the bars contain Virginia Creeper Trail section where facies asso- (Fig. 11). Megaripples which produced the inter- megaripples migrating parallel to the shoreline. ciations B and E are interbedded in a transitional nal cross-stratification climbed up the bounding The feldspathic, trough cross-bedded facies prob- interval (Fig. 3). surfaces. Coset boundaries are demarcated by ably formed in similar longshore channels or in The trough cross-bedded facies (St) consists concentrations of coarse-grained sandstone or

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Figure 10. Photographs of fades association E. (A) Symmetrical ripples. (B) Tabular-planar cross-bed with reactivation surface.

granule-to-pebble conglomerate. Rarely asym- of sinuous-crested megaripples in a time-velocity sist of starved ripples in mudstones; ripples con- metrical ripples define contacts between cosets. symmetrical flow regime (Allen, 1980). Cessa- sist of fine-grained sandstone to siltstone. Cross beds within cosets yield southeasterly pa- tion of sediment supply was followed by defla- Normally graded, clast-supported conglomer- leocurrent directions (Fig. 6F). tion of the megaripple field to produce the ate beds possess erosional bases with a few cen- tabular coset geometries (compare Rubin and timeters relief. Bed thicknesses range from 1.5 to Interpretation Hunter, 1982; Harris, 1987). Symmetrical rip- 4.8 m and decrease upward. Groove casts are ples capping cosets were generated by fair- common on basal erosional surfaces. Con- Compound cross-stratified sandstones reflect weather or storm reworking. glomerates contain clasts of well-rounded vein migration of a complex hierarchy of bedforms. Sand-wave and sand-ridge complexes typi- quartz, medium-grained sandstone, siltstone, Tabular-planar cross-bed sets were generated by cally are developed on tidal shelves (Allen, and mudstone. Quartz clasts range in size from migration of straight-crested megaripples or 1980; Walker, 1984; Johnson and Baldwin, fine sand to pebbles with diameters 2 to 4 cm. sand waves up gently inclined surfaces of larger- 1986). Evidence of fair-weather- or storm-wave Long axes of mudstone clasts are from 1.0 cm to scale bedforms such as sand ridges (compare reworking places this facies association in a 2.1 m in length and display soft-sediment de- Houbolt, 1968; Belderson and others, 1982). lower shoreface to inner-shelf setting. Rusophy- formation. Beds may be capped by horizontally The coset boundaries are comparable to E2 sur- cus and Paleophycus ichnofossils on bedding laminated siltstone containing small-scale load faces of Allen (1980); these are considered to planes in the trough cross-bedded sandstones and dewatering structures. form by erosion in the front of advancing sand support this interpretation; both develop below Black pyritic mudstones without bioturbation ridges. Crests of sand ridges typically are aligned fair-weather but above storm-wave base (Seil- are present above the conglomeratic interval in oblique to the strongest tidal current (Kenyon acher, 1969; Frey and Seilacher, 1980). the Virginia Creeper Trail section and sharply and others, 1981), whereas superimposed sand overlie facies association E in the Elk Creek sec- waves migrate in the direction of ebb or flood FACIES ASSOCIATION F: tion. Green, bioturbated mudstones occur at the flow. Paleocurrent data for this facies indicate OUTER SHELF base of the Hampton Formation at Balcony offshore or ebb flow; associated reactivation sur- Falls. The proportion and thickness of Tabc, faces may have been generated by subordinate Description Tbc, and Tc beds increase upward in the Hamp- flood currents. ton Formation at each locality and are accom- The tabular-planar, cross-bedded facies result- This facies association makes up the base panied by an increase in bioturbation of ed from migration of straight-crested mega- of the Hampton Formation and is present in intercalated mudstone with siltstone laminations. ripples in an offshore direction. Reactivation each of the measured sections (Fig. 3). Facies surfaces are best related to modification of association F consists of mudstones which typi- Interpretation megaripple crests by subordinate currents, im- cally contain siltstone laminations, and thin beds plying a tidal environment with time-velocity containing Tabc, Tbc, and Tc Bouma sequences. The lack of wave-produced structures in asymmetric flow (Klein, 1971). Nested troughs, A 15-m-thick interval made up of normally facies association F indicates that sedimentation orthogonal to the planar foresets indicate re- graded conglomerate beds and intercalated took place exclusively below wave base. On the working by longshore currents possibly related black pyritic mudstones is present at the base of basis of an upward transition into wave- to shoaling waves. the Hampton Formation in the Virginia Creeper produced facies, Simpson and Eriksson (1986) The trough cross-bedded sandstone facies is Trail section. argue that facies association F accumulated in more difficult to reconcile with a tidal regime, Tabc beds are graded from medium- and an outer-shelf setting. Tabc, Tbc, and Tc beds but its close association with the other two facies rarely granule- to fine-grained sandstone or silt- are interpreted as turbidites, and the associated, suggests a similar depositional setting. Cosets of stone. Tbc beds are graded from medium- to predominant mudstone facies as suspension trough cross beds have been related to migration fine-grained sandstone or siltstone. Tc beds con- deposits. Intensity of bioturbation was a func-

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the Iapetus Ocean. Evidence that rifting influenced sedimentation into Unicoi time includes the presence of basalts, abundant lithic clasts, feldspathic alluvial sediments, and thick alluvial- Figure 11. Schematic il- fan deposits. Incipient passive-margin sedimen- lustration of compound tation is recorded in transgression of the braid cross-stratified sandstone plain by shallow-marine sediments and subse- facies in facies association quent drowning of the siliciclastic shelf. E from Virginia Creeper Tectonic subsidence curves from the Cambro- section. Ordovician carbonate succession in southwestern Virginia have been used by Bond and others (1984) to determine stretching factors (/3) of 2.0 to 4.0, and it is likely that stretching commenced much earlier. The initial stage of stretching and associated rifting that preceded the opening of the Iapetus Ocean is recorded in the Crossnore tion of oxygen content at the sediment-water tion D and E are quartz arenites, with the excep- Plutonic Suite (690-640 Ma; Odom and Ful- interface. tion of the feldspathic trough cross-bedded facies lagar, 1984; Mose and Kline, 1986) and in sedi- Normally graded, clast-suported conglomer- in facies association D. On the tectonic discrim- mentary and volcanic strata of the Swift Run ates in association with the other gravity-flow ination plot of Dickinson and others (1983), ar- and Mount Rogers Formations and Lynchburg deposits are interpreted as the product of high- enites within facies associations A through C are Group (Odom and Fullagar, 1971; Rankin, concentration turbidity currents (Walker, 1978). concentrated near the transition from basement 1975; Wehr, 1985, 1986; Wehr and Glover, Such currents may be generated by slope fail- uplift to transitional continental fields, indicating 1985; Miller, 1986). Attenuated continental ures, seismic shock, sediment creep, or rivers in an active tectonic environment, whereas ar- crust therefore must have existed at the time of flood (Stow, 1985). The turbidity currents had enites from facies association D and E fall deposition of the Unicoi Formation. Stratigraph- sufficient velocity and strength to erode underly- within the craton interior field, indicating a sta- ic thicknesses of the Unicoi and lower Hampton ing sedimentary layers and derive mudstone and ble tectonic setting (Fig. 12). Formations in conjunction with the distribution sandstone clasts. Vein-quartz clasts are exotic to of underlying stratigraphic sequences (Fig. 13) the stratigraphic section and were introduced DEPOSITIONAL AND suggest that the Virginia Creeper Trail section from outside the depositional environment. The TECTONIC MODEL developed on more attenuated continental crust stratigraphically closest quartz-pebble conglom- than the Elk Creek section. The Balcony Falls erates are in the Unicoi Formation. The Unicoi Formation and the basal portion sequence probably accumulated on the least at- of the Hampton Formation are considered to tenuated crust. Differences in degree of attenua- tion may have controlled the distribution of SEDIMENT COMPOSITIONS record the late stages of rifting and the inception sedimentary environments within the various of an east-facing passive margin that bordered stratigraphic sections. The conglomerates in facies association A contain clasts of granite, potassium feldspar, and The contact between the proximal to medial subordinate volcanics. Vein quartz is the most braid-plain deposits of the Unicoi Formation common pebble type in conglomerates of facies and the glaciogenic sediments within the upper association B, comprising 50% to 95% of the clast population. Other clasts, in decreasing abundance, are granite, basalt, rhyolite, quartzite, porphyritic rhyolite, and gneiss. Polymictic • FACIES ASSOCIATION A, B AND C Craton Interior conglomerates are most common in proximity * FACIES ASSOCIATION D AND E to basalt flows. Sandstones in the Unicoi Forma- tion display an upward change in composition Transitional Continental from feldspathic to quartz arenites. Feldspathic Figure 12. Ternary tectonic arenites are dominant in facies associations A discrimination plot of Q (quartz), and B; these arenites contain abundant potas- F (feldspar), and L (lithics) sium feldspars and a minor lithic component (after Dickinson and others, composed of granite and low-grade metamor- 1983). Shaded area represents Basement Uplift phic fragments. Minor feldspathic wackes are continental block provenance. present. Sandstones in the basal Unicoi Forma- tion in the Balcony Falls section contain up to 10% volcanic grains. Siltstones in facies association B contain quartz, potassium feldspar, and up to 15% biotite. Sandstones in facies association C are dominated by feldspathic arenites with subordinate quartz arenite and feldspathic and quartz wackes. Sandstone in facies associa-

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NORTH SOUTH WEST EAST BALCONY ELK CREEK VIRGINIA CREEPER Figure 13. Diagrammatic recon- FALLS TRAIL struction of the late Precambrian- Cambrian rift to passive-margin transition in central and southern Virginia; time lines T3 and T4 are interpretive. Contacts between the Grenville basement and Unicoi For- mation are demonstrably autoch- thonous. Palinspastic restoration of the three sections, based on Boyer and Elliott (1982), gives an approx- imate North-South transect of the continental margin. The three sections are related in this figure in an interpretive cross section orthogonal to the inferred strike of the continental margin.

Lynchburg Group and Swift Run Formation Mount Rogers Formation in the Virginia grain size in facies associations A and B records (Wehr and Glover, 1985). The Catoctin Forma- Creeper Trail section may be unconformable long-term retreat and denudation of the source tion is considered to be the product of an exten- (Fig. 13, Tl), but the time represented by, or the area (compare Heward, 1978). Progressive infill- sional setting (Gathright, 1976; Rankin, 1976; areal extent of, this surface is unknown. The ing of the basin and resultant decrease in slope Conley, 1978; Wehr and Glover, 1985; Badger unconformity may not be a break-up unconfor- could account for the upward increase in frac- and Sinha, 1988). Catoctin volcanics and inter- mity as proposed by Wehr and Glover (1985) tional structures in facies association A (Fig. 4). calated alluvial sediments are overlain by a thin but could be a product of localized isostatic re- Widely dispersed paleocurrents in facies associa- equivalent of the Unicoi Formation comparable bound following the Mount Rogers glacial tion B (Fig. 6A) are a reflection of the tectonical- to that in the Balcony Falls section (Fig. 13). event. Detailed mapping of a similar type of ly active nature of the basin during accumulation Volcanic rocks are not present in the Balcony unconformable contact between the Proterozoic of facies associations A and B prior to T2 Falls section, but the volcanic debris in the basal Wildrose diamictite of the Kingston Peak For- (Fig. 13). Unicoi Formation is evidence for their previous mation and the passive-margin Noonday Do- Continued rifting is reflected in basaltic vol- existence at, or in proximity to, that locality. The lomite in the Panamount Range of California canic rocks and associated polymictic conglom- occurrence of granite, rhyolite, porphyritic rhyo- has demonstrated the local extent of the uncon- erates in the middle Unicoi Formation (Fig. 13, lite, gneiss, and quartzite clasts in proximity to formity previously thought to be a break-up un- T2); paleontological and geochronological data the basalts indicates uplift coeval with volcanism conformity and implying no regional uplift at suggest that rifting continued into Early Cam- and erosion of, at minimum, the upper Mount the transition (Miller, 1987). brian time. Basaltic volcanics are common in Rogers Formation. These lithologies occur as Rifting continued to influence sedimentation many rift settings (Crossley, 1979; Burke and clasts in the upper sedimentary member of the into Unicoi time and extended beyond the limits Kidd, 1980). Fossils recovered from the alluvial Mount Rogers Formation and also compose of the basin in which the Mount Rogers Group to marine transition in the Virginia Creeper Trail older stratigraphic units. accumulated (Fig. 13). Evidence for this faulting section (facies association B to E; Fig. 3) suggest Proximal to medial braided-alluvial sedimen- is reflected in (1) thick distal alluvial-fan sedi- that basalts closely associated with the proximal tation continued following basalt extrusion. Fa- ments of facies association A in the Elk Creek medial braid-plain deposits were extruded at or cies associations D and E accumulated during and Virginia Creeper Trail sections (Fig. 13) and near the Precambrian/Cambrian boundary. Ba- east-to-west transgression of the braid plain (Fig. (2) the immaturity of the sediments. Alluvial salts and associated braid-plain sediments in the 13, T3-T4). In the Virginia Creeper Trail sec- fans are most common adjacent to fault scarps Elk Creek and Virginia Creeper Trail sections tion (Fig. 3), proximal to medial braided alluvial and coarse-grained, sheet-flood deposits thus are represent a crude time line (Fig. 13, T2), and are sediments of facies association B and tidal sand- restricted to tectonically active areas (Hubert probably coeval with the Catoctin Formation to wave and sand-ridge deposits of facies associa- and Hyde, 1982; Nemec and Muszynski, 1982; the north of the study area (Fig. 1). Catoctin tion E developed on most attenuated crust and Wells, 1984). The over-all upward decrease in basalts were extruded at ca. 570 Ma onto the represent an environment analogous to a braid

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continent between 625 Ma and 555 Ma: New evidence and implica- delta (McGowen, 1970; McPherson and others, CONCLUSIONS tions for continental histories: Earth and Planetary Science Letters, 1987). Intercalation of sediments from the two v. 70, p. 325-345. Boothroyd, J. C., and Ashley, G. M., 1975, Processes, bar morphology and environments may be a result of small-scale sea- The Unicoi and lower Hampton Formations sedimentary structures on braided outwash fans, north-eastern Gulf of Alaska, in Jopling, A. V., and McDonald, B. C., eds., Glaciofluvial and level fluctuations or channel avulsion followed in the central Appalachians record the transition glaciolacustrine sedimentation: Society of Economic Paleontologists by subsidence. No such intercalation of alluvial from rift-to-passive margin sedimentation asso- and Mineralogists Special Publication No. 23, p. 193-222. Bott, M.H.P., 1979, Subsidence mechanisms at passive continental margins, in and marine sediments is developed in the Elk ciated with opening of the Iapetus Ocean. Thick Watkins, J. S., Montadert, L., and Dickerson, P. G-, eds.. Geological and geophysical investigations of continental margins: American Asso- Creek and Balcony Falls sections where distal alluvial-fan deposits, basalts, abundant lithic ciation of Petroleum Geologists Memoir 7, p. 3-9. braid-plain sediments of facies association C are clasts, and feldspathic sandstones in the lower Boyer, S. E., and Elliott, D., 1982, Thrust systems: American Association of Petroleum Geologists Bulletin, v. 66, p. 1196-1230. abruptly overlain by inner-shelf and lower and middle Unicoi Formation reflect rifting of Bradley, W. C., Fahnestock, R. K„ and Rowekamp, E. T., 1972, Coarse the late Proterozoic supercontinent. Geochrono- sediment transport by flood flows in Knik River, Alaska: Geological shoreface sand-wave complexes of facies associ- Society of America Bulletin, v. 83, p. 1261-1284. ation E, as well as tidal inlet deposits of facies logical and paleontological data indicate that Bull, W. B., 1972, Recognition of alluvial-fan deposits in the stratigraphie record, in Hamblin, W. K., and Rigby, J. K., eds., Recognition of association D at Balcony Falls. The feldspathic rifting continued into early Cambrian time. The ancient sedimentary environments: Society of Economic Paleontologists shoreface arenites capping the Unicoi Formation incipient phase of passive-margin sedimentation and Mineralogists Special Publication 16, p. 63-83. Burke, K„ and Kidd, W.S.F., 1980, Volcanism on Earth through time, in at Balcony Falls may reflect renewed faulting or is represented by transgressive deposits at the top Strangeway, D. W., ed., The continental crust and its mineral deposits: Geological Association of Canada Special Paper 20, p. 503-522. proximity to a fluvial source to the west. of the Unicoi Formation and is related to a Cas, R., 1979, Mass-flow arenites from Paleozoic interarc basin. New South second-order sea-level rise. On most attenuated Wales, Australia: Mode and environment of emplacement: Journal of As transgression continued, faults may have Sedimentary Petrology, v. 49, p. 29-44. crust to the east, initial transgressive facies con- Coleman, J. M., 1969, Brahmaputra River, channel processes and sedimenta- been reactivated to produce the conglomeratic, tion: Sedimentary Geology, v. 3, p. 129-239. gravity-flow deposits of facies association F at sisted of braid-delta and tidal sand-wave and Conley, J. F., 1978, Geology of the Piedmont of Virginia—Interpretations and sand-ridge deposits. As transgression progressed problems, in Contributions to Virginia geology—III: Virginia Division the base of the Hampton Formation in the Elk of Mineral Resources Publication 7, p. 115-149. cratonward, tidal sedimentation was supplanted Crittenden, M. D., Jr., Christie-Blick, N„ and Link, P. K„ 1983, Evidence for Creek section (Fig. 13). The thinning-upward two pulses of glaciation during the late Proterozoic in northern Utah sequence of beds in the conglomeratic interval by tide- and wave-influenced sedimentation. and southeastern Idaho: Geological Society of America Bulletin, v. 94, Drowning at the top of the Unicoi Formation is p. 437-450. may be related to fault-scarp retreat or reduction Crossley, R., 1979, Structure and volcanism in the South Kenya rift system: in fault-scarp relief. Overlying outer-shelf black indicated by outer-shelf black mudstones con- Rome, Italy, Academia Nazionale die Lincei, p. 89-98. taining Tabc, Tbc, and Tc beds and gravity-flow Davis. R. A., Jr., and Clifton, H. E„ 1984, Sea-level changes and preservation mudstones at this locality as well as those which potential of wave-dominated and tide-dominated coastal sequences: So- deposits at the base of the Hampton Formation. ciety of Economic Paleontologists and Mineralogists Annual Midyear abruptly overlie inner-shelf arenites in the Virgin- Meeting Abstracts, v. 1, p. 23. ia Creeper Trail section indicate anoxic condi- Deepening may have been enhanced by contin- Desloges, T. A., and Church, M., 1987, Channel and floodptain facies in a ued movement along listric faults throughout the wandering gravel bed river, in Ethridge, F. G., Flores, R. M., and tions related to rapid drowning, which may have Harvey, M. D., eds., Recent developments in fluvial sedimentology: incipient phase of passive-margin development. Contributions from the Third International Fluvial Sedimentology Con- been enhanced by continued movement on lis- ference: Society of Economic Paleontologists and Mineralogists Special tric faults and high thermal-subsidence rates; Most of the Hampton Formation and the overly- Publication No. 39, p. 99-109. ing Erwin Formation and Shady Dolomite re- Dickinson, W. R., Beard, L. S., Brakenridge, G. R., Eijavec, J. L., Ferguson, high initial thermal-subsidence rates occur im- R. C., Inman, K. F., Knepp, R. A., Lindberg, F. A., and Ryberg, P. T., cord progradation of the passive margin. 1983, Provenance of North American Phanerozoic sandstones in rela- mediately after cessation of rifting (Bond and tion to tectonic setting: Geological Society of America Bulletin, v. 94, others, 1984). In the Balcony Falls section, bio- p. 222-235. Donovan, D. T., and Jones, E.J.W., 1979, Causes of world-wide changes in sea turbated, outer-shelf facies resting on shoreface level: Geological Society of London Journal, v. 136, p. 187-192. ACKNOWLEDGMENTS Fichter, L. S., and Diecchio, R. J., 1986, Stratigraphie model for timing the deposits may reflect higher oxygen contents re- opening of the Proto-Atlanttc Ocean in northern Virginia: Geology, lated to shallower water depths than to the east. v. 14, p. 307-309. This paper constitutes part of the Ph.D. disser- Frey, R. W., and Seilacher, A., 1980, Uniformity in marine invertebrate ich- nology: Lethaia, v. 13, p. 183-207. The transgression recorded in the upper tation of the senior author. The research was Gathright, T. M., II, 1976, Geology of Shenandoah National Park, Virginia: Unicoi and Hampton Formations is considered funded by grants from Appalachian Basin Indus- Virginia Division of Mineral Resources Bulletin 86,93 p. Hampton, M. A., 1975, Competence of fine-grained debris flows: Journal of to represent the initial stage of the late Precam- trial Associates, The Geological Society of Sedimentary Petrology, v. 45, p. 834-844. Harland, W. B., 1983, The Proterozoic glacial record, in Medaris, L. G., Jr., brian to Late Cambrian, second-order sea-level America, and Sigma Xi. Permission was granted Byers, C. W., Michelson, D. M., and Shanks, W. C-, eds., Proterozoic rise of Vail and others (1977). The most reason- by the U.S. Department of Agriculture to under- geology: Geological Society of America Memoir 161, p. 279-288. Harms, J. C., Southard, J. B., and Walker, R. G., 1982, Structures and se- able cause of the transgression at the top of the take field work in the Jefferson National Forest. quences in clastic rocks: Society of Economic Paleontologists and Min- eralogists Short Couree 9, Calgary, 251 p. Unicoi Formation is mid-ocean-ridge inflation We benefitted from discussion with R. K. Harris, C. W., 1987, A sedimentologic and structural analysis of the Proterozoic (Pittman, 1978) following the initial break-up of Bambach, S. G. Driese, L. Glover III, C. W. Uncompahgre Group, Needle Mountains, Colorado [Ph.D. dissert.]: Blacksburg, Virginia, Virginia Polytechnic Institute and State Univer- the Eocambrian supercontinent (Donovan and Harris, J. G. Patterson, J. F. Read, and from the sity, 231 p. Hayes, M. 0- 1980, General morphology and sediment patterns in tidal inlets: Jones, 1979; Matthews and Cowie, 1979; Bond critical reviews of F. Schwab and J. P. Smoot. Sedimentary Geology, v. 26, p. 139-156. and others, 1984) coupled with high rates of Hein, F. J„ and Walker, R. G., 1977, Bar evolution and development of stratification in the gravelly, braided Kicking Horse River, B.C.: Cana- thermal subsidence. The initial transgression dian Journal of Earth Sciences, v. 14, p. 562-570. Heward, A. 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