Tomstown Dolomite (Lower Cambrian), Central Appalachian Mountains, and the Habitat of Salterella Conulata

Tomstown Dolomite (Lower Cambrian), central Appalachian Mountains, and the habitat of Salterella conulata

JUERGEN REINHARDT* Maryland Geological Survey, Baltimore, Maryland 21218

EDWARD WALL* Department of Earth and Planetary Sciences, The Johns Hopkins University, Baltimore, Maryland 21218

ABSTRACT o ro o Measured sections in the Tomstown Dolomite, Washington f- County, Maryland, indicate a considerably thinner section of car- c- bonate rocks than in Virginia. Salterella conulata Clark, found at PENNSYLVANI A three previously undescribed localities, is confined to a narrow MARYLAND stratigraphic interval and may have biostratigraphic value. S. conulata is a faunal component in the lagoon-bay portion of an Early Cambrian tidal flat complex. Key words: Lower Cambrian stratigraphy, carbonate sedimentology, biostratigraphy, paleoecology. INTRODUCTION

The stratigraphy and sedimentology of the Great Valley section in the central Appalachian Mountains has been summarized by Colton (1970), and more specifically for the Cambrian rocks by Palmer (1971). Byrd (1973), Byrd and others (1973), and Root (1968) have expanded our knowledge of the Tomstown (Shady) Dolomite beyond the original work of Stose (1909) and Butts (1940). The Tomstown and the overlying mixed carbonate and clastic rocks of the Waynesboro Formation mark the transition from upper Precambrian—Lower Cambrian clastic alluvial and marine sediments of the Chilhowee Group (Schwab, 1970, 1971) to the Middle Cambrian—Middle Ordovician carbonate platform section (Elbrook Limestone to St. Paul Group) described in part by Sando (1957), Matter (1967), Root (1968), and others. This report is a description of the Tomstown Dolomite, new localities for Salterella conulata Clark, and a paleogeographic scheme for late Early Cambrian time. Conclusions are based on detailed measured sections in the Tomstown Dolomite from Washington County, Maryland (Fig. 1).

STRATIGRAPHIC SETTING

The formational names for the Lower Cambrian rocks in the central Appalachians are shown in Table 1. The thickness estimates for the Tomstown in Pennsylvania (300 m) are based on the width of the outcrop belt (Root, 1968). Butts (1940) estimated the thickness of the Shady Dolomite to be between 485 to 600 m in Virginia. In the Austinville, Virginia, area it is 600 m thick (Brown and Weinberg, 1968). A composite stratigraphic section for the Toms- town Dolomite 150 m thick is based on three well-exposed localities in Washington County, Maryland (Fig. 2).

* Present address: (Reinhardt) U.S. Geological Survey, National Center Stop 928, Reston, Virginia 22092; (Wall) U.S. Geological Survey, Conservation Division, Metairie, Louisiana 70011. •

Figure 1. Location map of measured sections (shaded). Salterella conulata collection sites are lettered: A. Baltimore and Ohio Railroad cut immediately northwest and southeast of Maryland Rt. 34, Keedysville, Maryland (USGS 7947-CO). B. McCoy Farm 1.38 km (0.85 mi) southeast of Porters ville Bridge (520 contour) (USGS 7948-CO). C. Mill Farm pasture, 0.4 km (0.22 mi) southeast of Harpers Ferry-Mill Farm Road intersec- tion (460 contour) (USGS 8075-CQ).

Geological Society of America Bulletin, v. 86, p. 1377-1380, 7 figs., October 1975, Doc. no. 51006. 1377

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The fundamental question is whether our measured thicknesses are the same as the postlithification but pretectonic thicknesses or whether the Tomstown's involvement in the South Mountain fold is responsible for the observed thicknesses (Cloos, 1951). We have concluded that these observed thicknesses are approximately the same as the original thicknesses. In particular, the carbonate strata show only moderate recrystallization; ooids, peloids, and fossils are barely distorted (Figs. 4, 5). The exposure is sufficiently good to demonstrate that the measured sections are not repeated by thrust faults or thinned by normal faults. Salterella conulata, a problematic cone-shaped fossil, occurs in the two complete measured sections used to construct Figure 2. The fossil occurs approximately 30 m above the base of the Tomstown Dolomite in a zone less than 3 m thick. Yochelson (1970) suggested that S. conulata might be important for stratigraphie correlation. Our observations are at odds with his conclusion that S. conulata possibly occurs close to the top of the Tomstown Dolomite (Yochelson, 1970, Fig. 2).

LITHOLOGY

Salterella conulata occurs in a lithofacies that consists of irregular clots and pods of granular dolomite within a fine to medium crystalline, granular-weathering limestone matrix (Fig. 3). Fossils are more abundant in the dolomite than in the limestone matrix. Fossils are moderately recrystallized at all three localities (Fig. 4). The mottled limestone is generally thick and poorly bedded, al- though irregular thin beds are present in some outcrops. This poorly bedded limestone contrasts sharply with the two well- bedded lithologies mentioned below. Throughout the Tomstown Dolomite, two additional lithofacies are also prevalent: (1) peloidal or granular limestone, containing small- to large-scale sedimentary structures (Fig. 5); and (2) laminated dolomite, containing planar, irregular, and cross-laminations (Fig. 6). The mottled limestone lithofacies is less abundant in the upper half than in the lower half of the Tomstown section; it is absent from the section of the upper Tomstown at the Cavetown quarry (Fig. 2). Cyclicity is demonstrable only at some of the well- exposed localities; the arrangement of lithologies is shown in Figure 7.

DEPOSITIONAL ENVIRONMENTS

The Tomstown rock types and their arrangement strongly argue for a stable carbonate platform complex. Each of the three Figure 2. Composite section for the Tomstown Dolomite. Data for lower portions of section rely most heavily on the Keedysville section; TABLE 1. LOWER CAMBRIAN STRATIGRAPHIC NOMENCLATURE IN THE CENTRAL APPALACHIANS upper third of the section is based on the section at Cavetown. GREAT VALLEY SECTION C0NEST0GA VALLEY PENNSYLVANIA MARYLAND-PENNSYLVANIA VIRGINIA lithofacies has a modern analogue on the Bahama-Florida plat- Cloos (1951); Root (1968) Butts (1940) Stose & Stose (1944) forms. The cross-bedded peloidal limestone corresponds to marginal carbonate sand bodies described by Ball (1967). The mot- WAYNESBORO ROME LEDGERÎ FORMATION FORMATION FORMATION tled limestone resembles the bioturbated pelmicrite (pellet mud and grapestone) in the lagoon-bay portions of the complex; this zone is KINZERSÎ spatially the most important in the modern analogues (Ginsburg, TOMSTOWN* SHADY FORMATION 1956; Newell and others, 1959). The laminated dolomite contains DOLOMITE DOLOMITE VINTAGE a variety of sedimentary structures that probably occurred in sev- FORMATION eral subenvironments. The crinkled laminations, which contain sparse vertical disruptions, suggest supratidal levee-crest to levee- ANTIETAM ERWIN ANTIETAM backslope subzones of an algally dominated tidal flat (Ginsburg FORMATION FORMATION FORMATION and others, 1970). The planar and cross-laminated structures could result either from storm-swash deposits on the levee crest or low- energy strandline sedimentation. * Tomstown Formation in Pennsylvania (Root, 1968). 5 At least part of the unit 1s of Middle Cambrian age 1n the Lancaster, Salterella conulata is associated with the lagoon-bay lithofacies. Pennsylvania, area (Campbell, 1969). The fossils are concentrated in the dolomitic pods and clots of the

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Figure 4. A complete longitudinal section and two cross sections of Figure 3. Weathered clots of Salterella conulata Clark (arrows) in mot- Salterella conulata. Stylotization has modified the original fossil margins tled limestone, locality C (Mill Farm). (arrows). Mill Farm pasture, locality C. Bar = 1.0 mm.

Figure 5. Coarse peloidal limestone composed of limestone, dolomite, and quartz granules. This lithology is typically planar tabular to high-angle, Figure 6. Planar and ripple cross-laminated dolomite. Laminae are de- planar cross-bedded. Specimen from base of section in Cavetown quarry. lineated by detrital limestone particles. Top of section at Cavetown quarry.

mottled limestone. The geometry of these dolomitic pods suggests (Newell and others, 1959; Garrett, 1971) and in the Persian Gulf that they were shallow burrows or at least small depressions. The (Friedman and others, 1973). We suggest that Salterella may have shells are oriented in a random manner and show no abrasion. occupied a similar niche in Early Cambrian tidal flat complexes. These observations suggest in situ settling of shells post mortem. Possible life habits of Salterella are either planktonic-nektonic or CONCLUSIONS AND FURTHER SPECULATION benthonic. We concur with Yochelson (1970) that the growth laminae were laid down layer by layer with no chambers present. We suggest that the Tomstown Dolomite thickens and increases There is no hint of any buoyancy structure within the massive in lithologic variability from north to south and that it was depos- calcareous shell. This is perhaps the strongest evidence for a ben- ited on a stable carbonate platform. thonic life mode. Rodgers (1968) suggested that the platform margin during Cam- In modern carbonate bays and lagoons, a number of benthonic brian and Early Ordovician time might have been an abrupt organisms operate rather successfully as browsers and scavengers. platform-basin transition. Reinhardt (1974) demonstrated a car- Conspicuous among these are the cerithid gastropods, which are bonate ramp configuration in the Frederick Valley, Maryland, and larger and have a more complex shell structure than Salterella; a west-to-east migration of the carbonate platform between Middle gross shell shape and clustering of cerithid and Salterella shells in Cambrian and Early Ordovician time. death assemblages are similar. Cerithium is or has been a dominant The clearest indications of a platform edge in the Lower Cam- element in restricted carbonate bays and lagoons in the Bahamas brian strata are coarse, polymictic carbonate breccias in the Aus-

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252-260. Campbell, L. D., 1969, Stratigraphy and paleontology of the Kinzers For- LAMINATED DOLOMITE mation, southeastern Pennsylvania [M.S. thesis]: Lancaster, Pa., T j <~ 'J ' Parallel bedded ; containing Franklin and Marshall College, 78 p. '¡Cplanar, crinkled, and cross /SSyT ^ • - laminations. General absence of Cloos, Ernst, 1951, Stratigraphy of sedimentary rocks, in The physical features of Washington County: Maryland Geol. Survey Rept., p. ' ' ' ' ' ' ronrw pnrtirlo«. rnrp flat pebble 17-94. Colton, G. W., 1970, The Appalachian Basin — Its depositional sequences liXiL . •.'.•( îi MOTTLED LIMESTONE — and its geologic relationships, in Fisher, G. W., and others, eds., I — — ~ —~ Poorly, irregularly bedded; Studies of Appalachian geology, central and southern: New York, ^cTm T

Meter s - dolosllt ods 2 T-^TT P - ¡Salterella) from sea-marginal hypersaline pool, Gulf of Aqaba, Red Sea: Am. = t Assoc. Petroleum Geologists Bull., v. 57, p. 541-557. f—j^ "f ^ y GRANULAR LIMESTONE Garrett, Peter, 1971, The sedimentary record of life on a modern tropical _. . S —r ' ^ V^Well bedded with small to large- carbonate tidal flat, Andros Island, Bahamas [Ph.D. dissert.]: Balti- ( ^— . " ^^ scale current structures. Well more, Johns Hopkins Univ., 191 p. sorted carbonate particles (peloids, Ginsburg, R. N., 1956, Environmental relationships of grain size and con- S y > ^ J J oolites) and locally abundant stituent particles in some south Florida carbonate sediments: Am. ° / '' ,' ' Quartz sand. Assoc. Petroleum Geologists Bull., v. 40, p. 2384-2427. Ginsburg, R. N., Bricker, O. P., Wanless, H. R., and Garrett, P., 1970, Exposure index and sedimentary structures of a Bahama tidal flat: Figure 7. Lithofacies with representative sedimentary structures in the Geol. Soc. America Abs. with Programs, v. 2, p. 744-745. Tomstown Dolomite. Matter, Albert, 1967, Tidal flat deposits in the Ordovician of western Maryland: Jour. Sed. Petrology, v. 37, p. 601-609. tinville, Virginia, area (Brown and Weinberg, 1968) and in the Newell, N. D., Imbrie, J., Purdy, E. G., and Thurber, D. L., 1959, Or- ganism communities and bottom facies, Great Bahama Bank: Am. Thomasville-York, Pennsylvania, vicinity (Stose and Stose, 1944; Mus. Nat. History Bull., v. 117, p. 117-228. G. S. Gohn, 1974, personal commun,). Palmer, A. R., 1971, The Cambrian of the Appalachian and eastern New England regions, eastern United States, in Holland, C. H., ed., Cam- ACKNOWLEDGMENTS brian of the New World: London, Wiley-Interscience, p. 169—217. Reinhardt, Juergen, 1974, Sedimentology, stratigraphy and Cambro- We thank David Elliott for guidance in the field and support of Ordovician paleogeography of the Frederick Valley, Maryland: Mary- this project. E. L. Yochelson identified Salterella conulata in our land Geol. Survey Rept. Inv. 23, 74 p. thin sections. T. R. Waller offered advice on molluscan shell struc- Rodgers, John, 1968, The eastern edge of the North American continent ture. The manuscript was reviewed by A. R. Palmer, M. E. Taylor, during the Cambrian and Early Ordovician, in Zen, E-an, and others, and David Elliott, and an earlier version was reviewed by K. N. eds., Studies of Appalachian geology: Northern and maritime: New York, Wiley-Interscience, p. 141-149. Weaver, S. M. Stanley, J. D. Glaser, and E. L. Yochelson. Robert Root, S. I., 1968, Geology and mineral resources of southeastern Franklin Willson assisted in preparation of photomicrographs. County, Pennsylvania: Pennsylvania Geol. Survey Atlas 119 cd, 118 p. Sando, W. J., 1957, Beekmantown Group (Lower Ordovician) of Mary- REFERENCES CITED land: Geol. Soc. America Mem. 68, 161 p. Schwab, F. L., 1970, Origin of the Antietam Formation (late Ball, M. M., 1967, Carbonate sand bodies of Florida and the Bahamas: Precambrian—Lower Cambrian), central Virginia: Jour. Sed. Petrol- Jour. Sed. Petrology, v. 37, p. 557-591. ogy, v. 40, p. 354-366. Brown, W. H., and Weinberg, E. L., 1968, Geology of the Austinville- 1971, Harpers Formation, central Virginia: A sedimentary model: Ivanhoe district, Virginia, in Ridge, I. D., ed., Ore deposits of the Jour. Sed. Petrology, v. 41, p. 139-149. United States, 1933-1967: New York, Am. Inst. Mining, Metall., Sc Stose, G. W., 1909, Description of the Mercersburg-Chambersburg district, Petroleum Engineers, p. 169-186. Pennsylvania: U.S. Geol. Survey Atlas, folio 170, 19 p. Butts, Charles, 1940, Geology of the Appalachian Valley in Virginia: Vir- Stose, G. W., and Stose, A. J., 1944, Geology of the Hanover-York district, ginia Geol. Survey Bull. 52, pt. 1, 568 p. Pennsylvania: U.S. Geol. Survey Prof. Paper 204, 84 p. Byrd, W. J., 1973, Petrology of the Cambrian Shady Dolomite in North Yochelson, E. L., 1970, The Early Cambrian Salterella conulata Clark in Carolina, northeastern Tennessee, and southwestern Virginia [Ph.D. eastern North America: U.S. Geol. Survey Prof. Paper 683B, 10 p. dissert.]: Chapel Hill, Univ. North Carolina, 161 p. Byrd, W. J., Weinberg, E. L., and Yochelson, E. L., 1973, Salterella in the MANUSCRIPT RECEIVED BY THE SOCIETY OCTOBER 21, 1974 Lower Cambrian Shady Dolomite of southwestern Virginia (Cooper REVISED MANUSCRIPT RECEIVED MARCH 10, 1975 volume): New Haven, Conn., Am. Jour. Science, v. 273-A, p. MANUSCRIPT ACCEPTED MARCH 18, 1975

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