The Geology of Ohio—The Cambrian

No. 20 OHIOGeoFacts DEPARTMENT OF NATURAL RESOURCES • DIVISION OF GEOLOGICAL SURVEY

THE GEOLOGY OF OHIO—THE CAMBRIAN

The beginning of the Cambrian Period about 570 million years It is thought that the lower part of the Mt. Simon may represent ago is marked by the sudden, widespread appearance of multicel- an accumulation of quartz grains derived from a long period of lular marine animals having hard parts capable of fossilization. weathering of underlying Precambrian rocks. Transgression of the Were it not for this dramatic occurrence, the 65-million-year-long Cambrian seas across the shallow shelf on the edge of the craton Cambrian Period would not be regarded as the beginning of a new, reworked these sand deposits. and perhaps the most signifi cant, era in the history of the Earth. As the Cambrian sea continued to fl ood the craton during the Cambrian rocks also mark the fi rst widespread accumulation of opening of the Iapetus Ocean, the waters deepened across Ohio, and hydrocarbons—organic substances such as oil and gas—derived carbonate deposition began in eastern Ohio as the Rome Formation from once-fl ourishing life. accumulated. The Rome, which is primarily dolomite, is more than The Cambrian System was named for a succession of rocks in 700 feet thick in far eastern Ohio, becomes sandy in central Ohio, the Harlech Dome area of Wales by Adam Sedgwick in the 1830’s. and thins to the west. Overlying the Rome Formation in the east- The name “Cambrian” is derived from the Roman name for Wales: ern half of the state is the Conasauga Formation, which consists of Cambria. The apparent sudden appearance of fossils in rocks overly- shale, siltstone, fi ne-grained sandstone, and limestone in the south ing unfossiliferous Precambrian rocks was a dramatic discovery to and dark-gray to brown dolomite and interbedded fi ne- to coarse- early geologists in their attempts to divide geologic time. The base grained sandstone in the north. The Conasauga ranges from 40 to of the Cambrian System marked the beginning of the Paleozoic Era, more than 400 feet thick. the era of ancient life. Cambrian rocks underlie Ohio but are nowhere exposed at the Age System Western Ohio Central Ohio Eastern Ohio surface. Depth to Cambrian rocks ranges from about a thousand my Ordo- WELLS CREEK FM WELLS CREEK FM WELLS CREEK FM feet in southwestern Ohio to more than 10,000 feet in southeastern vician Ohio. These rocks therefore must be studied through information 505 ? Knox unconformity Beekmantown obtained from deep oil and gas wells and indirect geophysical tech- KNOX DOL KNOX DOL Rose Run KNOX

niques such as seismic refl ection, gravity, and magnetic intensity. Upper ? Copper Ridge GEOLOGIC HISTORY KERBEL FM

EAU CLAIRE FM CONASAUGA FM CONASAUGA FM By the beginning of the Cambrian Period the lofty Precambrian brian ROME FM

mountains, formed a billion years ago by continental collision, had iddle M been reduced by erosion to a gentle, rolling landscape. Local relief Cam ROME FM on the Precambrian surface was not much more than about 300 feet Rome sandstone facies at the time of deposition of Cambrian sediments. Toward the end of the Precambrian and continuing into the ? Cambrian, the Laurentian continental plate (North America) be- MT. SIMON SS MT. SIMON SS MT. SIMON SS

570 Lower gan to separate from an eastern continental mass, probably Baltica Pre- Middle Run Fm cambrian Grenville Province rocks (northern Europe), and the Iapetus Ocean formed. Ohio was on the Central Province rocks stable, passive margin of the Laurentian continent, which had the ancient Canadian Shield as its nucleus. Relationships of Cambrian rocks and underlying and overlying systems in Ohio. The posi- tions of the Cambrian-Ordovician boundary and the boundaries between Lower, Middle, and Upper Cambrian rocks are uncertain. Copper Ridge, Rose Run, and Beekmantown are informal subdivisions of the Knox Dolomite. None of these rocks are exposed at the Laurentia surface in Ohio. my = million years. Modifi ed from Janssens (1973). (North America) Siberia Ohio The Rome and Conasauga Formations grade westward into the Eau Claire Formation, which is composed of sandstone, siltstone, Gondwana shale, and dolomite. The Eau Claire ranges in thickness from about 200 feet to more than 500 feet. The Rome/Conasauga and Eau Claire Formations formed as off- shore, shallow shelf and prodelta deposits derived from a northwest Iapetus Ocean Baltica (Northern Europe) source area. Overlying these units in the central part of the state is the Kerbel Formation, an upward coarsening sandstone that is Continental confi guration in the Late Cambrian. Ohio was near the continental edge of Laurentia in south equatorial latitudes. Modifi ed from Riley and others (1993). interpreted to be a wedge of deltaic sediments. This north-south- oriented lobate deposit reaches a thickness of about 170 feet. The The basal Cambrian rock unit in most of Ohio is the Mt. Simon Kerbel is absent in southernmost, western, and eastern Ohio. Sandstone, a quartzose sandstone that is widespread across a During the Late Cambrian, the Kerbel delta foundered and car- multistate area. The Mt. Simon Sandstone is friable, fi ne to coarse bonate sediments (Knox Dolomite) began to accumulate as shelf grained, and in Ohio ranges from 0 to nearly 400 feet in thickness. deposits in the shallow, equatorial sea that covered Ohio. The Knox The Mt. Simon is unfossiliferous and its precise age is uncertain. Dolomite is more than 1,300 feet thick in the southern part of the Trilobites indicate that the units overlying the Mt. Simon are late state and thins to the north beneath a major regional unconformity. Middle Cambrian to early Late Cambrian in age; thus, the Mt. Simon Knox deposition continued throughout the remainder of the Cam- may be Early or Middle Cambrian in age. brian and into the Ordovician Period. The Cambrian-Ordovician continued ➮ the Rometrough wasthecontinental edgeandtheIapetusOcean.From Hansen(1997). south andeastofOhioaccumulatedagreat thicknessofCambriansediments.East time. Arrows indicatedirection ofsedimentdeposition.ThesubsidingRometrough Generalized paleogeographyofOhioandadjacentareas duringlateMiddleCambrian belt stretching from RossCountyonthesouthnortheastward to eastern Ohio,buttheprimaryexplorationarea isalonga linear exploration inOhio.TheRoseRunispresent beneathmost of the KnoxDolomite,hasbecomeaprimetarget forhydrocarbon the hydrocarbons inthehills. later accumulated. A capofimperviousOrdovician rock trapped veloped solutioncavities,knownasvugs,inwhichhydrocarbons the atmosphere andrainwaterduringtheOrdovician, theyde- (Knox unconformity).Whenthesedolomitehillswere exposedto subaerial erosion oftheKnoxDolomiteduringearlyOrdovician centration oferosional remnants (buriedhills)formedduringthe hundreds ofsmallreservoirs inMorrow County. rels ofoiland35billioncubicfeetgashavebeenproduced from set upinbackyards inCardington. Approximately 38millionbar- in the state foroilandgasproduction. Thousandsofwellswere drilled Cambrian rocks oneofthemostimportantgeologicsystemsin produced 45barrels ofoilperday. was in1919from theKnoxDolomiteinMarionCounty. Thiswell below thesurface.The to beexplored foreconomicbene Knox butnotmarkedbyasigni boundary therefore isuncertain,lyingwithintheupperpartof More recently, theRoseRunsandstone,asandstonebedwithin Morrow Countyandsurrounding areas are underlainbyacon- Beginning in1959,drillingMorrow Countyquicklymade Cambrian rocks inOhiohavebeenthelastofPaleozoicsystems fi rst halfofthe1960’sinafrenzied “oilboom”thatsawrigs The Divisionof GeologicalSurveyGeoFacts SeriesisavailableontheWorld Wide Web: http://www.OhioGeology.com (Eau Claire) sandy shelf 20¡S IN IMPORTANCE OFCAMBRIANROCKS MI ECONOMIC ANDENVIRONMENTAL (Conasauga prodelta KY shale) mud ROME TROUGH EMERGENT OHIO LAND Kerbel fi delta rst Cambrianhydrocarbon production fi cant unconformity. fi (Conasauga) tsbecauseoftheirgreat depth carbonate • ThisGeoFacts compiledbyMichaelC.Hansen •May1998 WVA shelf IAPETUS OCEAN PA NY

20¡S VA MD Shafer, W. E.,ed.,1994,TheOhioGeologicalSocietyanthology, theMorrow Riley, R. A., Harper, J. A., Baranoski,M.T., Laughrey, C.D.,andCarlton,R. Riley, R. A., 1994,Oilandgasexploration intheRoseRunsandstone:Ohio Lochman-Balk, Christina,1970,UpperCambrianfaunalpatternsonthe Hansen, M.C.,1997,ThegeologyofOhio—theCambrian:OhioGeology, Janssens, Arie, 1973,StratigraphyoftheCambrianandLowerOrdovician probably plantsexcept,perhaps, foralgaeandlichens. restrial environment, sofarasweknow, wasdevoidofanimalsand Cambrian. Theseanimalslivedinamarineenvironment. Theter- Cambrian rocks. Even graptolites, sponges,andahostofothergroups are knownfrom rock units.However, brachiopods,echinoderms,gastropods, index fossilsforcomparativedatingandcorrelation ofCambrian known from thissystem.Trilobites haveproved tobewonderful because thesearthropods constituteperhaps 60percent ofthefossils scene earlyintheCambrian. simple organisms incomparisontolifethatliterallyburstontothe teemed inthelatePrecambrian. Buttheseanimalsare relatively investigations ofthenineteenthcenturyhaverevealed thatlife capable offossilization,discoveriessincepioneeringgeologic spread appearanceofabundant,multicellularmarineorganisms injected impervious con high porosity andpermeability, anditisunderlainoverlainby because itisrelatively deep(2,500to10,000feet),ithascomparatively in Ohiofordisposalandlong-termstorageoftheseliquidwastes into theMt.Simon.Thisunitisoneofmore idealgeologicunits wells. Currently, 12wellsatfoursitesinOhioinjectwastechemicals unit fordisposalofliquidindustrialwastethrough ClassIinjection in Ohio(2,000,or85percent) havebeendrilledsince1988. 1965 inHolmesCounty;however, themajorityofRoseRunwells Ashtabula County. Gaswas Traditionally, theCambrianisknownas Age ofTrilobites Although thebeginningofCambrianmarks The basalCambrianMt.SimonSandstonehasbeenanimportant of centralOhio:OhioGeologicalSocietyPublication10,452p. County, Ohio“oilboom”1961-1967andtheCambro-Ordovician reservoir and NaturalGasResearch Consortium,257p. and westernPennsylvania:Morgantown, West Virginia, Appalachian Oil deposystems: theLateCambrianRoseRunsandstoneofeasternOhio W., 1993,Measuringandpredicting reservoir heterogeneity incomplex Geology, Spring1994,p.1,3-5. craton: GeologicalSocietyof America Bulletin,v. 81,p.3197-3224. Winter 1997,p. 1,3-5. rocks inOhio:DivisionofGeologicalSurveyBulletin64,197p. fl uids. fi ning layers(aquitards) thatprevent migrationof FURTHER READING fi CAMBRIAN LIFE shes swam intheseasbyendof fi rst produced from theRoseRunin fi rstwide-

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