Fluvial and Lacustrine Facies of the Early Mesozoic Culpeper Basin, Virginia and Maryland

19 Pages·1.838 MB·English

by  Joseph P. Smoot(auth.)

Fluvial and Lacustrine Facies
of the Early Mesozoic Culpeper
Basin, Virginia and Maryland
Seneca, Maryland to Stevensburg, Virginia
July 14, 1989
Field Trip Guidebook T213
Leader: Joseph P Smoot
American G~ophysicalUnion, Washington, D.C.
Published 1989 by American Geophysical Union
2000 Florida Ave., N.W., Washington, D.C. 20009
ISBN: 0-87590-618-4
Printed in the United States of America
COVER Lacustrine cycle in the Balls Bluff
Siltstone at the Culpeper Crushed Stone
Quarry (Stop 3). Hammer rests on the sharp
contact between a massive sandstone with
abundant root structures and black, laminated
shale. Overlying beds are progressively
thicker with more abundant desiccation fea
tures upsection returning to massive,
root-disrupted mudstone at the top. Rock
hammer is 35 em long.
LEADER:
Joseph P. Smoot
U.S. Geological Survey
926 National Center
Reston, VA 22092
IGC FIELD TRIPT213:
FLUVIALAND LACUSTRINEFACIES OFTHEEARLYMESOZOIC CULPEPER BASIN,VIRGINIAAND
MARYLAND
JosephP. Smoot
U.S. GeologicalSUlVey,Reston, Virginia
The Culpeper basin of northern Virginia and adjacent tonguesbasinwardwithfinergrainedrocksoftheBallsBluff.
Marylandis anelongate,north-northeast-trendinghalf-gra The Balls BluffSiltstoneis conformably overlainby sand
bencontainingathicksequenceofUpperTriassicandLower stonesandconglomeratesoftheTibbstownFonnationinthe
Jurassic nonmarine sedimentary and igneous rocks. The southernCulpeperbasinandtheCathatpinCreekFonnation
basiniscentrallylocatedinabeltofexposedearlyMesozoic inthecentralCulpeperbasin. Conglomeratesoccuraslenses
troughs(Figure1)thatroughlyparallelsthedominantAppa atthetopofthesefonnations,andaretheyoungestsedimen
lachian structural trend of the enclosing Precambrian and taryrocksinthe southernCulpeperbasin.
Paleozoic rocks and extends discontinuously from South TheupperCulpeperGroupofEarlyJurassicageconsistsof
Carolinato Canada. The basins withinthis beltformed by aseriesofbasaltflows intercalatedwithsedimentaryrocks.
extensionalprocessesduringtheinitialstagesofcontinental Thelowennostunit,theMountZionChurchBasalt,consists
fragmentationthatultimatelyledtotheopeningoftheAtlan oftwo or more tholeiitic basalt flows. The.Mount Zion
ticOcean. Somepreexistingthrustfaultsthatinfluencedthe ChurchBasaltisoverlainbyred-brownsandstone,siltstone,
shapeand distributionofthebasinswere apparently reacti andgraymudstoneoftheMidlandFormation,whichintum
vated as nonnal faults duringthis extensionalevent (Lind is succeeded by the Hickory Grove Basalt. The Hickory
holm, 1978;Ratcliffe andBurton, 1985). The sedimentary Groveconsistsofatleasttwoormoreflowsthatlocallyare
rocks and basalt flows in these exposed basins are collec separatedbylenticularsandstone and siltstonebodies. The
tively referred to as the Newark Supergroup (Froelich and HickoryGroveBasaltisoverlainbysandstoneandinterbed
Olsen,1985). ThesedepositsareintrudedbynumerousEarly ded withred-brown and gray-greensiltstoneoftheTurkey
Jurassic tholeiitic diabase dikes and sheets that thermally RunFonnation, whichis cappedbythe SanderBasalt. The
alteredtheenclosingsedimentarystrata. Sander Basalt comprises the thickest series of flows and
The Culpeper basin has an exposed area ofabout 2,750 containsthemostextensivelenticularsandstoneandsiltstone
km2. The strata dip for the most part toward a western intercalations.TheoverlyingWaterfallFonnationconsistsof
nonnal-faultmarginand awayfrom aneasternunconfonn interbeddedsandstone, siltstone,conglomerate, andseveral
ablemargin(Figure2). A numberofbroadfoldsinthestrata thincalcareousshalezones.
aretruncatedbythewesternborderfaultanddiabasesheets Primarily on the basis ofpalynofloral evidence (Comet,
haveupliftedareasofsedimentaryrocks,causingrepetition 1977),thestrataoftheCulpeperGrouparebelievedtorange
ofsection. inagefrom atleastthelateCarnianageoftheLateTriassic
Thesedimentaryrocksandintercalatedbasaltflowsinthe (about 225 Ma) to the late Sinemurian or possibly early
Culpeperbasinform the CulpeperGroup (Lindholm, 1979; PliensbachianageoftheEarlyJurassic(about193Ma)with
Lee andFroelich,inpress) (Figure3). ThelowerCulpeper theTriassic-Jurassicboundary (about208 Ma) a shortdis
Group, mostlyofLateTriassic age, is subdivided into four tancebelowtheMountZionChurchBasalt. Thisagerange
fonnations:theManassasSandstone,theBallsBluffSiltstone, is supported by comparisons of additional paleontological
theTibbstownFormation, andthe CathatpinCreekForma data (fish and reptile fossil assemblages and ichnofossil
tion. TheManassasSandstonecontainsthreeconglomerate suites) aswellasgeochemical,isotopic-age,andpaleomag
members that lie at the base of the Culpeper Group in netic datafrom basalts, to data from similarrocks in other
differentareas alongtheeasternmarginofthebasin.These NewarkSupergroupbasins. DiabaseintrusionsintheNew
membersallgradeupwardandlaterallyintothemainarkosic ark Supergroup have a mean age of 198.42.1 Ma as deter
red sandstone (Poolesville Member) ofthe Manassas. The mined from40Ar/39Ar age spectraby Sutter(1985). This
BallsBluffSiltstoneconfonnablyoverlies,gradesinto, and age, combinedwithpaleomagneticdatabyRaymondetal.
intertongueswithsandstoneoftheManassas.Itoccupiesthe (1982) and supported by field geologic relations (Froelich
medial part of the basin and is predominately red-brown and Gottfried, 1988), suggests that the diabase intrusions
siltstoneandsandstoneintercalatedwithgreen-grayfossilif weremostlycontemporaneouswiththeextrusivebasaltsand
erous mudstone. TheLeesburgMemberofthe Balls Bluff thatthis emplacementoccurredoverarelatively shorttime
Siltstoneisapoorly-sorted,boulder-to-cobbleconglomerate intelValintheEarlyJurassic. Thethreeseriesofbasaltflows
dominatedbyclastsofCambrian-Ordoviciancarbonaterocks, in the Culpeper basin have been correlated to similar se
that occurs adjacent to the western border fault and inter- quences in the Newark, Hartford, and Pomperaug basins,
T213: 1
EXPLANATION
1. Wadesboro(N.C.-S.C.)
2. Sanford(N.C.'
3. Durham(N.C.)
4. DavieCounty(N.C.)
5. DanRiverand
Danville(N.C.-Va.)
6. Scottsburg(Va.)
7. Basinsnorthof
Scottsburg(Va.)
8. Farmville(Va.)
9. Richmond(Va.)
10. Taylorsville(Va.)
11. Scottsville(Va.)
12. Barboursville(Va.)
13. Culpeper(Va.-Md.)
14. Gettysburg(Md.-Pa.)
15. Newark(N.J.- Pa.- N.Y.)
16. Pomperaug(Conn.)
17. Hartford(Conn.-Mass.'
18. Deerfield(Mass.)
19. FundyorMinas
(NovaScotia-Canada)
20. Chedabucto(Nova
Scotia-Canada)
100
I
I I
300 400 KILOMETERS
FIGURE 1 DistributionofexposedearlyMesozoic basinsin
easternNorthAmerica. (FromFroelichandOlsen, 1985).
suggesting that they represent regionally extensive events distributionoftine-grainedlacustrinestratainthecentralpart
(LeeandFroelich,inpress; Smootetal., 1988)~ ofthe basinwith coarse conglomerates concentrated along
ThedepositionalenvironmentsrepresentedintheCulpeper thewesternborderfaultandfluvialsandstonesconcentrated
Groupexhibitconsiderablelateralandstratigraphicvariabil alongtheeasternunconfonnablemarginsuggestsplaya-lake
ity. Due to the relatively poor exposure, a considerable deposits in a closed basin that intertongue with coarse
numberoffundamental sedimentary relationshipshave not grainedalluvialfans derivedfrom themountainouswestern
beenresolved. Most published depositional models ofthe borderandwithdepositsofriversenteringfromalowerrelief
Culpeperbasin(forinstance,Lindholm,1979;Froelichetal., easternmargin. Theagesofthedifferentsedimentaryunits,
1982;Gore,1983,LeeandFroelich,inpress)suggestanarid
however, suggestthatsomeofthe facies distributions may
to semiaridclosed-basinsetting. The aridityofthedeposi reflect temporal changes in the depositional environment.
tional environment is suggested by the arkosic nature of Forinstance, the restrictionofManassas Sandstone fluvial
sandstones, andbythecommonoccurrenceofsynsedimen depositstotheeasternmarginofthebasinmaybeintetpreted
tarycarbonatenodules(caliche) andeuhedralcrystalmolds astheexposureoftheloweststrataoverlyingabasaluncon
after gypsum in some red siltstones and mudstones. The
fonnity. Inthisintetpretation,thefluvialsandstonesextend
T213: 2
Diabase
Jurassic 8a alt Series
and Sedimentary Rocks
"'''''''''''''
""'"'''''"'''''''''''''''"'''''''"'' C tharpln Creek
"'''''''''-',''''''' Formation
"''''''''':'''
GO~·:m~:~·k
t&gW1 Portionofb••ln.hown.
D
Bell. Bluff Siltstone
.~~:~;
Leesburg Member
fMW?t)
Mene.... Sendstone
+ +
+ + +
+++++ -Pre-Triassic In basin
+ +
5 miles
10 kilometers
FIGURE2a Lithologicmapofthenorthernportionofthe Culpeperbasin. Heavylines
along marginsofthebasindenotefaultboundaries, lighterlinesindicateunconformable
contacts. (BasedonthemapofLee andFroelich,inpress).
completelyacrossthebasininthesubsurface,indicatingthat otherNewarkSupergroupbasinsthattheclimatefluctuated
initially the basin had an open drainage (Smoot, 1985). frequentlyduringLateTriassicandEarlyJurassicdeposition
Sedimentary feahlres inthe overlying Balls BluffSiltstone (Olsen, 1986;SmootandOlsen, 1988). Evidenceforexten
lacustrine deposits, however, suggest closed basin condi sivevegetation,swampyfloodplains,andbroadmeandering
tions. The lakes changed radically in area and in depth rivers is common in all of the stratigraphic units in the
through tilne and were commonly completely desiccated. Culpeperbasin,suggestingthatthe ariddepositionalcondi
There is mounting evidence from the Culpeper basin and tionsmayberestrictedto certainstratigraphiclevels.
T213: 3
10 kilometers
5 miles
Portionofbasinshown.
Diabase
Jurassic Basalt Series
and Sedimentary Rocks
,","
CathFaorrpminatiCorneek "",","'"",',","""
"""
'
~
Goose Creek
Member
•
Wtt%J Tibbstown
••n..... S.nd.tone Formation
D
+ +
Pre-Triassic in basin +++++ B.II. Bluff Siltstone
+ + +
+ +
FIGURE2b Lithologicmapofthesouthern portionofthe Culpeperbasin. Heavylines
along marginsdenotefaultboundaries, lighterlinesindicateunconformablecontacts.
(Basedon themapofLee andFroelich,inpress).
Thisfieldtripisdesignedtoillustratesomeofthevariabil alargequarrynearthetownofCulpeper. Thesebedsinclude
ity ofdepositional facies inthe Culpeperbasin. The three occurrencesofstrataboundbase-metaldeposits enrichedin
stops are limited to UpperTriassic sedimentary rocks and copper,zinc, andlead.
require that we travel almost the full length of the basin Awordofwarningtothosetakingthistrip. Allthestops
(Figure2). Stops laand Ib are influvial sandstonesofthe haveluxuriantgrowthsofpoisonivy,particularlyduringthe
Manassas Sandstone exposed in bluffs along the Potomac summermonths. Forthoseunfamiliarwithpoisonivy, any
River. These stops show two different styles of fluvial skincontactwiththisplantmaycausethedevelopmentofa
deposition in the Manassas and also an example ofbase skinrashthatisveryitchy. Longpantsandlong-sleeveshirts
metalmineralizationinthesandstones. Stop2is arailroad arerecommendedtominimizecontactwithpoisonivyandas
cutintheLeesburgMemberoftheBallsBluffSiltstonevery protectionfromtheoftenpresentmosquitoes. Hikingboots
near to the western border fault. This stop illustrates the arestronglyrecommended,particularlyforthefirstandlast
natureofthe"borderconglomerates",includingdebris-flow stops,wherewewillbewalkingaroundlarge,looseboulders.
deposits and shallow-stream deposits. Stop 3 is in cyclic HardhatsarerequiredatStop3.Wewillhavesomeextrahats
lacustrinemudstonesoftheBallsBluffSiltstoneexposedin available,butparticipantsareurgedtobringtheirown.
T213: 4
Waterfall Formation
Sander Basalt
.u-
Turkey Run Formation
UJ
.Uc.oJ. D. Hickory Grove Basalt
.., :::»
::I Midland Formation
0
a:
Mount Zion Church Basalt
CJ (
? ? Goose Creek
a: Tibbstown Catharpin Member
W
Formation Creek Formation
D.
-
W
u D.
.-
...J
UJ :::» Balls Bluff
UJ Leesburg
..C.-O. 0 Siltstone
Member
t-
Manassas Sandstone
FIGURE3 Stratigraphic unitsoftheCulpeperGroup. Notallformal members are shown. (AfterLee
andFroelich,inpress).
Roosevelt Bridge across the Potomac River and into Vir
ROADLOG ginia. This is the approximate location ofthe Fall Zone,
whereCretaceousandyoungerstrataoftheAtlanticCoastal
ThefieldtripbeginsattheWashington,D.C.,Convention Plaintotheeastunconformablyoverlapcrystallinerocksof
Center located on R Street, N.W., between 8th and 11th thePiedmont.
Streets. MeasuredmileagebeginsattheintersectionofRand
10thStreets. Incremental mileage is givenin parentheses. 2.5 (0.7) ExitrightontotheGeorgeWashingtonMemorial
Thefieldtripstops arelocatedonFigure2. Parkway. FollowsignstotherightfortheGeorgeWashing
tonMemorialParkwaynorth. Inapproximatelyonemilethe
towersofGeorgetownUniversitywillbevisibletotheright
Cumulative acrosstheriver, andoutcropsofconglomerategneissofthe
Milage Description UpperPrecambrian-LowerCambrianSykesvilleFonnation
canbeseenontheleft.
0.0 IntersectionofRand10thStreets,N.W.infrontof 11.4 (8.9)ExitrightontoRte1-495northtowardFrederick.
theWashingtonConventionCenter. Proceedsouthon10th
Street. Continueforfourblocksto ConstitutionAvenue. 12.0 (0.6) AmericanLegionMemorialBridgeacrossthe
PotomacRiver.
0.5 (0.5) Intersection of 10th Street and Constitution
Avenue. TumrightandproceedwestonConstitutionAve 13.9 (1.9) Exit39forRte190(RiverRoad). Exitrightthen
nue. follow signsforRte 190westtoPotomac.
1.8 (1.3) Intersection ofConstitution Avenue and 23rd 14.6 (0.7) IntersectionoftheentrancerampwithRte 190
Street. Proceedacross23rdStreetandontorampleadingto west. MergeontoRte190andproceedwestovertherolling
T213: 5
POTOMAC RIVER
~----I
1 mil~
._ ~ ...-'" VA ..... 1_k_ilo_m_e_te_r _
FIGURE4 LocationsofStops laand IbintheManassasSandstonenearSeneca,Maryland.
terrainofPiedmontmetamorphic rocks. ofthecanaltotheseriesofsandstoneledgesalongthebluff
westofastreamgully(Figure4).
26.1 (11.5) Junction ofRte 190 and Rte 112. Turnleft
followingRiverRoad(Rte112). In0.2milesyouwillcross
the unconfonnable contactbetween the Triassic Manassas STOP la-POOLESVILLE MEMBER OF THE
SandstoneandtheunderlyingPaleozoicrocks (Figure4). MANASSAS SANDSTONEALONGTHEC&O CANAL
26.5 (0.4) IntersectionofRte112withRiley'sLockRoad. Theseexposuresillustrateonestyleofdepositionfoundin
Tum left on Riley's Lock Road and proceed into Seneca theManassasSandstone. Thesandstoneledgesareseparated
CommunityPark. bycoveredintelVals which are predominatelysiltstoneand
mudstone. Therocksaregenerallydipping10to20degrees
27.3 (0.8) Parkinparkinglotandproceedsouthonfootup westward. Thevariationsindipofadjacentsandstoneledges
thewoodenstairwayandoverthewoodenfootbridgetothe canbeseenfromthepath. Thisprobablyreflectsirregulari
Chesapeakeand OhioCanalTowpath. Proceedwestonthe tiesinthebeddingshapesanddifferencesintheinclinationof
towpathacrossthewoodenbridgeoverSenecaCreek. About beddingduringdeposition. Ofthesixmajorsandstoneledges
50meterspastthebridge,anotherpathtothenorthintersects intheareathatwewillexamine,allbutthethesecondfrom
fromtheright. ContinueleftonthetowpathsouthoftheC&O the bottom consist offining-upward sequences with sharp
Canal. Thebroad areaofthe canalto yourright is Seneca basalcontacts. Theseledgeshavedune-scaletroughcross
Basin. Bargeswereloadedherewithstonecutfrom nearby beds (greaterthan20cmthick) overlainby beds of·ripple
quarries. scalecross-lamination. Theyareinterpretedasfluvialchan
nel-filldeposits. Lindholmetale(1979)indicatedthatthenet
28.0 (0.7) STOP1a. Pickyourwayacrossthedryportion flowdirectionintheManassasSandstoneinVirginiawasto
T213: 6