175 ABSTRACT Microfossils from an outcrop of the coral reef and rudist-bearing calcareous upper member of the Mal Paso Formation just north of Chumbítaro, State of Michoacán, Mexico, indicate a deepening trend and transition from nearshore through outer shelf depositional environments upward through the sampled stratigraphic interval. The microbiota is mostly composed of species of calcareous algae and foraminifera. The identified calcareous algae are: Pseudolithothamnium album Pfender, 1936; Cayeuxia kurdistanensis Elliott, 1957; Acicularia americana Konishi and Epis, 1962; and Dissocladella sp. cf. D. savitriae Rama Rao and Pia, 1936. The species of foraminifera are: Nezzazata sp. cf. N. isabellae Arnaud-Vanneau and Sliter, 1995; Buccicrenata subgoodlandensis (Vanderpool, 1933); Cuneolina parva Henson, 1948; Pseudolituonella sp.; Praechrysalidina sp.; and Rotalipora appenninica (Renz, 1936). In addition, a species of stromatoporoid is illustrated and an indeterminate tube-shaped calcitic microorganism is described as an incertae sedis. From the base of the section upward, four biofacies are defined by the co-occurrences of these taxa: a benthic foraminiferal assemblage, a coral assemblage, a caprinid - dasycladacean assemblage, and a coral - miliolid assemblage. This report documents the first detailed examination of the microbiota of the calcareous upper member of the Mal Paso Formation. Data from this analysis of the microbiota supplement earlier paleoenvironmental interpretations based on studies of macrofossils, mainly scleractinian corals, rudists, and other mollusks, and carbonate facies relationships. The combined stratigraphic ranges of the microfossil species identified from this measured section of the Mal Paso Formation support an age determination of late Albian. The occurrence of Rotalipora appenninica (Renz, 1936), a planktic foraminiferan, in the uppermost portion of the exposed stratigraphic section is especially significant because its presence indicates a marked deepening of the depositional environment which can be correlated with the onset of the global late Albian marine transgression and drowning of Tethyan carbonate platforms that is known as the R. appenninica - event. A late Albian age was also suggested by previous studies of other taxonomic groups that have been discovered in the same stratigraphic section, particularly the species of the rudist bivalve genus Mexicaprina Coogan, 1973. Key words: foraminfera, algae, paleoenvironment, Mal Paso Formation, Cretaceous, Albian, Mexico. Microfossils, paleoenvironments and biostratigraphy of the Mal Paso Formation (Cretaceous, upper Albian), State of Guerrero, Mexico Harry F. Filkorn 1,* and Robert W. Scott 2 1 Physics and Planetary Sciences Department, Los Angeles Pierce College, 6201 Winnetka Avenue, Woodland Hills, California 91371 USA. 2 Precision Stratigraphy Associates and University of Tulsa, 149 West Ridge Road, Cleveland, Oklahoma 74020, USA. * fi[email protected]Revista Mexicana de Ciencias Geológicas, v. 28, núm. 1, 2011, p. 175-191 Filkorn H.F., Scott, R.W., 2011, Microfossils, paleoenvironments and biostratigraphy of the Mal Paso Formation (Cretaceous, upper Albian), State of Guerrero, Mexico: Revista Mexicana de Ciencias Geológicas, v. 28, núm. 1, p. 175-191.
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Microfossils, paleoenvironments and …Aptian benthic foraminifera have been reported from the nearby El Cajon locality (Omaña-Pulido and Pantoja-Alor, 1998) which is stratigraphically
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Microfossils, paleoenvironments and biostratigraphy of the Mal Paso Formation 175
ABSTRACT
Microfossils from an outcrop of the coral reef and rudist-bearing calcareous upper member of the Mal Paso Formation just north of Chumbítaro, State of Michoacán, Mexico, indicate a deepening trend and transition from nearshore through outer shelf depositional environments upward through the sampled stratigraphic interval. The microbiota is mostly composed of species of calcareous algae and foraminifera. The identified calcareous algae are: Pseudolithothamnium album Pfender, 1936; Cayeuxia kurdistanensis Elliott, 1957; Acicularia americana Konishi and Epis, 1962; and Dissocladella sp. cf. D. savitriae Rama Rao and Pia, 1936. The species of foraminifera are: Nezzazata sp. cf. N. isabellae Arnaud-Vanneau and Sliter, 1995; Buccicrenata subgoodlandensis (Vanderpool, 1933); Cuneolina parva Henson, 1948; Pseudolituonella sp.; Praechrysalidina sp.; and Rotalipora appenninica (Renz, 1936). In addition, a species of stromatoporoid is illustrated and an indeterminate tube-shaped calcitic microorganism is described as an incertae sedis. From the base of the section upward, four biofacies are defined by the co-occurrences of these taxa: a benthic foraminiferal assemblage, a coral assemblage, a caprinid - dasycladacean assemblage, and a coral - miliolid assemblage. This report documents the first detailed examination of the microbiota of the calcareous upper member of the Mal Paso Formation. Data from this analysis of the microbiota supplement earlier paleoenvironmental interpretations based on studies of macrofossils, mainly scleractinian corals, rudists, and other mollusks, and carbonate facies relationships.
The combined stratigraphic ranges of the microfossil species identified from this measured section of the Mal Paso Formation support an age determination of late Albian. The occurrence of Rotalipora appenninica (Renz, 1936), a planktic foraminiferan, in the uppermost portion of the exposed stratigraphic section is especially significant because its presence indicates a marked deepening of the depositional environment which can be correlated with the onset of the global late Albian marine transgression and drowning of Tethyan carbonate platforms that is known as the R. appenninica - event. A late Albian age was also suggested by previous studies of other taxonomic groups that have been discovered in the same stratigraphic section, particularly the species of the rudist bivalve genus Mexicaprina Coogan, 1973.
Microfósiles del miembro superior calcáreo de la Fomación Mal Paso colectados en un afloramiento justo al norte de Chumbítaro, Estado de Michoacán, indican una tendencia la profundización y una transición de un ambiente de depósito de plataforma interna a uno de plataforma externa hacia la porción superior del intervalo estratigráfico muestreado. La microbiota se compone mayoritariamente de especies de algas calcáreas y foraminíferos. Las algas calcáreas identificadas son: Pseudolithothamnium album Pfender, 1936; Cayeuxia kurdistanensis Elliott, 1957; Acicularia americana Konishi y Epis, 1962; y Dissocladella sp. cf. D. savitriae Rama Rao y Pia, 1936. Las especies de foraminíferos son: Nezzazata sp. cf. N. isabellae Arnaud-Vanneau y Sliter, 1995; Buccicrenata subgoodlandensis (Vanderpool, 1933); Cuneolina parva Henson, 1948; Pseudolituonella sp.; Praechrysalidina sp.; and Rotalipora appenninica (Renz, 1936). Adicionalmente, se ilustra una especie de estromatopórido y se describe como incertae sedis un organismo calcítico tubular no determinado. Desde la base y hacia la porción superior de la sección se definen cuatro biofacies por la concurrencia de estos taxa: un ensamble de foraminíferos, un ensamble de corales, un ensamble de caprínidos– dasycladáceas, y un ensamble de corales-miliólidos. Este reporte documenta el primer examen detallado de la microbiota del miembro superior calcáreo de la Formación Mal Paso. Los datos derivados de este análisis de la microbiota complementan interpretaciones paleoambientales previas basadas en estudios de microfósiles, principalmente corales escleractinios, rudistas, y otros moluscos, así como en relaciones de facies carbonatadas.
Los alcances estratigráficos combinados de las especies microfósiles identificadas en la sección medida de la Formación Mal Paso apoyan la determinación de una edad del Albiano tardío. La presencia de Rotalipora appenninica (Renz, 1936), un foraminífero planctónico, en la porción más alta de la sección estratigráfica expuesta es especialmente significativa dado que indica una marcada profundización del ambiente de depósito, el cual puede ser correlacionado con el inicio de la trasgresión marina global del Albiano tardío y con la inundación de las plataformas carbonatadas del Tethys conocido como evento R. appenninica. Una edad correspondiente al Albiano tardío había sido también sugerida en estudios previos de otros grupos taxonómicos que fueron descubiertos en la misma sección estratigráfica, particularmente las especies del género de rudistas Mexicaprina Coogan, 1973.
Palabras clave: foraminíferos, algas, paleoambiente, Formación Mal Paso, Cretácico, Albiano, México.
INTRODUCTION
The Albian Mal Paso Formation is a significant, thick lithostratigraphicunitinsouthwesternMexico(Pantoja-Alor,1959,1992).TheMalPasoFormationisdividedinformallyintotwoparts,alowersiliciclasticandvolca-niclasticunitandanuppercarbonateinterval.Theuppercarbonateintervalhasyieldeddiverseandabundantcorals,mollusks,echinoids,foraminifera,andcalcareousalgae(García-BarreraandPantoja-Alor,1991).Itwasdepositedinatectonicallyactiveisland-arcsettingduringthelateAlbianrelativesea-levelrise.Asimilarmicrofossilas-semblageisknownfromAlbianshallow-watercarbonatesinAhuacatlán,Querétaro(Buitrón-Sánchezet al.,1995).AptianbenthicforaminiferahavebeenreportedfromthenearbyElCajonlocality(Omaña-PulidoandPantoja-Alor,1998)whichisstratigraphicallybelowtheMalPasoFormation.Thissectionisadatapointrelevanttohypoth-esesaboutthereplacementofcoralcommunitiesbyrudistcommunities.
syncline,approximately0.75kmnorthoftheruralvillageofChumbítaro,Michoacán(Figure1).Ameasuredstrati-graphicsectionatthislocalityiscomposedofacontinuoussequenceoflimestonestrataabout200mthick.Thisintervalcontains horizons with significant insituaccumulationsofoysters,reefcorals,stromatoporoids,andrudistbivalves(Figure2).Thestrataofthissectionhavebeenassignednumbersfromthebaseupward,unitsMP1throughMP18,inordertofacilitatelithologicdescriptionsandasarefer-enceframeforthepositionsoffossiliferoushorizonsandlithologicsamples.Thinsectionspreparedfromlitho-logicsamplestakenthroughoutthissectionhaveyieldedadiversemicrobiotathatcomplementsthepaleontologicalandpaleoecologicaldataderivedfrompreviousstudiesofothermacrofaunalgroupsoftheMalPasoFormationincludingscleractiniancorals(FilkornandPantoja-Alor,2004,2009),rudistbivalves(Filkorn,2002a),gastropods(Buitrón-SánchezandPantoja-Alor,1994,1996,1998),andechinoids(García-BarreraandPantoja-Alor,1991).The23thinsectionsexaminedinthisstudy(Table1)alsoincludedthreefromsamplesofbedsbelowthebaseoftheuppermember,intheclasticlowermemberoftheforma-tion(sampleslabeledHFLMP1,2,and3),butthesethinsectionsarebarrenofmicrofossils.All20ofthethinsec-tionsofsamplesfrombedsintheuppermembercontained
Microfossils, paleoenvironments and biostratigraphy of the Mal Paso Formation 177
90°95°100°105°110°
MEXICOTampico
AcapulcoTeloloapanZihuatanejo
Puerto Vallarta
Guerrero Terrane MexicoCity*
110° 105° 100° 95° 90°
TEXAS
250 km
20°
25°
15°
30°
15°
20°
25°
30°
Angao
La Estancia
Huetamo
San Lucas
TuritzioCharacoCumburindio
Las Eras
Highway 51
Zirándaro Chumbítaro
Ciudad Altamirano
Balsas River
MICHOACÁN
MICHOACÁN
GUERRERO
GUERRERO
X
fossillocality
5 km
Study area
2009).Atthescaleoftheoutcropandinhandsample,thesedimentsdepositedbetweenthelargercoralcoloniesandmollusks in the coral reef horizon could be classified as an echinoid spine wackestone or floatstone with local pack-stone.Notably,echinoidtestswerenotobservedinthesamebedandtheobviouspresenceofthemajorityofthelargertaxainthereeffaciesisnotevidentfromexaminationofthe fine-grained bioclastics of the intra-reef rock. Thus, the microbiotaofthecoralreefhorizoniswithinthecaprinid
identifiable microfossils (Table 1). Because facies of the Mal Pasochangegreatlylaterallyandverticallyatotherlocalities(García-BarreraandPantoja-Alor,1991),theconclusionspresentedhereinpertaintothissiteonly.
PALEOECOLOGY
ThestudiedsectionoftheuppermemberoftheMalPasoFormationrecordsamajorlateAlbianmarinetrans-gressionanddrowningoftheareafromthebaseofthesectionupwardthroughthehighestsampledbed.Theshiftfromcontinentalsiliciclasticsandvolcaniclasticsinthetopofthelowermembertopredominantlycalciumcarbonatesedimentsinthebasalpartoftheuppermembermarksasignificant shift in depositional setting and begins the tran-sitionofenvironmentsupwardthroughthesectionfromlagoonaltobackreef,coral-stromatoporoidreef,caprinidrudistforereef,distalreefbioclastics,andultimatelydepos-itsindeeperwater.Thepresenceofsmallcoloniesofcorals(Ovalastreasp.,Preverastraeasp.)andstromatoporoids,rudistbivalves,miliolidsandotherbenthicforaminiferasuspended in a fine-grained matrix in the lower part of thesectionindicatenormallyquietandrelativelyshallowmarineconditionsofalagoonalorinnershelfsetting.Abedwithabundantrecrystallizedfragmentsofabranchingdendroidcoralspeciesandaspeciesofredalga(unitMP7)occursjustbelowanin situdepositofoystershells(unitMP8).Abovethishorizon,thesedimentologyanddiversebiotaofsmall,isolatedcorallaofmassiveandphaceloidcolonialcoralspecies,speciesoftherudistRadiolites,andspeciesofothermolluskssuchasnerineidgastropodsandthepectinidbivalveNeithea[Pecten]roemeriHill,1889,indicate an increased affinity with a coral reef environment. Themicrobiotafromthissamepartofthesection(unitsMP9-MP12)isinthecaprinid-dasycladbiofaciesandmainlycomposedofadiverseassemblageofspeciesofcalcareousgreenalgae(CayeuxiaandDissocladella),encrustingredalgae(Pseudolithothamnium)andbenthicforaminifera(Table1).Thus,thebiotaclearlysupportstheinterpretationofdepositioninashallowmarineenvironmentthatwaswellwithinthedepthlimitsofthephoticzone.TherudistspeciesMexicaprinaalatahasitslowestrecognizedoccurrenceinthisstratigraphicsectionoftheuppermemberinunitMP12whereitoccurswithNeithearoemeriandabundantquanti-tiesofothermolluscanbioclastics.
Figure1.Mapofthelocationofthestudyarea.TheoutlinemapofMexicoin the upper part of the figure shows the Guerrero Terrane and three of its partsinsouthwesternMexico,theZihuatanejo,TeloloapanandHuetamosubterranes.ThestudyareaislocatedintheHuetamosubterraneandindicatedbythesmallblackrectanglewithanarrow.Thismarkedregionisenlargedandshowninthesketchmapofthestudyareashowninthelower part of the figure. The fossil locality is in the State of Guerrero andapproximately0.75kmnorthoftheruralvillageofChumbítaro,StateofMichoacán.TheterraneboundariesarefromFreydieret al.(1997)andCenteno-García(1994).ThedetailedmapofthestudyareaisbasedonMexican1:50,000scaletopographicmapsheetsCoyucadeCatalán (E14A74, first edition, 1976, second printing, 1981) and Huetamo (E14A64, first edition, 1976, third printing, 1989).
Filkorn and Scott178
covered
oyster bed
calcareoussandstone
Mexicaprina cornuta,miliolids, red algae
Cayeuxia,Dissocladella,Pseudolithothamnium
Acicularia
Mexicaprina alatavalves, miliolidsgreen and red algae
–dasycladbiofaciesandthealgal–caprinidmicrofacies,similartothatofthestratigraphicallyadjacentbeds,andtheidentified elements are mostly caprinid rudist fragments, calcareousgreenalgae(Dissocladella)andbenthicfora-minifera(miliolids,Buccicrenata,Cuneolina).
underlyingcoralreefhorizon.Thus,thestratigraphicrela-tionship coupled with the identified biota indicate that the Mexicaprina floatstone was deposited in a forereef setting adjacenttothecoralreefenvironmentandthatthephysi-calconditionsofthehabitatalsowerewithinthelimitsoftheessentialecologicalrequirements(lightintensity,watertemperatureandbathymetry)ofcalcareousgreenalgae.ThedistributionoftheMexicaprinavalvesandcolonialcoralsinthestratigraphicintervalwhichspansunitsMP12-MP14,i.e.,Mexicaprinavalvestransportedintoandbehindthecoralreef,butnocoralstransportedintheoppositedirec-tionandforwardofthereeffrontintotheforereeffacies,suggeststhattheprevailingwatercurrentsandwaveenergywerefromanoffshoredirectionandtowardthereeffront,aswouldbeexpectedinmostcoastalsettings.
AbovethetopofunitMP14,inunitMP15,thetaxo-nomicdiversitydecreases,calcareousalgaeareabsent,andthemicrobiotaisprimarilycomposedofbenthicforamin-ifera(miliolidsandCuneolina).Inaddition,thebeddingchangesnoticeablyinthestrataaboveunitMP14,wherestratabecomethin-tomedium-beddedandanintercalatedargillaceousmudstoneandwackestonecomponentisintro-duced,concurrentwithadecreaseinmacrofossilsandanincrease in the degree of sorting of fine-grained bioclastics. ThetopofunitMP14isthehighestobservedoccurrenceofMexicaprinavalvesinthisstratigraphicsection.
Figure2.StratigraphiccolumnofthemeasuredsectionoftheUpperAlbiancalcareousuppermemberoftheMalPasoFormationatthelocal-itymarkedinFigure1.Thebedshavebeenassignednumbersfromthebaseofthesectionupward,unitsMP1throughMP18,andthehorizonsof studied thin section samples and identified biota are indicated. The topofthestratigraphicsectionatthislocalityistruncatedbythepres-entsurfaceoferosion.Theapproximatecoordinatesofthebaseandthetopofthemeasuredstratigraphicinterval,asdeterminedfromthegridmarksontheCoyucadeCatalántopographicmapsheet,are:latitudeN18°29’14.2”,longitudeW100°42’41.9”;andlatitudeN18°29’23.5”,longitudeW100°42’28.7”,respectively.
Microfossils, paleoenvironments and biostratigraphy of the Mal Paso Formation 179
Sample Horizon Microfacies
Num
ber
of T
axa
Qua
rtz
Cal
care
ous A
lgae
Enc
rust
ing
Alg
ae
Ben
thic
For
amin
ifera
Rud
ists
Cor
als+
Stro
mat
opor
oids
Buc
cicr
enat
a
Cun
eolin
a
Nez
zaza
ta
Mili
olid
s
Rot
alip
ora
Aci
cula
ria
Cay
euxi
a
Dis
socl
adel
la
Pseu
dolit
hoth
amni
um
Bio
faci
es
Env
iron
men
t
M10 MP18 Pkst,coral-foram-peloid 8 10 10 F R F R
Cor
al-m
iliol
id
Out
erS
helfM09 MP18 Pkst,coral-foram-peloid 7 10 20 F R F
M08 MP18 Pkst,coral-foram-peloid 7 10 20 F R F M07 MP18 Pkst,coral-foram 8 10 5 20 F R F HFMP18 MP18 Pkst,coral-foram 7 20 20 O R O
HFMP17 MP17 Pkst,caprinid 7 2 2 20 R R R
Cap
rinid
-das
ycla
d
Mid
dle
Shel
fBui
ldup
HFMP15 MP15 Pkst,foram 5 11 0 R O M49 MP14 Pkst,algal-caprinid 10 22 1 20 5 R C M48 MP14 Pkst,algal-caprinid 10 7 20 R F HFMP13SW MP13 Wkst,algal-caprinid 9 10 3 5 R R R F HFMP12 MP12 Pkst,algal-caprinid 13 7 1 1 10 5 S S F RHFMP11 MP11 Wkst,algal 9 10 1 5 10 R F HFMP10 MP10 Wkst,algal 7 10 6 R F O HFMP9U MP9 Wkst,mollusk 10 1 2 5 S R R HFMP9L MP9 Pkst,caprinid 10 6 1 1 30 0.5 R F R R
HFMP7A MP7 Pkst,coral-strom 8 0.5 1 0.5 30 S R Coral
HFMP6 MP6 Gnst,bioclast 5 2 1 1 R S
Ben
thic
fora
m
Inne
rShe
lf
HFMP3 MP3 Gnst,bioclast 8 1 0.5 6 1 F R R S HFMP2 MP2 Gnst,bioclast 7 5 6 F R R HFMP1-2x3 MP1 Wkst,caprinid 9 20 5 S R R
coralreefhorizon(unitMP13),butnoechinoidtestswerefoundinthesamebed.Thecombinationofthesekindsofoccurrenceshasbeeninterpretedasevidencethattheregu-larechinoidslivedintheshallow-watercoralreefhabitatwheretheywerede-spinedandthattheirwholetestswerelatertransportedintodeeperwaterbyweakbottomcurrentsflowing in an offshore direction.
Thestratigraphicallyhighestbedexposedinthismea-suredsectionoftheuppermemberisa1m-thickdepositoffine-grained wackestone, unit MP18. This bed has yielded twospeciesofscleractiniancorals:Blastozopsammiaguer-reroterionFilkornandPantoja-Alor,2004,athin-branched,ramose,colonialdendrophylliid,andParacycloseriseffre-natusFilkornandPantoja-Alor,2009,asmall,disc-shapedorcupolate,solitarycunnolitid.Thedendrophylliidspe-ciesB.guerreroterionbelongstoagroupofscleractinian
coralsthatisrecognizedasprimarilyinhabitingdeep-orcold-waterenvironments,thereforethisspeciespresum-ablylackedthekindofalgalsymbiontsthataretypicalofmodernreef-buildingcorals(i.e.,itwasazooxanthellate)andlivedbeneaththelowerlimitofthephoticzone.Thesmall,solitarycoralspeciesP.effrenatusalsowasnotareef-build-ingformanditislikelythatittoowasazooxanthellateandexisted in deeper water. The microbiota identified from unit MP18 is particularly significant because it is predominantly composedofbenthicforaminifera(Cuneolina,Nezzazata,miliolids),calcareousalgaeareabsent,andonespeciesofplankticforaminifera,Rotaliporaappenninica(Renz,1936) makes its first appearance in the section (see Table 1). Thelackofcalcareousalgae,acommonbioticcomponentoftheunderlyingstratainthissection,suggeststhatthedepositionalenvironmenthadbecomedeeperupwardinto
ThecalcareousalgaeandforaminiferaspeciesfromthissectionofthecalcareousuppermemberoftheMalPasoFormationsupportalateAlbianagedetermination.TheplankticforaminiferalRotalipora appenninicaZoneintheupperpartofthesection(unitMP18)hasbeencorrelatedwithlatestAlbianammonitezones(PremoliSilvaandSliter,2002; Scott, 2009). This zone is defined as the interval above the first occurrence (FO) of the nominate species up to the FOofRotalipora brotzeni(Sigal)thatisatthebaseoftheCenomanianStage.ThespeciesBuccicrenata subgood-landensis(Vanderpool,1933),Cuneolina parvaHenson,1948,Nezzazatasp.cf.N.isabellaeArnaud-VanneauandSliter,1995,Pseudolituonellasp.,andPraechrysalidinasp.ofthebenthicforaminiferalassemblageallhavelongerstratigraphicranges,buttheyalsoareconsistentwithalateAlbianage.Thus,thestratigraphicrangesoftheforamin-iferasupportthepreviousagedeterminationofthestrataatthislocalitythatwasmainlybasedupontheoccurrenceofspeciesofthecaprinidrudistMexicaprinaCoogan,1973(Filkorn,2002a).ThisgenuscharacterizestheuppermostAlbianrudistzoneintheGulfCoast(ScottandFilkorn,2007).
TheupperMalPasoFormationatthissiterecordsthelatestAlbianrelativeriseinsealevelandamarinetrans-gressionwhichcorrelatewithawidespreadsea-levelcycle.InnorthTexas,thePawpawandMainStreetformationsintheupperpartoftheWashitaGroup,cycleWA5,representa siliciclastic flooding and shoaling into a carbonate shelf (Scottet al.,2003).ThislatestAlbiansea-levelcycleisalsorecordedinEurope(Amédro,2008;AmédroandRobaszynski,2008).However,aswithmanyotherareasinsouthwesternMexico,depositionoftheMalPasoFormationwasalsoaffectedbylocaltectonics(García-BarreraandPantoja-Alor,1991).
thishorizonoftheuppermemberandthatthesubstratewasnowbelowthephoticzone.ThepresenceoftheplankticforaminiferanspeciesR.appenninicainunitMP18isex-tremely significant because it adds further support to this interpretation.TheRotaliporaappenninicaZonespansanintervalofcarbonateplatformdrowningintheTethystypicallyreferredtoastheR.appenninica-event(Grötschet al.,1993).OncarbonateplatformsintheMediterraneanregionthisstratigraphicintervaloverliesakarsticcontactthatrepresentsasequenceboundary.Thus,theentirefossilassemblagefromunitMP18supportstheinterpretationthattheenvironmentofdepositionwasinadeeper-watersetting.Thisinterpretationcompletesthestratigraphictransitionofdepositionalenvironmentsupwardthroughtheuppermem-beroftheMalPasoFormationatthislocalityandsupportstheoverallinterpretationthatthissectionrecordsamajormarinetransgression.
Tosummarize,thefourbiofaciesintheMalPasoFormationaredefinedby theco-occurrenceoffossilgroups.Instratigraphicorderfromthebasetheyare:thebenthicforaminiferalassemblage;thecoralassemblage;thecaprinid-dasycladassemblage;andthecoral-miliolidassemblage(Table1).Thesebiofaciesdocumentaprogres-sionfromnearshoretooffshoredepositionalenvironmentsat this locality.Thebenthicforaminiferalassemblagerepresentstheinnershelfmiliolidassemblage(SliterandBaker,1972)andthemiddletoexternalcarbonateshelf(Tronchetti,1984).IntheMiddleEastthegenerarangeindepthfrom10to65m(BannerandSimmons,1994).ThecalcareousalgalassemblageofPseudolithothamnium album,Cayeuxia kurdistanensis,Acicularia americana,andDissocladellawithoutudoteaceanalgaecomposetheDasyclad-Rhodophyteassemblageandrepresentsdepthsof15to30m(BannerandSimmons,1994).Thesamemodelprojectsplankticforaminiferaappearingatouterplatformdepthsbelowabout60m.Thestratigraphicsuccessioninspecies diversity and composition reflects a progressive changeinenvironmentfromnearshorewithquartzsandinflux to shallow shelf followed by late stage deepening to approximately60to65m.
Theco-occurrenceofrudistsandcolonialcoralsandtheirformationofbuildupscanbeusedtotesthypothesesposedtoexplaintheapparentreplacementofcoralpaleo-communitiesbyrudistpaleocommunities.OnehypothesisproposesthatlateAlbianrudistsout-competedcoralsinahighsalinity“Supertethys”zone(KauffmanandJohnson,1988).Anotherhypothesis,so-called“Strangelove”pumpwithnicheexclusion,proposesthatAlbiancoralsoccupieddeeperpartsoftheshelfthanrudistsandwerestressedrepeatedlybyAlbiananoxiceventsOAE1b,1c,and1d(Scott,1995).Ifthepresentpositionofthissectionrela-tivetonorthernMexicoisthesameassuggestedbytheLate Aptian plate reconstruction of Mann (1999, fig.16), it waswellwithinthe“Supertethys”hypersalinezone.Thuscoralswerenotexcludedbythelocalsalinity,whichwasapproximately‘normal’assuggestedbythediversefossil
Microfossils, paleoenvironments and biostratigraphy of the Mal Paso Formation 181
InstitutodeGeología(=IGM),UniversidadNacionalAutónomadeMéxico,MexicoCity.Theillustratedspeci-mens are denoted by IGM specimen numbers in the figure captions.Thesupragenericclassificationofcalcareousalgaeisasyetdeveloping(SaundersandHommersand,2004). Thus, we use the classification of Wray (1978). For foraminifera, the suprageneric classification of Loeblich andTappan(1988)isfollowed.
PhylumRhodophytaClassRhodophyceae
OrderCryptonemialesFamilySquamaraceaeHauck,1885
GenusPseudolithothamniumPfender,1936
Pseudolithothamnium album Pfender, 1936Figure3.1,3.2
1936Pseudolithothamnium albumPfender,1936,p.304-308, pl. 19, figs. 1-5.
Remarks.Pseudolithothamnium albumencrustsmicritenodulesandlargebioclastsinthebasalbedoftheuppermemberoftheMalPasoFormationwhereitoccurswiththerudistMexicaprina cornuta.Thethalliare0.089–0.179mmthick and composed of curved tubes 7 to 21 μm in diameter. FulldescriptionsareprovidedbyJohnson(1969)andKussandConrad(1991).Stratigraphic range.ThisspecieshasbeenreportedfromUpperAlbianstratainTexas(Johnson,1969)andMaastrichtiantoPaleocenestratainEgypt,SinaiandJordan(KussandConrad,1991).IntheuppermemberoftheMalPasoFormation,thisspecieshasbeenrecognizedinseveralhorizonsinthelowerpartofthesection,belowthemaincoralreefinterval(MP13).
Description.Elongate,curvedtubewithaninternaltubeoraxis,circulartoovalincrosssection;diameter:outertube,0.232–0.214mm,innertube,0.125mm;wallsofeachtubecomposedofequantmicrogranularcalciteandperforatedbyminuteovalpores.Specimenswhichlackacentraltubepossess a wall composed of fine radial calcite fibers that originatefromcentralaxis;darkerconcentricgrowthringscloselyspaced.Remarks.SpecimensofthistaxonwererecognizedinthinsectionsamplesfromunitsMP9,MP10andMP12ofthecalcareousuppermemberoftheMalPasoFormation.
Everticyclammina Redmond, 1964, also in thesubfamilyBuccicrenatinae,ismorelenticularandlesscompressed thanBuccicrenata,has twowhorls in itsplanispirallycoiledstageanditschambersarewedge-shaped;theagglutinateouterwallhassmallalveolaeandseptaareunperforated;andthesinglesimpleapertureiswide(LoeblichandTappan,1988;BannerandHighton,1990).
PseudocyclamminaYabeandHanzawa,1926,inthesubfamilyChoffatellinaeMaync,ischaracterizedbyitstypespeciesP. lituus(Yokoyama,1890),whichhastwotothreewhorlsinitsplanispirallycoiledstagewithnar-rowarcuatechambers;numerouscloselyspacedalveolaetransectboththeagglutinatewallandthesepta;andthecribrate aperture fills the face of the last septum (Loeblich andTappan,1988).
1999Pseudocyclammina hedbergiMaync,ScottandFinch,1999, fig. 4D, E.
2001 Buccicrenata subgoodlandensis (Vanderpool),BouDagher-Fadel, p. 168-169, pl. 1, figs. 5-7.
Diagnosis. Test planispiral to uniserial, four to five hemi-sphericalchambersincoil;uniserialpartwithonetothreeellipsoidalchambers;simpleterminalaperture.Remarks.ThefiveotherspeciesofBuccicrenataaredescribedbyBouDagher-Fadel(2001).MeasurementsofspecimensfromtheWalnutFormation:length0.71to4.73mm,coildiameter1.38mmto2.69mm,testthickness0.58mm(Vanderpool,1933;LoeblichandTappan,1949).ThespecimensfromtheMalPasoFormationrangeinlengthfrom0.98mmto1.79mm,axialwidth0.64to1.27mm,andproloculusdiameter0.125mm.Stratigraphic range.ThetypespecimensarefromtheMiddleAlbianWalnutFormationinsouthernOklahoma(Vanderpool,1933).ThespeciesrangesthroughouttheAlbianinTethyansectionsfromNorthAmericatotheMiddleEast.InTexasthespeciesoccursintheLowerAlbianGlenRoseFormationandintheLowertoUpperAlbianWalnutandGoodlandformationsoftheFredericksburgGroup. In Florida the species was identified as Lituola sub-goodlanensiswhereitoccurswithCoskinolinoides texanusKeijzerintheFredericksburgGroup(ApplinandApplin,1965).InMexicoandHondurasthespecieswasreportedinMiddletoUpperAlbianstrataasP. hedbergibyScottandGonzález-León(1991)andScottandFinch(1999).IntheUnitedArabEmiratesandOmanitrangesthroughouttheLowertoUpperAlbianNahrUmrFormation(SimmonsandHart,1987;BannerandHighton,1990;BouDagher-Fadel,2001).InSpainthespeciesisreportedfromUpperAlbiantoLowerCenomanianstrata(GonzálezFernándezet al.,2004).
Type species. Cuneolina pavoniad’Orbigny,1846,p.253.Emended Diagnosis. “Testfree,somewhatcompressedparallel to the plane of biseriality and conical to flabel-liform.Theembryonicapparatusconsistsofaglobulartoovalprotoconchcoveredbyadeuteroconchthatisdividedbybeamsandrafters.Thefollowingbiserialstageconsistsofbroad,lowchambersthataredividedbyradialbeamsextendingfromtheouterwalltowardtheplaneofbiserial-ityand,sometimes,byhorizontalrafters.Thewallisag-glutinatedormicrogranularandkeriothecal.Theapertureconsistsofarowofporesatthebaseoftheseptalface”(Arnaud-VanneauandSliter,1995,p.554).
Cuneolina parva Henson, 1948Figure4.9-4.12
1948Cuneolina pavoniad’Orbignyvar.parvavar.nov.Henson, p. 624-627, pl. XIV, figs. 1-6, pl. XVII, figs. 7-12, pl. XVIII, figs. 12-14.
1991Cuneolina walteriCushmanandApplin,1947,notp. 30, pl. 10, figs. 4, 5a, b; Scott and González-León, p. 58, fig. 5O.
1995Cuneolina parvaHenson,Arnaud-VanneauandSliter,p. 554, pl. 4, figs. 6-9.
Diagnosis. Test moderate size, biserial flabelliform to tri-angular,apicalanglegenerallylessthan90°.Wallagglu-tinatedandcomposedofuniformmicritegrainswithsmallalveolae.Proloculusattestapexslightlyeccentricandoval;uptotwelvebiserialarcuateconcentricchambersexpandfromapex;chambersdividedintochamberletsbyradialseptathatalignwithseptainadjacentchambers,butseptainsertedbetweenolderseptainyoungerabapicalchambers;oneortwoshortlateralpartitionsprojectfromouterwallofconcentricchambersandinsertanarrowconcentriclinebetweenchambersepta;septalfacescurvedtowardmedianwallbetweenchambers.Remarks.AstratigraphicsuccessionofspeciesrangesfromAptiantoMaastrichtianandhasbeendiscriminatedbytestsizes,apicalanglesandsutures(Henson,1948;Arnaud-VanneauandSliter,1995).However,thetwospe-ciesreportedmostoftenareCuneolina pavoniad’Orbigny,1846,andCuneolina parvaHenson,1948(Arnaud-VanneauandSliter,1995;Arnaud-VanneauandPremoliSilva,1995).Cuneolina pavonia is flabelliform and tends to be slightly largerandhaveagreaterapicalanglethanC. parva,whichis conical to flabelliform (Table 3; Figure 5, graphs a, b).
ThemeanapicalangleofparatypesofC. parvais48° andrangesfrom18°to96°(Henson,1948).ThechambersofC. pavoniaarewiderthanthoseofC. parvaandC. pavoniahastwotothreetransverseraftersinthechamberscomparedtoonetotwoinC. parva(Arnaud-VanneauandSliter,1995;ArnaudVanneauandPremoliSilva,1995).Cuneolina parvaisreportedfromAlbian-CenomanianstrataandC. pavoniafromuppermostAlbiantoSantonian.TheAptianspecies,Cuneolina sliteriArnaudVaneauandPremoliSilva,1995,hasasmalltriangulartoconicaltestandlacksthehorizontalrafters in its first chambers.
TheMiddleCenomanianFloridaspeciesCuneolina walteri Cushman and Applin, 1947 (pl.10, figs. 4, 5a, 5b) is also illustrated by Applin and Applin (1965, pl. 2, fig. 2). ThisnamewasusedforspecimensinMexico(ScottandGonzález-León,1991)andinHonduras(ScottandFinch,1999).ThedimensionsofthetypesofC. walteriareintherangeoftopotypesoftheoldernameC. pavonia(Figure5,graphsA,B)andiftheinternalstructureofC. walteriislikethatofC. pavonia,thentheformerwouldbeajuniorsyn-onymofthelatter.ThesizesoftheMexicanandHonduranspecimensareinthesizerangeofC. parvaandoverlapwiththesmallerspecimensofC. pavonia.However,measure-mentsofCuneolinaspecimenstakenfromthinsectionstendtobeincompletebecausemostspecimensareunorientedandthecrosssectionsareobliquetothegrowthaxes.Thus,distinguishingspeciesofCuneolinabysizesofthinsectionspecimenshassomedegreeofuncertainty.
Description.Testuniserial,conical,withatrochospiralproloculus,twojuvenileovatechambersandfourarcuatechambers.Testlength0.52mm,width0.34mm.Walloftestmicrocrystallinecalciteandsolid.Proloculusasingleovalchamberwithathickprismaticwall;innerdiameter0.053mm,outerdiameter0.120mm.Remarks.Thespeciesdescriptionisbasedonaspeci-menfromthecalcareousuppermemberoftheMalPasoFormation.ThespecimenismuchsmallerthananddiffersfromPseudolituonella reichelibytheabsenceoftubularpillarsontheadapicalsideoftheapertures.ItisverysimilartoaspecimenintheUpperAlbiansectioninthePyreneesidentified as Pseudolituonellasp.?(Peybernes,1976,pl.34,fig. 12), which is 1.62 mm in length and 0.85 mm in width. TheolderandlargerPseudolituonella gavonensisFoury,1968,hasastreptospiralearlystage.Stratigraphic range.AccordingtoLoeblichandTappan(1988)thegenusrangesfromCenomaniantoCampanianintheMediterraneanpartoftheTethys.However,P. gavonesisisaBarremianspecies.InEurope,Pseudolituonellasp.?hasbeenreportedwiththeUpperAlbianNeorbitolinopsis conulus(Douvillé)(Peybernes,1976).TheMexicanspeci-menisfromunitMP10ofthecalcareousuppermemberoftheMalPasoFormation.
Type species.Praechrysalidina infracretaceaLupertoSinni,1979.
Praechrysalidina sp.Figure4.15,4.16
Description.Testsmall,conical,withoverlappingaperturalfaces.Testlength0.119–0.385mm,width0.098–0.210mm. No interior marginal flaps or other structures present. Proloculusnotobserved.Remarks.Thespeciesdescription isbasedonspeci-mensfromthecalcareousuppermemberoftheMalPasoFormation.Banneret al.(1991)placedPraechrysalidinainthefamilyChrysalidinidaeNeagubecauseofitsinferredphylogeneticrelationshipwithChrysalidinaratherthaninthefamilyAtaxophragmiidaeSchwager,asdidLoeblichandTappan(1988).TheMalPasospecimensaresimilartothejuvenileportionofaspecimenfromtheAlbianofIraq(Banneret al., 1991, fig. 9). The Mexican specimens do notshowthecharactersofPraechrysalidina infracretaceaandtheyaresmaller;additionalmaterialisneededinordertomakeamoredetailedspeciesdescriptionandamoreaccuratespeciesdetermination.
Microfossils, paleoenvironments and biostratigraphy of the Mal Paso Formation 189
497-501, pl. 30, figs. 1, 3, not fig. 2 (prior synonymy provided).
2002Rotalipora appenninica(Renz,1936),PremoliSilvaand Sliter, p. 71, pl. 8, figs. 1a-1c, 2a-c, 3a-c, pl. 9, figs. 1a-1c, 2a-c, 3a-c.
Description.Testchamberstrapezoidalinoutline;keelwell-developedalongperipheralmargin.DimensionsoftwospecimensobservedinthinsectionM10:testdiameter0.378mm,height0.126mm;andtestdiameter0.252mm,height0.105mm.Remarks.ThebriefdescriptionisbasedontwospecimensobservedinthinsectionM10fromunitMP18.TheoverallformoftheMexicanspecimensismostsimilartothatofR. appenninica,exceptthewallisthickerthannormal,es-peciallyintheearlywhorls(MarkLeckieandBrianHuber,personalcommunications,December2009).Thethickenedwallmaybetheresultofrecrystallization.Stratigraphic range.TheMexicanspecimensarefromunitMP18,nearthetopofthemeasuredsectionofthecalcareousuppermemberoftheMalPasoFormation.ThefullrangeofR.appenninicaisUpperAlbiantoLowerCenomanian.TheRotalipora appenninica Zone is defined as the interval above the first occurrence (FO) of the nominate species and the FO ofRotalipora brotzeni(Sigal)anditcorrelateswithlatestAlbianammonitezones(PremoliSilvaandSliter,2002).
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