Depositional Environment of Coral – Rudist Associations in the Upper Cretaceous Cardenas Formation ( Central Mexico )

In late Cretaceous times rudist bivalves became the most important benthic carbonate producers in both the Tethyan and Caribbean realms (KAUFFMAN, 1973; ROSS & SKELTON, 1993). They predominantly built widespread monoto paucispecific associations but were also able to co-exist successfully with corals yielding complex and diverse coral–rudist reefs (e.g., KAUFFMAN & SOHL, 1973; MASSE & PHILIP, 1981; LAVIANO, 1984; CAMOIN et al., 1988; SCOTT et al., 1990; HÖFLING, 1997; SANDERS & BARON-SZABO, 1997; SANDERS & PONS, 1999; Depositional Environment of Coral–Rudist Associations in the Upper Cretaceous Cardenas Formation (Central Mexico)

Herein we present a detailed description of an approximately 170 m thick sequence selected from an approximately 900 m thick section of the Cardenas Fm.It is exposed in a river bed called the Arroyo de la Atarjea (MYERS, 1968), situated approximately 3 km north of the city of Cardenas (Fig. 1).MYERS (1968) first described the composition and tectonic structures of the Cardenas Fm. from this locality.The selected sequence contains one radiolitid assemblage and 23 coral-rudist reefs.Subsequently, we use the term "reef" in the general sense of "calcareous deposits created by essentially in place sessile organisms" (RIDING, 2002, p. 165).

GEOLOGIC SETTING AND LITHOLOGIES OF THE CARDENAS FORMATION
The Upper Cretaceous Cardenas Formation exposed in central Mexico (State of San Luis Potosí) is a mixed clastic-carbonate sedimentary sequence approximately 1800 m in thickness.Laterally, the sedimentary rocks of the Cardenas Fm. pass into the deep water marls of the coeval Mendez Formation, which in turn overlie marl and limestone units of the San Felipe Formation (Fig. 2) (SOHL et al., 1991;VEGA et al., 1995).The Cardenas Fm. conformably overlies the uppermost limestone units of the Valles-San Luis Potosí (V. S. L. P.) carbonate platform (Tamasopo Formation), which are of Cenomanian to Santonian age (BASAÑEZ-LOYOLA et al., 1993).The Cardenas Fm. is overlain by the red beds of the Tabaco Formation, which are of unknown age (MYERS, 1968).According to our results, the Tabaco Fm. consists of sandstones, claystones, and varicoloured conglomerates that conformably overlie the Cardenas Fm.Tectonically the sediments of the Tamasopo Fm., the Cardenas Fm., and Tabaco Fm. are part of the fold and thrust belt of the Sierra Madre Oriental of eastern Mexico (TORRE, 1964;FUENTE-NAVARRO, 1964;SUTER, 1987).
The sedimentary rocks of the Cardenas Fm. were deposited during the Campanian-Maastrichtian time period (MYERS, 1968;CAUS et al., 2002), when a facies change from carbonate dominated deposition to mainly clastic sedimentation took place in northern and central Mexico (LOPEZ-RAMOS, 1985;SOHL et al., 1991;MORAN-ZENTENO, 1994;YE, 1997).In central Mexico this facies change is represented by the transition from the limestone units of the V. S. L. P. carbonate platform to the mixed clastic-carbonate sediments of the Cardenas Fm. and the marls of the Mendez Fm. (SOHL et al., 1991;MORAN-ZENTENO et al., 2000;GOLDHAMMER & JOHNSON, 2001).The input of clastic sediments during the Upper Cretaceous may be attributed to early orogenic phases of the Sierra Madre Oriental (SOHL et al., 1991;YE, 1997).Due to this uplift underlying older sedimentary units of the V. S. L. P. carbonate platform were eroded and deposited as clastic sediments in the foreland basin east of the Sierra Madre Oriental (Fig. 3) (YE, 1997).In the Cardenas region, the shallow water units of the Cardenas Fm. were deposited in a narrow depositional area in front of the Sierra Madre Oriental, whereas the Mendez Fm. represents the deeper parts of the foreland basin farther east.In the Tertiary, the main orogenic phases of the Sierra Madre Oriental involved the sediments of the Cardenas Fm., resulting in their deformation into broad anticlines and synclines (TORRE, 1964;FUENTE-NAVARRO, 1964;MYERS, 1968).
MYERS (1968) subdivided the Cardenas Fm. into 3 lithological members (Fig. 5; for explanation of symbols see Fig. 4), which comprise clastic intervals consisting of marls, silt-and sandstones as well as conglomeratic layers intercalated in the sandstone beds at the base of the upper member.The lower and upper members are characterized by rudist-and coral-rudist limestones, which are absent in the middle member of the Cardenas Fm. (MYERS, 1968).

LITHOLOGIES OF THE INVESTIGATED SECTION
The sediment succession investigated herein is 175 m thick and belongs to the upper member of the Cardenas Formation.The section is generally well exposed despite a few intervals that are partly covered by soil and calcretes (Fig. 6).Sedimentary rocks consist of interlayered marls, fine-grained sandstones, oolithic and bioclastic grainstones, and algal packstones.These sediments are described in detail below.In addition, 5 units (u1-u5) with rudist and coral-rudist associations, intercalated with the clastic sediments, are described in the following sections.The age of the section is considered to be Maastrichtian, based on the ammonite Sphenodiscus pleurisepta (CONRAD) found in the lower member of the Cardenas Fm. that is also exposed in the Arroyo de la Atarjea.
In the lower part of the section fine-grained sandstones are predominant and are intercalated with  1.5-6 m thick oolithic and bioclastic grainstones.The sandstone beds are between 20 cm and 3 m in thickness and characterized by lamination and cross-bedding.The sandstones are fine-grained calclithites (PETTIJOHN et al., 1984) and consist of more than 50% non-skeletal limestone clasts and abundant quartz clasts.The clasts are subangular to subrounded and well sorted.Matrix is absent.Generally, the sandstones are nonfossiliferous but two 3.5 cm and 0.5 m thick sandstone layers below unit u1 contain slightly fragmented radiolitids.
A bioturbated horizon separates these sandstone layers from the overlying reef.
The allochems in the oolithic grainstones consist of superficial ooids with a radial microframe and coated grains, which are well rounded and sorted.The nuclei of the ooids are miliolid foraminifera, quartz fragments, and molluscan shells.Interstitial spaces of the components are filled by dog-tooth spar cement and blocky calcite cement.The uppermost 10 cm of the oosparite bank are bioturbated.

RUDISTS AND CORALS OF THE CARDENAS FORMATION
Since BÖSE (1906) documented rudist species of the Cardenas Formation in his monograph, they have been the object of numerous articles (e.g., BÖSE &  , 1927;MYERS, 1968;MUELLERRIED, 1930;ALENCASTER et al., 1999;SCHAFHAUSER et al., 2002).In addition, they are also well-known from the Caribbean region (CHUBB, 1971;MITCHELL, 2002;STEUBER, 2002;STEUBER et al., 2002) as well as from coeval sediments of Chiapas in southern Mexico (ALENCASTER, 1971) and the Yucatan carbonate platform in Guatemala (SCOTT, 1995;STEUBER, 2002).Their taxonomy was described in detail by many authors, including BÖSE (1906), MYERS (1968), andALENCASTER (1971).In contrast, the coral fauna of the Cardenas Fm. is nearly unknown.A list of all coral species collected during this field work from several outcrops of the upper member of the Cardenas Fm. is presented in Table 1.
Table 1 Coral species collected from the upper member of the Cardenas Formation.

Radiolitid assemblages
In the investigated section the lowermost rudist-bearing bed (r1) is approximately 1 m thick and contains single specimens or clusters of small radiolitids, which reach maximum heights of approximately 5 cm (Fig. 7).The sediment consists of a radiolitid floatstone with packstone matrix that contains rare benthic foraminifera (rotaliidae, miliolidae), coralline algae as well as fragments of echinoderms.The bioclasts are unsorted and angular, suggesting only minor current energy.Rudists are not in life position, but parautochthonous embedding is suggested by the low degree of shell fragmentation.

Coral-rudist reefs
The majority of the reefs in the Cardenas Formation are mixed coral-rudist reefs (r2-r4, r6, r7, r10-r14) (Figs.7-8).Their thickness ranges from 0.5 to 2 m.Upsection, in units u3 and u4, the reefs continuously decrease in thickness.Reefs r3 and r4 are laterally exposed.They are lens-shaped biostromes and reach lateral extensions of approximately 14 m.Within the units the reefs are separated by marls, sandy marls or fine-grained sandstones.The latter contain abundant actaeonellid gastropods.In unit u3 plagioptychids are also very common.
Generally, this reef-type contains a diverse coralrudist fauna.The rudists are dominated by radiolitids (T.floriformis, Bournonia cardenasensis (BÖSE), Biradiolites sp.) and plagioptychids (Coralliochama gboehmi BÖSE, Coralliochama sp.), but hippuritids (Hippurites muellerriedi TRECHMANN, Hippurites ceibarum MUELLERRIED) were also observed, which occur in thickets of more than 10 species.The radiolitid Tampsia and plagioptychid Coralliochama reach maximum heights of approximately 40 cm.Hippuritids are up to 15 cm in height.The coral fauna encompasses columnar, massive, and encrusting growth-forms.Species comprise abundant Multicolumnastraea cyathiformis as well as Siderastrea adkinsi, Actinhelia elegans, Cladocora gracilis, Dictuophyllia conferticostata, Dermosmiliopsis orbignyi, Placocoenia major, Actinacis parvistella, and Actinacis haueri.In reef r2 a large number of Neithea, a non-rudist bivalve, settled onto the corals.The reef fauna of the coral-rudist reefs is generally well preserved and appears in growth position.Only in reef r6 and on the top of r2 rudists and branched corals are reworked but still well preserved and unfragmented, thus indicating the absence of significant transport.Upsection from the reworked layer of r6, the prior reef fauna is reestablished.In thin sections coralline algae frequently encrust both rudists and corals, whereas hippuritid shells suffered strong bioerosion.
Clinger morphotypes (SKELTON & GILI, 1991) of plagioptychids build the pioneer fauna at the base of reefs r3 and r4 (Fig. 9).They settled in scour marks that are incised into the underlying sandstones and might have provided a stable substrate for the overlying reef-fauna.Reef sediment consists of coral-rudist rudstones with packstone and wackestone matrix that contains unsorted and angular fragments of rudists, corals, gastropods, and non-rudist bivalves as well as detrital quartz.Microfossil constituents comprise abundant dasycladaceans, ?gymnocodiaceans, and benthic foraminifera (miliolids and rotaliids) as well as coralline algae and ostracods with articulated valves.
In addition, rodoliths are abundant components in the packstones and wackestones of reef r2 and present nuclei of radiolitid and plagioptychid fragments.

Coral-dominated reefs
The reefs r17-r24 in the uppermost section (unit u5) are dominated by corals (Fig. 10).Upsection, reef-thickness decreases from 1-2 m at the base to 15-40 cm at the top.The 9 reefs are separated by layers of marls, which are 1-25 cm in thickness.
The number and diversity of rudists in coral-dominated reefs is considerably lower than in the underlying coral-rudist reefs (r1-r16).The rudist fauna consists of radiolitids (Biradiolites sp.), plagioptychids, and sparse hippuritids (e.g., H. ?perkinsi).Rudists are small and reach up to 10 cm in height.Although the diversity of corals is very low, they are the main reef builders.The coral fauna is conspicuously dominated by columnar and ramose growth forms of the species M. cyathiformis and D. conferticostata, but massive growth forms of the species A. cellulosa also occur.Numerous specimens of Neithea were found on top of the corals and are characteristic components of the coral-dominated reefs.

Reefs
Except for the absence of radiolitids the faunal composition is similar to the coral-rudist reefs.The rudist fauna is exclusively composed of hippuritids (H.ceibarum, Hippurites sp.) and plagioptychids whereas coral species are generally the same as in the coral-rudist reefs.Reef sediment is also similar to the Fig. 7 Detailed illustration of limestone unit u1 (see also Fig. 6).
coral-rudist reefs, except for the absence of coralline algae and ostracods.

DEPOSITIONAL ENVIRONMENT OF THE CARDENAS FORMATION
Rudist and coral-rudist associations of the Cardenas Formation developed in areas of low water energy and on soft substrates, indicated by low fragmentation of the fauna and by the angular and unsorted bioclasts in the wackestones and packstones.The abundance of dasycladacean algae and miliolid foraminifera in the sediments suggest rudist and coral growth in lagoonal environments.High-energy events produced reworked layers of rudists and corals (e.g.,r2,r6,r17;Figs. 7,8,10).
In the lower part of the section, a high input of terrigenous clastic sediments is indicated by the abundance of sandstone units which prevented the development of extended reefs.Only clusters of small radiolitids developed and corals are completely absent.Moreover, the radiolitid-bearing sandstone units below the radiolitid assemblage r1 are also devoid of coral fragments.Radiolitids might have been able to sustain changes in water chemistry, salinity, and aerial exposure (KAUFFMAN & SOHL, 1973), thus indicating a nearshore environment for these assemblages, possibly in the upper subtidal area of restricted lagoons.This theory is also supported by the presence of laminated and cross-bedded sandstones below these units which are interpreted as shoreface zone deposits (TUCKER, 1990a).
In the upper part of the section the decreasing number of sandstone units indicates less influence of clastic sediments resulting in the development of abundant diverse mixed coral-rudist reefs (Figs. 6, 8).The diverse coral fauna indicates stenohaline conditions and deeper waters than is suggested by the radiolitid assemblage below.Nevertheless, the mixed coral-rudist and coral-dominated reefs developed in shallow water environments or even in open lagoons, which is indicated by the presence of abundant miliolid foraminifera and dasycladacean algae.In the coraldominated reefs, the absence of detrital quartz and the predominance of columnar and ramose corals indicate low water energy (CHAPPELL, 1980) and reef growth in sheltered lagoonal areas, which were not influenced by the input of clastic sediments (GÖTZ, 2001).
In the hippuritid-dominated reefs (r5,r8,r9,r15,r16;10) the absence of ostracods and radiolitids suggests that this reef type reflects the deepest environments of all reefs in the investigated section.They correspond to the hippuritid reefs described from the mixed siliciclastic-carbonate units of the Gosau basin in Austria, where hippuritid-dominated reefs grew on the inner shelf (SANDERS & BARON- SZABO, 1997;SANDERS & PONS, 1999).Also, on the Turonian-Upper Santonian Adriatic Carbonate Platform hippuritid reefs occur in the deeper parts of the outer platform (MORO, 1997).The actaeonellidbearing sandstones that are intercalated with the hippuritid-dominated reefs and the mixed coral-rudist Fig. 10 Detailed illustration of limestone unit u5 (see also Fig. 6).
reefs might represent mobile sand bodies that separated the lagoonal from inner shelf environments (SANDERS & PONS, 1999).
In the Cardenas Fm. plagioptychid rudists are not restricted to reefal limestones.They occur in life position with a diverse molluscan fauna in marls and sandstones.The clinger plagioptychids, Coralliochama, form a pioneer fauna at the base of the coral-rudist reefs r3 and r4 in scour marks that are incised into the underlying clastic units.Plagioptychids also occur within mixed coral-rudist, coral-dominated, and hippuritid reefs.However, radiolitid-assemblages are devoid of plagioptychids.This indicates that plagioptychids tolerated input of terrigenous clastic sediment into their habitat, but on the other hand they were probably less tolerant to changes in salinity and temperature.

FACIES DEVELOPMENT WITHIN THE SECTION
Environments of rudist-and coral-rudist associations are similar in the investigated section, but allow definition of 8 deepening (D1-D8) and 4 shallowingupward sequences (S1-S4) (Fig. 6).Reef thickness decreases in transgressive intervals and increases in regressive sequences (Figs. 8,10).We grouped these facies sequences in 4 idealized sedimentary cycles, comprising 2 deepening-and 2 shallowing-upward cycles (Fig. 11).In the section most of the cycles are incomplete.Unfortunately, the composition of these idealized cycles is obscured by sandstone units (e.g., D3, D7 -Fig.6).The sand derived from the uplifting Sierra Madre Oriental to the west and was episodically transported into the Cardenas area, where these beds are superimposed on the cyclic background sedimentation.
The first idealized deepening-upward cycle (Fig. 11a) consists of basal cross-bedded and laminated calclithites of the shoreface which pass into lagoonal radiolitid assemblages, that are in turn overlain by oolithic grainstones.The well-rounded and sorted ooids indicate shallow water depths and form oolite bars that were deposited seaward of the lagoons on the inner shelf (TUCKER, 1990b, c).This cycle type occurs in the lowermost part of the section and is represented by facies sequences D1 and D2 (Fig. 6).The sequences are separated from the following cycles by bioturbated horizons.
The second type of deepening-upward cycle (Fig. 11b) is represented by facies sequences 8,10) and comprise cross-bedded shoreface calclithites which pass into lagoonal marls or limestones, which contain miliolid foraminifera and dasycladacean algae.Upsection they are overlain by coral-rudist associations (coral-rudist, coral-dominated or hippuritid-dominated reefs) passing into thin bedded marls and sandstones, which are rich in gastropods (turritellids), plagiopty-chids (Coralliochama sp.), and non-rudist bivalves.Our interpretation is that these interlayered marls and sandstones represent the deepest part of the depositional area and were deposited on the inner shelf indicated by the diverse molluscan fauna, sparse miliolid foraminifera, and the absence of calcareous algae.
The abundance of sandstones makes interpretation of facies sequence D3 difficult.Limestones containing rudist fragments, miliolid foraminifera, and dasycladacean algae are interpreted as the talus of rudist dominated lagoonal assemblages.The diverse fauna in the uppermost part of the sequence is similar to the inner shelf fauna described above from the deepening-Fig.11 Idealized sedimentary cycles observed in the investigated section (Fig. 6).Frequently, however, these cycles are incompletely developed and obscured by sandstone units (see text).
In the right column, facies are marked that occur in the deepening-and shallowing-upward sequences of the investigated section (for explanation of symbols see Fig. 4).
upward cycle (Fig. 11b).The lack of coral-rudist reefs may be due to the high input of clastic sediments, which was indeed tolerated by the inner shelf fauna comprising gastropods, plagioptychids, and non-rudist bivalves.Therefore, we interpret this sequence as a transgressive succession, which passes from lagoonal to inner shelf environments like the idealized deepeningupward cycle of Fig. 11b.At its base, the first idealized type of shallowingupward cycle (Fig. 11c) consists of hippuritid-dominated reefs from the inner shelf, which are overlain by actaeonellid sand bar deposits.Upsection mixed coral-rudist reefs of the inner lagoon follow.
The second type of shallowing-upward cycle (Fig. 11d) occurs only in the uppermost rudist unit u5 (Figs. 6, 10) and differs from the shallowing-upward cycle described above by the lack of sand bar deposits.Coral-dominated reefs grew in sheltered areas with less sediment input (see chapter 6): actaeonellid-bearing sand bar deposits are replaced by marls.
Shallowing-upward cycles commonly terminate in layers containing reworked reef fauna (S2, S4; Figs. 8, 10).We interpret these reworked layers as having been derived from reefs situated above the storm wave base, which were therefore subject to erosion.
The investigated section of the Cardenas Fm. consists of interlayered fine-grained sandstones, marls, and limestones.In the lower part of the section, sediments of the shoreface and lagoonal sediments are predominant and are represented by cross-bedded and laminated sandstones, and radiolitid assemblages.In the upper part of the section the number of sandstone units decrease, whereas limestone units are abundant.In addition, units of marl layers occur, which contain a diverse inner shelf fauna, indicating a general deepening-upward trend from the base of the section towards the top.Such transgressive phases together with the low sediment input permitted the development of the diverse coral-rudist reefs in the Cardenas area.

CONCLUSIONS
The investigated section of the upper Cardenas Formation is of Maastrichtian age and comprises a transgressive sequence that consists of sandstones, marls, and limestones.The limestone units contain 4 types of rudist and coral-rudist associations.Radiolitid assemblages were deposited in restricted lagoons.Mixed coral-rudist and coral-dominated reefs developed in open stenohaline lagoons.Reefs dominated by corals are indicative of sheltered areas with less input of clastic sediments.Hippuritid-dominated reefs grew seaward of the lagoons on the inner shelf.Only plagioptychids occur in both the coral-rudist associations and the clastic sediments of the outer shelf, but are absent in the radiolitid assemblages of restricted lagoons.Plagioptychids were able to live in environments strongly influenced by changing sediment input, but were probably less tolerant to salinity or temperature changes of the seawater.
The investigated transgressive sequence encompasses several deepening-and shallowing-upward facies sequences, which can be grouped in idealized types of deepening-and shallowing-upward cycles.The deepening-upward cycles pass from cross-bedded sandstones of the shoreface into calcareous algal or radiolitid dominated limestones and marls of restricted lagoons.They are in turn overlain by lagoonal coral-rudist associations and marls of the outer shelf containing diverse molluscan assemblages (gastropods, plagioptychids and non-rudist bivalves).
The shallowing-upward cycles consist of basal hippuritid-dominated reefs that pass into actaeonellid sand bar deposits or marls, which are overlain by coralrudist reefs.
This cyclic background sedimentation was obscured by the episodic input of clastic sediments derived from the uplifting Sierra Madre Oriental to the west.This sediment input decreases in the upper part of the transgressive sequence, resulting in an increasing number of coral-rudist reefs.So rudist reefs of the Cardenas Formation flourished in times of low sediment input during transgressive phases.

Fig. 1
Fig. 1 Location of the investigated section in the Arroyo de la Atarjea riverbed (MYERS, 1968).

Fig. 2
Fig. 2 Stratigraphy of the formations of the Cardenas area.The marls of the Mendez Formation interfinger with the mixed clastic-carbonate units of the Cardenas Formation.The Cardenas Fm. overlies the shallow water carbonates of the Valles-San Luis Potosí carbonate platform (Tamasopo Formation).The Cardenas Fm. is covered by the red beds of the Tabaco Formation.Similar to the marl and limestone units of the San Felipe Formation, the age range and geologic context of the Tabaco Fm. is not well defined in central Mexico.

Fig. 3
Fig. 3 Palaeogeography of the Gulf of Mexico in the Late Creta-ceous (modified after PINDELL et al., 1988, andSOHL et al., 1991).The mixed clastic-carbonate sediments of the Cardenas Formation and the marls of the Mendez Formation were deposited in the foreland basin east of the Sierra Madre Oriental.In southern Mexico the rudist-bearing Yucatan carbonate platform developed.

Fig. 5
Fig.5Schematic illustration of lithology, lithostratigraphic units and faunal content of the Cardenas Formation as described by MYERS (1968) (for explanations of symbols see Fig.4).

Fig. 6
Fig. 6 Lithologic succession and fossil content of the investigated section of the Cardenas Formation.The section contains 5 limestone units illustrated separately in Figs. 7, 8, 10.The whole sedimentary succession is interpreted as a transgressive facies sequence that consists of 8 deepening-upward (D1-D8) and 4shallowing-upward (S1-S4) facies sequences (for explanation of symbols see Fig.4).

Fig. 9
Fig. 9 Clinger morphotypes (SKELTON & GILI, 1991) of plagioptychids build the pioneer fauna at the base of reef r3.They settled in scour marks that are incised into the underlying sandstone units.