Guerrero-Morelos Platform Drowning at the Cenomanian–Turonian

Cretaceous Research (1997) 18, 661 – 686
Guerrero-Morelos Platform drowning at the
Cenomanian – Turonian boundary, Huitziltepec
area, Guerrero State, southern Mexico
*†Ulises Herna´ ndez-Romano, ‡§Noemi Aguilera-Franco,
*Martı´n Martı´nez-Medrano and *Jaime Barcelo´ -Duarte
* Area de Exploracio´ n de Recursos Energe´ ticos del Subsuelo , Divisio´ n de Estudios de Posgrado , Facultad de
Ingenierı´ a , UNAM , Ciudad Universitaria , CP 04510 , Me´ xico DF , Mexico
† Present address: Postgraduate Research Institute for Sedimentology , University of Reading , PO Box 227 ,
Whiteknights , Reading , RG6 6AB , UK
‡ Micropaleontologı´ a del Mesozoico , Subdireccio´ n de Tecnologı´ a de Exploracio´ n , Instituto Mexicano del
Petro´ leo , Eje Central Norte La
´ zaro Ca´ rdenas No. 152 , CP 07730 , Me´ xico DF , Mexico
§ Present address: Department of Geology , Imperial College of Science , Technology and Medicine , Royal
School of Mines , Prince Consort Road , London SW7 2BP , UK
Revised manuscript accepted 7 February 1997
Facies successions in three stratigraphic sections (Barranca del Tigre, Axaxacoalco, and Zotoltitla´n,
Guerrero State, southern Mexico) that comprise middle Cenomanian to lower Turonian rocks of the
central part of the Guerrero-Morelos Platform, indicate the drowning of some parts of the platform
near the Cenomanian – Turonian boundary. In the western part of the study area (Barranca del Tigre
section), Cenomanian shallow-marine limestones (mainly subtidal facies) with abundant benthic
fauna (mainly foraminifera) pass upwards to a 7-m-interval of Turonian open-marine nodular
limestones with few benthic organisms and then to dark-grey and black laminated pelagic limestones
and marls. In the west-central part (Axaxacoalco section), Cenomanian restricted shallow-marine
limestones (intertidal to subtidal facies) change abruptly upwards to Turonian black and dark-grey
laminated pelagic limestones and marls with only pelagic fauna (calcisphaerulids, planktonic
foraminifera, and radiolaria). In the eastern part of the study area (Zotoltitla´n section), Cenomanian
restricted shallow-marine limestones (mainly intertidal facies) are overlain by 45 m of open-marine
limestones showing up-section a rapid decrease in benthic diversity until only calcisphaerulids and
echinoderm fragments occur. Overlying these rocks, there are 65 m of nodular shaly limestones in
thick strata with renewed benthic fauna, 80 m of shaly and silty limestones intercalated with
claystones and some siltstones, and finally, dark grey / black laminated pelagic limestones and marls.
The facies successions in the stratigraphic sections suggest the progressive drowning of the
Guerrero-Morelos Platform around the Cenomanian – Turonian boundary. We attribute the drowning
of some parts of the platform to the occurrence of the Cenomanian– Turonian anoxic event. The
impingement of anoxic waters over the platform could produce the drastic reduction of the carbonate
producing benthos observed in the stratigraphic sections and therefore a reduction in carbonate
accumulation rates. Subsidence and the late Cenomanian-earliest Turonian sea-level rise were then
able to drown the platform. This occurred first in the western part, where subtidal conditions
dominated and an irreversible drowning occurred, allowing the deposition of organic-rich pelagic
sediments over pre-existing shallow-marine carbonates. At the same time, the eastern part, where
intertidal conditions dominated, changed to open-marine conditions, shallow first and deep later.
Here, a temporal restoration of shallow open-marine carbonate sedimentation resulted, but eventually
this region was also drowned, probably by the interplay of terrigenous-clastic supply to the platform
and a new impingement of anoxic waters.
÷ 1997 Academic Press Limited.
KEY WORDS: Cenomanian – Turonian boundary; Guerrero-Morelos Platform; platform drowning;
anoxic event; Mexico.
1. Introduction
The Guerrero-Morelos Platform is represented by more than 800 m of Albian –
Cenomanian shallow-marine limestones (Morelos Formation) that have extensive
outcrops in the states of Morelos and Guerrero, southern Mexico. The study
0195 – 6671 / 97 / 050661 1 26 $25.00 / 0 / cr970078
÷ 1997 Academic Press Limited
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U. Herna´ ndez-Romano et al.
Figure 1. Location of the study area. The tips of the arrows mark the location of the measured
stratigraphic sections.
area is located in the central part of Guerrero (Figure 1), and in the central part
of the Guerrero-Morelos Platform. In this area, a sequence of Aptian – Albian
evaporites (Huitzuco Anhydrite) underlies the shallow-marine limestones of the
Morelos Formation, which are in turn overlain by pelagics, open-marine
limestones and terrigenous clastic rocks, all of them being different facies of the
Mexcala Formation (Figure 2). The limestones of the Morelos Formation were
Figure 2. Cretaceous lithostratigraphic units of the central part of the Guerrero-Morelos Platform.
This study is focused on the upper part (upper Cenomanian) of the limestones of the Morelos
Formation and the lower part (Turonian) of the Mexcala Formation, which has almost
everywhere argillaceous limestones in the lower part and shales, siltstones and sandstones in the
middle and upper parts.
Guerrero-Morelos Platform drowning at the Cenomanian– Turonian boundary
663
Figure 3. Schematic paleogeographic reconstruction for the Cenomanian of the Guerrero-Morelos
Platform, based on data from Ontiveros-Tarango (1973), de Cserna et al . (1978; 1982),
Urrutia-Fucugauchi (1988), Bo¨ hnel et al . (1989), Gonza´lez-Pacheco (1991), Meneses-Rocha et
al. (1994), and Ruiz-Violante & Basa´n˜ ez-Loyola (1994).
deposited on an epeiric carbonate platform more than 150 km wide and 250 km
long attached to a Paleozoic metamorphic hinterland in the east (Figure 3).
The Late Cretaceous to early Tertiary Laramide compressional event produced
a series of N – S oriented folds and thrust faults in these rocks, with later normal
faulting related to NE – SW extensional stress. Strike-slip faulting is also present
and probably occurred later in the Tertiary. Paleomagnetic studies in this area
indicate a paleolatitude of 288N during the Cenomanian with no major
movements relative to the North American craton since then (Bo¨ hnel et al.,
1989; Urrutia-Fucugauchi, 1988).
Several workers have studied these rocks (Bohnenberger-Thomas, 1955; Fries,
1960; Bolivar, 1963; de Cserna, 1965, 1981; Olea-Gomezcan˜ a, 1965; Da´ vilaAlcocer, 1974; Cha´ vez-Quirarte, 1980a, 1980b; de Cserna et al., 1978, 1980;
Salinas-Prieto, 1986; Sabanero-Sosa, 1990), but detailed stratigraphic study of
the central part of the Guerrero-Morelos Platform started only a few years ago,
with diagenetic, environmental and paleogeographical approaches (Gonza´ lezPacheco, 1988, 1991; Go´ mez-Rodrı´guez, 1991; Martı´nez-Medrano et al., 1992;
Martı´nez-Medrano, 1994; Herna´ ndez-Romano, 1995; Aguilera-Franco, 1995).
During the Aptian and early Albian, deposition of evaporites (Huitzuco
Anhydrite) took place in a sabkha. At the same time, eastward (landward) from
the sabkha, alluvial sandstones and conglomerates (Zicapa Formation) were
deposited in a coastal plain, and limestones (Acahuizotla Formation) were
deposited seaward in a carbonate platform at the western part (Figure 3) (de
Cserna et al., 1980; Gonza´ lez-Pacheco, 1991; Martı´nez-Medrano et al., 1992;
Martı´nez-Medrano, 1994).
The environments (and facies) migrated eastward during late Albian long-term
sea-level rise. Shallow marine conditions transgressed the sabkha, establishing
carbonate deposition on an epeiric platform. Deposition of shallow-marine
limestones and dolomites (Morelos Formation) lasted until the early Turonian,
when pelagic limestones (lower Mexcala Formation) were deposited over some
parts of the platform (Gonza´ lez-Pacheco, 1991; Martı´nez-Medrano, 1994,
Herna´ ndez-Romano, 1995). This drastic change in facies is the subject of this
paper.
664
U. Herna´ ndez-Romano et al.
During the Turonian to Santonian(?), after the establishment of pelagic
conditions, carbonate deposition gradually lessened until terrigenous-clastic
sediments predominated, producing a coarsening upward deltaic sequence
composed of limestones and shales in the lower part of the Mexcala Formation;
shales, siltstones and sandstones in the middle, and sandstones and conglomerates in the upper part (Herna´ ndez-Romano, 1995).
2. Description of the stratigraphic sections
Three stratigraphic sections were measured across the boundary between the
Morelos and Mexcala formations, which closely coincides with the Cenomanian –
Turonian boundary. A total of 340 thin sections from 184 samples collected were
petrographically studied to define sediment composition, fossil content, and
ultimately, sedimentary conditions. A brief description of the stratigraphic
sections is given below.
2.1. Barranca del Tigre section
This section was measured 4 km west of Huitziltepec, at the point where the
ravine Barranca Tepetlatipa changes from a north – south direction to almost
east – west and becomes Barranca del Tigre. This sequence is part of the east
flank of a north – south oriented anticline. The total measured thickness is 265 m,
of which 110 m correspond to the upper part of the Morelos Formation and
155 m to the lower part of the Mexcala Formation (Figure 4).
The Morelos Formation is composed of intercalation of bioclastic (benthic
foraminifera, mollusc fragments, ostracods) and peloidal wackestones and
packstones, 0.1 – 2.5 m thick. Mollusc fragment grainstones, floatstones and
rudstones are also present as well as a radiolitid bafflestone close to the top of the
Morelos Formation. Dolomites are rare, although limestones with varying
degrees of dolomitization are common.
The predominant fauna consists of benthic foraminifera (mainly miliolids,
nezzazatids, and soritids), ostracods, mollusc fragments (pelecypods and gastropods), and echinoderm fragments. The association of benthic foraminifera
Pseudorhapydionina laurinensis (De Castro), Nezzazatinella picardi (Henson),
Moncharmontia apenninica (De Castro), Biconcava bentori (Hamaoui & SaintMarc), and Pseudolituonella reicheli (Marie) gives a late Cenomanian age
(Berthou, 1973; Bilotte, 1985; Rosales et al., 1994; Aguilera-Franco, 1995)
(Figure 5). Besides the benthic foraminifera, calcareous algae that had not been
reported from Mexico at this stratigraphic level are also present. Such species
are: Acicularia endoi (Praturlon), Acroporella radoicici (Praturlon), Cylindroporella
cf. sudgeni (Elliot), and Clypeina sp. The calcisphaerulids Pithonella ovalis
(Kaufmann), Stomiosphaera sphaerica (Kaufmann), and Calcisphaerula innominata
(Bonet), are very rare in these rocks.
Terrigenous-clastic content in the limestones is very low and a distinctive
volcaniclastic interval is noted at 70 – 75 m from the base of the section.
Sedimentary structures include abundant burrows, tabular and trough cross
lamination, and occasional wavy lamination.
At the contact with the Mexcala Formation, benthic foraminifera disappear,
calcisphaerulids are very abundant, and whiteinellids and heterohelicids appear.
An increase in clay and organic-matter content is evidenced by the claystone and
Guerrero-Morelos Platform drowning at the Cenomanian– Turonian boundary
665
Figure 4. Lithology, paleontology, and depositional environments in the Barranca del Tigre section
(see also Figure 5). Note that several groups of benthic organisms do not occur above the
Cenomanian – Turonian boundary. The echinoderms occurring above this boundary are predominantly crinoids and most planktonic foraminifera are globigerinids. Symbols used in Figures 4 – 9
are also shown.
shale interbeds and dark-grey to black colour of the rocks, respectively. The
limestones become nodular and thinner. The transition from the Morelos
Formation to the Mexcala Formation takes place in an interval of about 7 m.
666
U. Herna´ ndez-Romano et al.
Figure 5. Stratigraphic distribution of microfossils in the Barranca del Tigre section. Structures and
lithology as in Figure 4.
After the interval of nodular limestones, the Mexcala Formation consists of
thin (5 – 20 cm thick) shaly limestones (calcisphaerulids, planktonic foraminifera
and radiolarian wackestones and packstones) intercalated with calcareous shales.
Globigerinids are abundant and keeled forms of planktonic foraminifera are
extremely rare.
Guerrero-Morelos Platform drowning at the Cenomanian– Turonian boundary
667
The lower part of the measured interval of the Mexcala Formation, contains
Hedbergella planispira (Tappan), Heterohelix moremani (Cushman), Whiteinella
archaeocretacea (Pessagno), W . paradubia (Sigal), Dicarinella sp., D. cf. hagni
(Scheibnerova), and the calcisphaerulids Pithonella ovalis (Kaufmann),
Stomiosphaera sphaerica (Kaufmann), Calcisphaerula innominata (Bonet), Risserella
rablingae (Trejo), Pithonella perlonga (Andri), and Navarrella castroi (Trejo)
(Figure 5). The last three calcisphaerulid species appear in this interval and are
sparse, while the former three are very abundant, in contrast to Cenomanian
strata where they are very rare. This group of fossils indicates an early Turonian
age (Caron, 1985; Soto, 1981; Rosales et al., 1994). The lower – middle Turonian
is represented by rocks bearing Helvetoglobotruncana helvetica (Bolli) (Caron,
1985; Soto, 1981; Sliter, 1989) (Figure 5). Limestones predominate in the lower
part of this formation, but upwards they are rhythmically intercalated with
calcareous shales.
2.2. Axaxacoalco section
This section was measured on a road cut of the Mexico City – Acapulco Highway
(Autopista del Sol), four kilometres north of Axaxacoalco (Figure 1). The total
measured thickness is 140 m, of which 125 m correspond to the Morelos
Formation and 18 m to the Mexcala Formation (Figure 6).
The Morelos Formation consists of bioclastic and intraclastic wackestones and
packstones, mollusc fragment floatstones, and benthic foraminiferal / intraclastic
grainstones. Shaly limestones are common and dolomites are rare. An interval
with high content of detrital quartz and clay was found at 75 – 83 m from the base
(Figure 6). Bed thickness varies from 0.2 to 2 m.
These rocks are characterised by abundant benthic fauna (miliolids, nezzazatids, lituolids, soritids, discorbids, rotaliids, ostracods, and molluscs). Echinoderm fragments are common. The benthic foraminifera Biplanata peneropliformis
(Hamaoui & Saint-Marc), Biconcava bentori (Hamaoui & Saint-Marc),
Pseudolituonella
reicheli
(Marie),
Nezzazatinella
picardi
(Henson),
Pseudorhapydionina laurinensis (De Castro), Moncharmontia apenninica (De
Castro), Murgeina apula (Luperto-Sinni), Trochospira avnimelechi (Hamaoui &
Saint-Marc) indicate a late Cenomanian age (Berthout, 1973, Bilotte, 1985;
Rosales et al., 1994; Aguilera-Franco, 1995) (Figure 7). Some species of
calcareous algae are also sparsely present: Acicularia endoi (Praturlon),
Salpingoporella dinarica (Radoicic), Marinella lugeoni (Pfender), and Cayeuxia
kurdistanensis (Elliot). These species had not previously been reported in Mexico
at this stratigraphic level. Pithonella ovalis (Kaufmann) is rarely present.
The sedimentary structures present consist of burrows, wavy and parallel
lamination, fenestrae, and scour and fill structures and desiccation cracks at
some levels. The contact between the Morelos and Mexcala formations is marked
by the change to pelagic rocks. The transition occurs in an interval of less than
one meter and is characterised by a considerable increase in clay content,
dark-grey to black colour, and a lack of benthic foraminifera.
Following the transition interval, the Mexcala Formation consists of thin
(2 – 10 cm thick), black marly beds of wackestones and packstones with abundant
planktonic foraminifera (globigerinids) and calcisphaerulids intercalated with
claystones. Keeled planktonic foraminifera are very rare. The pelagic beds
contain the planktonic foraminifera Whiteinella archaeocretacea (Pessagno) , W .
innornata (Bolli), Helvetoglobotruncana helvetica (Bolli) , H. praehelvetica
668
U. Herna´ ndez-Romano et al.
Figure 6. Lithology, paleontology, and depositional environments in the Axaxacoalco section (see
also Figure 7). As in the Barranca del Tigre section, most groups of benthic organisms disappear
at the Cenomanian – Turonian boundary. Patterns and symbols as in Figure 4.
Guerrero-Morelos Platform drowning at the Cenomanian– Turonian boundary
669
Figure 7. Stratigraphic distribution of microfossils in the Axaxacoalco section. Structures and
lithology as in Figure 4.
(Trujillo) , Marginotruncana renzi (Gandolfi) , M. cf. sigali (Reichel) , and M. cf .
pseudolinneiana (Pessagno) (Padilla-Avila & Sa´ nchez-Rios, pers. comm., 1995)
and the calcisphaerulid Navarrella castroi (Trejo), indicating an early-middle
Turonian age (Soto, 1981; Trejo, 1983; Caron, 1985; Sliter, 1989) (Figure 7).
Since no evidence of erosion was found at this level, we believe that the
670
U. Herna´ ndez-Romano et al.
lowermost Turonian is extremely condensed or was not deposited. Other
bioclasts that occur in these rocks are radiolarians and thin-shelled mollusc and
echinoderm fragments. The upper part of the section consists of a rhythmic
alternation of pelagic limestones and claystones and / or shales. Parallel lamination
is common throughout this part of the Mexcala Formation.
2.3. Zotoltitla´ n Section
This section was measured one kilometre to the north-east of Zotoltitla´ n, along a
ravine (Figure 1). The total thickness of the measured interval is 383 m, of which
178 m belong to the upper part of the Morelos Formation and 205 m to the lower
part of the Mexcala Formation (Figure 8).
The Morelos Formation consists of bioclastic and intraclastic wackestones to
packstones, 0.3 – 1 m thick. The main bioclasts are benthic foraminifera and
ostracods, with occasional beds in which mollusc fragments, dasycladacean algae,
and echinoderm fragments predominate. Rocks with different degrees of dolomitization are common as well as some dolomites. Thin beds of calcareous
claystones are rare. The formation contains the association Moncharmontia
apenninica (De Castro), Merlingina cretacea (Hamaoui & Saint-Marc), Biconcava
bentori (Hamaoui & Saint-Marc), Nezzazatinella picardi (Henson),
Pseudorhapydionina laurinensis (De Castro), P. dubia (De Castro),
Pseudocyclammina rugosa (d’Orbigny), Pseudolituonella reicheli (Marie), Murgeina
apula (Luperto-Sinni), and Nezzazata simplex (Omara), indicating a late Cenomanian age (Figure 9). Some calcareous algae are also present. As in the Morelos
Formation of the other sections, the calcisphaerulids are very rare.
Highly bioturbated horizons are very common. Also, parallel and wavy
lamination, tabular cross lamination, fenestrae, and desiccation cracks occur in
these rocks at several levels. Towards the contact with the Mexcala Formation,
the clay content increases, strata acquire a nodular aspect and most of the
Cenomanian benthic foraminifera disappear; only biserial textulariids and scarce
small nezzazatids persist into the Mexcala Formation.
In this section, the lowest part of the Mexcala Formation is represented by
shaly limestones (bioclastic packstones with abundant calcisphaerulids and
echinoderm fragments) with abundant complete and well-preserved benthic
macrofauna (echinoderms, molluscs, corals) and cephalopods in nodular strata
10 – 40 cm thick. Planktonic foraminifera are also present in these rocks and
include Hedbergella delrioensis (Carsey), Whiteinella sp., W. archaeocretacea
(Pessagno), Dicarinella sp., D. hagni (Scheibnerova), and D . difformis (Gandolfi),
indicating early Turonian age. The calcisphaerulids Pithonella perlonga (Andri),
Navarrella castroi (Trejo), Risserella rablingae (Trejo), and Bonetiella sp. appear at
this level (Figure 9), occurring with the species recognised in the underlying
Morelos Formation, which are very abundant in this part of the Mexcala
Formation.
Benthic organic diversity decreases upwards until only calcisphaerids and rare
echinoderms and globigerinids can be observed in rocks around 200 – 210 m
above the base. This interval consists of a rhythmic alternation of thin-bedded
shaly limestones (calcisphaerulid wackestones) with calcareous claystones. Above
this interval (220 – 290 m), limestone beds become thicker again, being nodular
floatstones with molluscs and udoteacean algae [Boueina pygmaea (Pia)] intercalated with thin strata of calcareous claystones. At this level, beds consisting
mostly of oyster shells are common.
The interval dominated by limestones is followed by a sequence of alternating
Figure 8. Lithology, paleontology, and depositional environments in the Zotoltitla´n section (see also
Figure 9). In contrast to the other sections, benthic organisms show more variety in the lowest
Turonian. A decrease in the benthonic organic community is reflected in the thinning of the beds
at about 190 m. A temporal recovery of the benthic flora and fauna (platform progradation?) is
seen from 230 to about 290 m and is reflected by thick, predominantly calcareous beds. From
there upwards the amount of benthic organisms present in the rocks progressively decreases and
planktonic foraminifera become dominant. Patterns and symbols as in Figure 4.
672
U. Herna´ ndez-Romano et al.
Figure 9. Distribution of microfossils in the Zotoltitla´n section. Structures and lithology as in
Figure 4.
Guerrero-Morelos Platform drowning at the Cenomanian– Turonian boundary
673
thin beds (0.1 – 0.3 m) of shaly limestones (bioclastic packstones with mollusc
fragments, echinoderm fragments, calcisphaerulids, and planktonic foraminifera)
with calcareous claystones and locally siltstones (290 – 370 m). Woody fragments
are present in the upper part of this interval.
Finally, at the top of the section (above 370 m), parallel laminated shaly pelagic
limestones (wackestones with calcisphaerulids and planktonic foraminifera) are
intercalated with calcareous claystones. Woody fragments can commonly be
observed in these rocks. The presence of Helvetoglobotruncana sp. and Loeblichella
sp. indicate an early – middle Turonian age (Figure 9).
3. Lithofacies
Considering lithologic characteristics, 12 lithofacies were defined, 8 for the
Morelos Formation and 4 for the Mexcala Formation (Table 1). The characteristics of the Morelos Formation indicate that sediments were deposited in a
carbonate platform with varying degrees of restriction. In contrast, the fossil
content of the Mexcala Formation indicates open-marine conditions. Three
facies associations or subenvironments were defined for each formation and are
described below. The distribution of these facies associations along the measured
sections can be observed in Figure 10.
3.1. Facies associations of the Morelos Formation
Three facies association representing three different subenvironments can be
differentiated in the Morelos Formation.
Intertidal to supratidal (lithofacies 1 and 2, Table 1). This subenvironment is
represented by calcareous claystones and argillaceous laminated wackestones and
packstones with intraclasts and scarce bioclasts. Organisms show low diversity
and frequency and are mainly euryhaline (ostracods, miliolids, rotaliids). Part of
the lamination is due to cyanobacterial binding, but in some cases this is not
evident. Desiccation cracks are commonly present as well as fenestrae. Beds are
0.1 – 0.5 m thick and show varying degrees of dolomitization. Rocks with these
characteristics are present in the Morelos Formation in the Axaxacoalco and
Zotoltitla´ n sections, while in the Barranca del Tigre section were not observed
(Figure 10).
Intertidal to subtidal (lithofacies 3 to 6, Table 1). Rocks assigned to this
subenvironment are bioclastic, intraclastic and / or peloidal wackestones to
grainstones with occasional subaerial exposure indicators (desiccation cracks,
fenestrae, early dolomitization). Organisms are mainly euryhaline (ostracods,
miliolids, rotaliids) and show low diversity, sometimes occurring in highfrequency, almost monospecific accumulations of benthic foraminifera. Molluscs
and green algae are also present. These rocks show varying degrees of
dolomitization. Bed thickness is usually 0.3 – 0.7 m. This facies association in the
Morelos Formation is dominant in the Zotoltitla´ n section and common in the
Axaxacoalco section. It is also present in the Barranca del Tigre section (Figure
10).
Subtidal (lithofacies 7 and 8, Table 1). This subenvironment is represented by
bioclastic wackestones and packstones, thoroughly bioturbated, and with a
diverse benthic association dominated by euryhaline organisms (ostracods,
miliolids, rotaliids, textulariids, other benthic foraminifera, molluscs, echinoderms, and green algae). Sponge spicules are common locally. Subaerial
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U. Herna´ ndez-Romano et al.
Table 1. Main lithofacies present in the Cenomanian of the Morelos Formation (lithofacies 1– 8)
and in the Turonian of the Mexcala Formation (lithofacies 9 – 12).
Guerrero-Morelos Platform drowning at the Cenomanian– Turonian boundary
675
Figure 10. Correlation of the subenvironments interpreted from facies associations. Intertidal :
lithofacies 1 and 2, Intertidal -subtidal : lithofacies 3-6, Subtidal : lithofacies 7 and 8, Open shelf :
lithofacies 9 and 10, Prodelta : lithofacies 11, Pelagic : lithofacies 12. See Table 1 for description of
lithofacies. The orientation is almost west-east. The datum is the Cenomanian – Turonian
boundary event that caused a marked change in fossil associations and sedimentary conditions.
See Figure 1 for location of the sections.
exposure features are absent. Beds are usually thicker (0.3 – 1.5 m) than the beds
of the intertidal – subtidal and intertidal – supratidal facies associations. Rocks with
these characteristics are dominant in the Morelos Formation in the Barranca del
Tigre section, common in the Axaxacoalco section, and infrequent in the
Zotoltitla´ n section (Figure 10).
3.2. Facies associations of the Mexcala Formation
Three main facies associations can be recognised in the measured portions of the
Mexcala Formation:
Open shelf (lithofacies 9 and 10, Table 1). The open shelf is represented by
argillaceous echinoderm / mollusc / calcisphaerulid wackestones and packstones
with nodular aspect. Benthic (biserial textulariids) and planktonic (globigerinids)
foraminifera are common as well as brachiopods, corals, ammonoids, codiacean
algae and sponge spicules. Also included in this subenvironment are rocks
constituted almost exclusively by calcisphaerulids, with just scarce echinoderm
fragments and planktonic foraminifera. Beds are usually 0.15 to 0.4 m thick.
This facies association, when present, occurs in the lower part of the Mexcala
Formation, directly overlying the Morelos Formation (Figure 10).
Prodelta (lithofacies 11, Table 1). Rocks assigned to this subenvironment
consist of silty claystones and shaly siltstones with scarce calcisphaerulids and
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U. Herna´ ndez-Romano et al.
benthic foraminifera (biserial textulariids). Silt-size monocrystalline quartz is the
dominant terrigenous component. Echinoderm and mollusc fragments are rare as
are planktonic foraminifera. Bed thickness is usually 0.1 – 0.3 m. The occurrence
of occasional woody fragments and the characteristics of the facies of the middle
and upper part of the Mexcala Formation in the eastern part of the study area
indicate that this facies corresponds to distal prodelta (Herna´ ndez-Romano,
1995). This facies association is present in the Mexcala Formation intercalated
with the open shelf facies in the upper part of the Zotoltitla´ n section (Figure 10).
Pelagic (lithofacies 12, Table 1). This subenvironment is represented by
dark-grey to black, laminated globigerinid / calcisphaerulid wackestones and
packstones. Keeled forms of planktonic foraminifera are very rare and radiolaria
are occasionally abundant. Bed thickness is usually less than 0.15 m. This facies
is present overlying the open shelf facies of the lower part of the Mexcala
Formation in the Barranca del Tigre and Zotoltitla´ n sections. In the Axaxacoalco
section it overlies directly the intertidal-subtidal facies of the Morelos Formation
(Figure 10).
4. Discussion
Our observations in the field indicate that structural deformation in the study
area did not affect the relative original position of the stratigraphic sections with
respect to each other. The total effect of compressional and extensional
deformation events is an ENE – WSW shortening. The lateral movements are
more difficult to evaluate, but major structures of this type do not cross the area
between the sections.
Taking into account these considerations, we have correlated the stratigraphic
sections (Figure 10). The datum in all of them is the latest Cenomanian – earliest
Turonian event that caused a marked change in the fossil association and
sedimentary conditions. The subenvironments represented by lithofacies associations have also been correlated, although this is to a certain extent more
speculative.
Below the Cenomanian – Turonian boundary event, all sections are represented
by shallow-marine limestones, with shallower and more restricted facies that
include intertidal and supratidal environments toward the east (Zotoltitla´ n
section), and subtidal environments in the west (Barranca del Tigre section).
Above the Cenomanian – Turonian boundary event, open-shelf carbonates are
present in the Barranca del Tigre and Zotoltitla´ n sections, while in the
Axaxacoalco section this facies is not present; instead, pelagic limestones directly
overlie restricted shallow-marine limestones. The occurrence of open shelf facies
in the Barranca del Tigre section and its absence in the Axaxacoalco section may
be due to topographic irregularities in the platform. The open shelf facies are best
developed in the eastern part of the study area where they locally contain
abundant normal marine benthos and are interdigitated with prodelta siltstones,
while in the central and western parts, when present, they are relatively thin
intervals with scarce benthic organisms. Open-shelf limestones in the Barranca
del Tigre and Zotoltitla´ n sections are overlain transitionally by pelagic limestones
with characteristics similar to those of the pelagic rocks in the Axaxacoalco
section; however, in the pelagic and upper open shelf rocks of the Zotoltitla´ n
section, woody fragments are relatively common.
The sequence of facies observed in the sections indicate that at least some
Guerrero-Morelos Platform drowning at the Cenomanian– Turonian boundary
677
parts of the Guerrero-Morelos Platform were drowned at the end of the
Cenomanian. At the same time, the conditions changed to open-marine in the
areas that remained in shallow water, but eventually, they also drowned.
The causes of this drastic change in sedimentary conditions in this part of
Mexico have not been explored by other workers. However, similar changes at or
close to the Cenomanian – Turonian boundary have been reported from other
parts of the country (Basa´ n˜ ez-Loyola et al., 1993) and the world (Jenkyns, 1991;
Burchette, 1993; Caus et al., 1993; Simo, 1993; Philip et al., 1995). Throughout
the geologic record, three main conditions (and combinations of them) have
arisen as causes of platform drowning: rapid subsidence, rapid rise in sea level,
and adverse environmental conditions for carbonate producing organisms
(Schlager, 1981). We now explore these factors and their possible effects in the
Guerrero-Morelos Platform.
4.1. Subsidence
The sedimentary record in this area indicates almost continuous subsidence since
the Aptian, allowing the deposition of the Huitzuco Formation (thickness
unknown, probably .100 m) and the Morelos Formation (more than 1000 m in
this area as indicated by some exploration wells). No major hiatuses or erosion
surfaces have been found nor reported in the Aptian – late Cenomanian sequence
of this area. Fries (1960) reported an unconformity between the Morelos (early
Cenomanian) and Cuautla (early Turonian) formations in the northern part of
the Guerrero-Morelos Platform; however, this unconformity has not been found
in more recent studies (Gonza´ lez-Pacheco, 1991).
Considering a static sea-level, subsidence fast enough to exceed sedimentation
rates in carbonate platforms can be related to active faulting (Schlager, 1981) or
to flexural subsidence in foreland basins (Dorobek, 1995). Rapid subsidence of
young or rejuvenated oceanic crust is not considered to produce platform
drowning (Schlager, 1981; Bergersen, 1995) and is not applicable to our case.
During active faulting, some parts of the platform subside while others remain
in shallow conditions. Shallow water sedimentation takes place in platforms
elongated in the direction of the structural trend, and pelagics and resedimented
carbonates are deposited on top of the shallow-marine deposits of the subsided
blocks. Features indicating this process have not been recognised in the area nor
reported from other parts of the Guerrero-Morelos Platform.
The effects of flexural subsidence could have been important in the demise of
shallow-marine carbonate sedimentation in the western part of the study area
(and probably of the entire Guerrero-Morelos Platform) and the establishment of
open, shallow-marine conditions in the east. During the Early Cretaceous, a
volcanic arc developed along the Pacific coast of southern Mexico, between the
Guerrero-Morelos Platform and the paleo-Pacific ocean. Whether this volcanic
arc was attached to the craton or was allochthonous and accreted to it is still
under discussion (Ramı´rez-Espinosa et al ., 1991; Tardy et al., 1992; Sa´ nchezZavala, 1993). This has important implications with respect to the tectonic
setting and evolution of the Guerrero-Morelos Platform since it could have
developed in a backarc basin or on a passive margin. The youngest volcanic rocks
in the arc are late Cenomanian (Sa´ nchez-Zavala, 1993), indicating an important
change in the tectonic conditions by this time that interrupted volcanic activity.
The Pacific margin of southern Mexico experienced compression from the west
that produced intense folding and thrusting during the latest Cretaceous – early
678
U. Herna´ ndez-Romano et al.
Tertiary. This compression was either due to the collision and accretion of the
island arc and the subsequent change in the vergence of subduction from
westward to eastward (Tardy et al., 1992), or to a change in the subduction
conditions in an already established eastward-verging subduction zone.
The timing of the beginning of the compression and deformation has not been
constrained. The high terrigenous content in the lower Turonian of the La
Esperanza area (Figure 1) (Aguilera-Franco, 1995) indicates that uplift and
erosion occurred eastwards (out of the study area) by early Turonian. However,
we have no evidence indicating that this uplift was due to the development of a
forebulge as a response to the initial phases of development of a thrust belt on the
Pacific side. The fact is that between this zone with uplift and the zone with the
volcanic and volcano-sedimentary rocks, part of the Morelos Platform was
drowned and the area closer to the uplifted area remained in shallow, openmarine conditions. Detailed stratigraphic and structural studies are needed to
support or reject this idea.
4.2. Rise in sea level
Rapid sea-level rise can produce the same effect as rapid subsidence. Pulses of
sea-level rise fast enough to surpass carbonate platform sedimentation rates are
those of glacial-eustatic origin. Despite some paleoclimatic models (Sloan &
Barron, 1990) and geologic evidence (Francis & Frakes; 1993) that indicate that
the climate during the Cretaceous was not as equable and stable as usually
thought, during the Cretaceous glacial-eustasy seems to have been negligible
(Morner, 1980; Hart et al. , 1993). Moreover, during the latest Cenomanian –
early Turonian, sea-level reached one of the highest levels in the geological record
(Haq et al ., 1988) and ocean surface-water temperatures were at a maximum
(Arthur et al ., 1985). In these conditions short-term, climate-driven sea-level
changes are expected to be of low amplitude (Read, 1995), with few possibilities
of drowning carbonate platforms by themselves. Nevertheless, Arthur et al.
(1987) estimated a rapid increase in the area of shelf and epicontinental seas
around the Cenomanian – Turonian boundary that could have been produced by
an equally fast rise in sea level.
Although world oceanic crust production was high (especially in the Pacific)
during the Cenomanian – Turonian, it was already in decline after a peak in the
middle Albian (Larson, 1991; Arthur et al ., 1985). For the late Cenomanianearly Turonian there are no reports of major pulses of oceanic volcanism to
account for sea-level rise as proposed by Tarduno et al . (1991) for the Aptian.
If short-term, high amplitude sea-level fluctuations did occur around the
Cenomanian – Turonian boundary, it is not clear what could produce them. We
believe that sea-level rise contributed to the drowning of the Guerrero-Morelos
Platform, but was the combination with other factors what ultimately caused the
demise of shallow-water carbonate sedimentation around the Cenomanian –
Turonian boundary?
4.3. Adverse environmental conditions
In optimum conditions, carbonate production can keep up with almost any
amount of tectonic subsidence or eustatic sea-level rise (Wilson, 1975).
However, adverse environmental conditions can negatively affect carbonate
producing organisms and abate carbonate sedimentation rates, forcing the
platform to retrograde and / or drown.
Guerrero-Morelos Platform drowning at the Cenomanian– Turonian boundary
679
The timing of the drowning in the stratigraphic sections (first in the west and
later in the east) and the continuation of shallow, open-marine carbonate
sedimentation in the east (Zotoltitla´ n section, Figure 10) indicates that the inflow
of terrigenous-clastic material coming from the east (Herna´ ndez-Romano, 1995)
was not an important factor in the drowning of some parts of the GuerreroMorelos Platform around the Cenomanian – Turonian boundary. However, the
terrigenous clastic input probably played an important role in determining the
drowning of the Zotoltitla´ n area later in the Turonian. The higher proportion of
clay in the lower part of the Mexcala Formation may be only apparent because
this material managed to occupy a greater proportion in these rocks once benthic
carbonate production was shut down by other factors. Clay was probably
supplied at the same rate during the late Cenomanian and earliest Turonian. A
real increase in the supply of clay and silt grade terrigenous material to the
western part of the platform probably took place later in the Turonian. It is
important to note that significant amounts of quartz sand and silt as well as clay
can be found in the upper part of the Morelos Formation in the Axaxacoalco
(75 – 82 m above the base, Figure 6) and Zotoltitla´ n (145 m above the base,
Figure 8) sections; nevertheless, ‘healthy’ carbonate sedimentation can be
inferred from rocks above these intervals.
Subaerial exposure occurred prior to the drowning in the Axaxacoalco section
and probably in the other sections. This subaerial exposure was not exceptionally
intense, but could help a fast relative rise in sea level to drown this part of the
platform by removing carbonate-producing organisms and therefore shutting
down carbonate production. Since subaerial exposure was a common event
during deposition of the Morelos Formation and the platform was not drowned
during subsequent floodings, in this case, and in the absence of other agents to
stop carbonate production, the crucial factor would be an exceptionally rapid
relative rise of sea level to submerge the platform below the optimum depth of
carbonate production (20 – 40 m) before this could resume (Schlager, 1989).
The invasion of oxygen-poor waters to the platform has been invoked as an
explanation for the demise of some carbonate platforms (Jenkyns, 1991;
Basa´ n˜ ez-Loyola et al., 1993). The deposition of black, organic-rich, laminated
sediments lacking bioturbation, and the absence of benthic organisms or the
presence of depauperate forms, indicates the existence of dysaerobic or anoxic
conditions during deposition (Schlanger et al., 1987). Numerous works have
documented the existence of an expanded oxygen-minimum zone (named
‘oceanic anoxic event (OAE)’ by Schlanger & Jenkyns, 1976) in the world oceans
at particular times during the Cretaceous. The most studied of these is the
Cenomanian – Turonian anoxic event (de Graciansky et al., 1984; Schlanger et
al., 1987; Jarvis et al. , 1988; Jenkyns, 1991; Caus et al. , 1993; Ulicä ny´ et al. ,
1993; Ross & Skelton, 1993; Peryt & Wyrwicka, 1993; Hart et al. , 1993).
The end of shallow-water carbonate sedimentation and the establishment of
pelagic conditions at a time when one of the highest peaks in sea-level was
reached during early Turonian (de Graciansky et al. , 1984; Schlanger et al.,
1987; Arthur et al., 1987; Ferrandini, 1988; Haq et al. , 1988; Jenkyns, 1991;
Hancock, 1993; Segura et al. , 1993), imply that these events are linked.
Schlanger & Jenkyns (1976) proposed that during transgressions, wide areas
formerly occupied by coastal plains and lowlands are invaded by marine water
considerably increasing the area and volume of shallow epicontinental and
680
U. Herna´ ndez-Romano et al.
marginal seas favourable for life. This leads to an increase in organic activity and
the production of organic carbon, and therefore an increase in demand for
oxygen in the water column (by aerobic organisms and for degradation of organic
matter). Besides this, the existence of an equable global climate during the
Cretaceous did not allow well-oxygenated polar water to replace oceanic anoxic
water. These factors favoured the formation of an expanded oxygen-minimum
layer (Schlanger & Jenkyns, 1976).
An alternative theory by Arthur et al. (1987) attributes the expansion of the
oxygen minimum zone to the enhanced production of warm saline bottom water
(WSBW) in the shallow shelves and epicontinental seas created by the
Cenomanian – Turonian transgression. This WSBW formed in shallow shelves of
areas with net evaporation, became both more saline and more dense, and sunk
to varying depths of the ocean depending on the density contrast and volumes.
The higher temperature of these water masses did not allow them to carry the
same amounts of dissolved oxygen, so they became anoxic sooner than polar
waters do today, given enough residence time (Brass et al., 1982). With the
increase in the area of shelves and epicontinental seas, rates of WSBW increased
as did the rates of upwelling of deep oceanic waters, increasing sea surface
fertility and productivity, which in turn could cause short-term expansion and
intensification of a mid water oxygen-minimum zone (Arthur et al., 1987).
The pelagic rocks of the lower part of the Mexcala Formation are dark-grey to
black in colour, possibly due to the high content of organic-matter. These rocks
commonly show millimetre-scale parallel lamination indicating absence of
bioturbating organisms during and after deposition. Close to the Cenomanian –
Turonian boundary, benthic flora and fauna are drastically diminished (Figures
4, 6) or replaced temporarily by an open marine assemblage (Figure 8) which
also shows a paucity shortly after the establishment of the open-marine
conditions, then a slight recovery, but finally also disappears from the sediments
(Zotoltitla´ n section, Figure 8). Some benthic organisms that persist and are
widespread in the Mexcala Formation (biserial textulariids) have been reported
as tolerant of low oxygen levels (Koutsoukos et al., 1990). Other benthic
foraminifera that also persist from the Morelos Formation occur in the Mexcala
Formation but show a significant reduction in the size of the test. All these
characteristics indicate that oxygen-poor (probably anoxic) waters affected the
Guerrero-Morelos Platform close to the Cenomanian – Turonian boundary.
Paleoclimate models for the mid-Cretaceous (Barron, 1985; Arthur et al.,
1987) predict a zone of upwelling along the Pacific side of southern Mexico, a
factor that could help to raise the upper surface of the oxygen-minimum zone to
impinge in shallow areas of the Guerrero-Morelos Platform.
Paleomagnetic studies in the Morelos Formation in an area close to the
Barranca del Tigre section, indicate a paleolatitude of 28 Ú 38N for the
Albian – Cenomanian, and also indicate that the terrane on which the GuerreroMorelos Platform was developed has not experienced significant latitudinal
translations relative to the North American craton since the mid-Cretaceous
(Bo¨ hnel et al., 1989; Urrutia-Fucugauchi, 1988). According to the North
American apparent polar wander path, the latitude of North America (and of the
Guerrero-Morelos Platform) has decreased since the Cretaceous. Taking into
account that Cretaceous carbonate platforms have been reported from higher
latitudes than 308N, the drift of the Guerrero-Morelos platform to higher
latitudes as a cause for the demise of shallow-marine sedimentation is unlikely.
Guerrero-Morelos Platform drowning at the Cenomanian– Turonian boundary
681
We suggest that the drowning of the western part of the Guerrero-Morelos
Platform and the change to open, shallow-marine conditions in the eastern part
was mainly caused by the interplay of the Cenomanian – Turonian oceanic anoxic
event and the subsidence profile of the platform. During the late Cenomanian,
shallow-marine conditions with varying degrees of restriction dominated the
whole area (Figure 11A). Subtidal conditions dominated in the west and
intertidal in the east; however periodic subaerial exposure occurred even in the
west-central parts of the area (Axaxacoalco section). During the latest
Cenomanian – earliest Turonian, and after a short period of subaerial exposure in
some areas (Figure 11B), oxygen-poor waters probably invaded the western parts
of the Guerrero-Morelos Platform with the rise in sea level (Figure 11C). These
oxygen-poor waters impinged over some parts of the platform causing the demise
of benthic organisms that were the main carbonate-sediment producers during
the Cenomanian. The elevation of the upper limit of the oxygen-minimum zone
was probably aided by upwelling along the Pacific side of the platform, so that
oxygen-poor waters could invade the subtidal parts of the platform. The demise
of the carbonate-producing benthos drastically reduced sedimentation rates,
allowing the relative rise in sea level to take the sediment surface to depths where
carbonate production could not resume. This was more easily achieved in areas
previously exposed to subaerial conditions, since they were devoid of carbonateproducing organisms.
Another factor that could have influenced the early drowning of the western
parts of the platform was probably an enhanced subsidence of the western part
and uplift of the eastern areas out of the study area as a flexural response to the
initiation of a thrust belt along the Pacific margin. As benthic organisms were
eradicated, deposition was limited to the skeletons of planktonic (calcisphaerulids, planktonic foraminifera, and radiolaria) and nektonic (Saccocoma sp.)
organisms living in the oxygenated surface waters, and wind blown clay- and
silt-sized material, even in shallow environments.
In the western part of the study area (Axaxacoalco and Barranca del Tigre
sections), the adverse conditions lasted until the shelf was below the euphotic
zone and carbonate sedimentation could not be restored once the anoxic
conditions retreated. When this part of the shelf was drowned, the conditions in
the eastern part (Zotoltitla´ n section) changed from restricted shelf to open shelf
(most probably a ramp) once the western restriction disappeared (Figure 11D).
The Zotoltitla´ n region also underwent a drastic change in the fossil assemblage
from benthic foraminifera-dominated to calcisphaerulid / echinoderm-dominated
(Figure 8, above 180 m). Oxygen-poor waters seem to have been less noxious in
this area where, despite the disappearance of most species of Cenomanian
benthic foraminifera, they were replaced by new benthic organisms that were
tolerant to low-oxygen levels or better adapted to the open marine conditions. A
reduction in sedimentation rates in the earliest Turonian can also be inferred
from the reduction in bed thickness and this possibly caused an increase in depth,
but this deepening was less than in the western part, or anoxic conditions were
not very severe and permitted shallow carbonate sedimentation to resume (Figure
8, 230 – 290 m above the bottom). However, a subsequent increase in the inflow
of terrigenous clastic material to this part of the platform, and probably a new
impingement of oxygen-poor waters, again reduced carbonate-producing organisms and drowned the eastern parts of the platform by middle Turonian times
(Figure 11E).
682
U. Herna´ ndez-Romano et al.
Figure 11. Proposed model for the drowning of the Guerrero-Morelos Platform. Arrows mark the
location of the stratigraphic sections: BT, Barranca del Tigre; A, Axaxacoalco; Z, Zotoltitla´n. (A)
Conditions that predominated during the Cenomanian, with a relatively deep oxygen-minimum
zone (O2 min). (B) During the late Cenomanian, the oxygen-minimum zone was expanded and
became shallower; prior to the drowning, subaerial exposure took place in some parts of the
platform. (C) With the rise in sea level, oxygen-poor waters impinged over the platform,
exterminating the carbonate-producing benthos. (D) Subsidence and sea-level rise drowned the
platform after carbonate-production was shut down. Open marine deposition started in the east.
(E) Terrigenous clastic supply and a new impingement of oxygen-poor waters also caused the
demise of shallow-marine sedimentation in the eastern parts. In all three stages subsidence was
greater in the west than in the east and this profile was probably enhanced in the postCenomanian. See text for detailed explanation.
Guerrero-Morelos Platform drowning at the Cenomanian– Turonian boundary
683
6. Conclusions
The facies successions (shallow, restricted marine to pelagic or shallow, restricted
marine to open marine and then to pelagic) in stratigraphic sections from the
central part of the Guerrero-Morelos Platform indicate progressive drowning of
the platform close to the Cenomanian – Turonian boundary.
We attribute the drowning of some parts of the Guerrero-Morelos Platform to
the occurrence of the Cenomanian – Turonian anoxic event. The impingement of
anoxic waters over the platform, possibly aided by upwelling along the Pacific
margin, could have produced the drastic reduction and changes in the carbonate
producing benthos observed in the stratigraphic sections, and therefore a
reduction in carbonate accumulation rates. Subsidence and the late
Cenomanian – earliest Turonian sea-level rise were then able to drown the
platform. This occurred first in the western part, where subtidal conditions
dominated and an irreversible drowning occurred, allowing the deposition of
organic-rich pelagic sediments over pre-existing shallow-marine carbonates. An
enhanced subsidence of the western part could also have contributed to the faster
drowning of this area. At the same time, the eastern part, where intertidal
conditions dominated, changed to open-marine conditions, shallow first and deep
later. The fossil assemblage in this area also reflects adverse conditions, possibly
because of low-oxygen levels. Here, a temporal restoration of shallow openmarine carbonate sedimentation resulted, but eventually this region was also
drowned, probably by the interplay of terrigenous-clastic supply to the platform
and a new impingement of anoxic waters.
Acknowledgements
This work was sponsored by the Area de Exploracio´ n de Recursos Energe´ ticos
del Subsuelo, Divisio´ n de Estudios de Posgrado, Facultad de Ingenierı´a,
UNAM. We thank Patricia Padilla-Avila and M. Antonieta Sa´ nchez-Rios who
kindly studied the planktonic foraminifera of some samples from the Zotoltitla´ n
and Axaxacoalco sections. We thank J. L. Wilson, W. V. Sliter and an
anonymous reviewer for their corrections, comments and suggestions which
greatly improved this paper.
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