Impact-Induced Sediments at the K-T Boundary: Offshore Campeche and Chiapas-Tabasco Region, Southeastern Mexico
Published:December 01, 2007
- PDF LinkChapter PDF
J.M. Grajales-Nishimura, 2007. "Impact-Induced Sediments at the K-T Boundary: Offshore Campeche and Chiapas-Tabasco Region, Southeastern Mexico", The Paleogene of the Gulf of Mexico and Caribbean Basins: Processes, Events and Petroleum Systems, Lorcan Kennan, James Pindell, Norman C. Rosen
Download citation file:
An important carbonate breccia interval represents a significant portion of the Cretaceous-Tertiary (K-T) boundary sedimentary succession accumulated under deep-water conditions in the western part of the Yucatan platform, offshore Campeche, during the Chicxulub impact event. The K-T boundary sedimentary succession is located approximately 300 km westward from the center of the Chicxulub structure. This sedimentary succession consists of a single graded deposit subdivided into three main units that from base to top includes: (1) a basal 50 to 300 m-thick, coarsegrained carbonate breccia; (2) a 10 to 20 m-thick, finegrained carbonate breccia, and; (3) a 25 to 30 m-thick, interval of sand and silt to clay-sized constituents, mostly abundant ejecta material. Additionally, a 10-20 m-thick, fine-grained calcareous breccia is recognized within the ejecta material-rich layer (unit 3) in some wells. The K-T boundary sedimentary succession is bounded at its base by a deep-water, shaly-calcareous facies of Upper Maastrichtian age and at its top by similar rocks of lower Paleocene age. Similar sedimentological characteristics and stratigraphic relationships are observed in analog outcrops in the Sierra de Chiapas (El Guayal, State of Tabasco; and Bochil, Chilil and Soyalό, State of Chiapas). Lithoclasts of the calcareous breccias are derived dominantly from platform-interior and platform-margin environments and only a few from deep-water settings. Ejecta material in unit 3 includes: shocked quartz, quartz with ballen structure, shocked plagioclase, altered melt rock, and rare pelitic schist fragments. Wireline log data, distribution, and stratigraphic relationships indicate a base-of-slope apron geometry for the thick carbonate breccia deposit.
The stratigraphic architecture and distribution of impact material within the K-T boundary sedimentary succession suggest the following sequence of events and products that probably occurred within a very short time span following the Chicxulub impact:
Unusually strong seismic shaking induced the collapse of the platform margin, resulting in an enormous debris flow (units 1 and 2),
Arrival and deposition of ballistic impact ejecta (unit 3), and
Reworking and deposition, possible induced by tsunami currents (carbonate breccia within unit.
Units 1 and 2 represent the most important oil reservoirs at the Campeche Bay oil fields, and unit 3 is the seal layer. Unit 4 is a dark clay bed deposited during a global decrease in ocean productivity following the meteorite impact.
The offshore zone along the western margin of the Yucatan platform, also known as the Bay of Campeche, is currently the most prolific oil-producing province in southeastern Mexico (Fig. 1; Meneses de Gyves, 1980; Santiago-Acevedo, 1980; Santiago-Acevedo et al., 1984). The Cantarell Field is the largest oil field in Mexico. By 1999, this field had produced more than 6,934 million barrels of oil and 2,954 billion cubic feet of gas, and it contains additional recoverable reserves of 10,176 million barrels of oil and 5,169 billion cubic feet of gas (Pemex Exploratiόn y Productiόn, 1999). Production comes from three stratigraphic levels including Upper Jurassic limestone, Cretaceous carbonates, and Upper Cretaceous-Paleocene calcareous breccias (Pemex Exploratiόn y Productiόn, 1999). About 70% of the total current production in the Cantarell Field comes from the carbonate breccias. These carbonate breccias are part of a thick sedimentary succession capped by a horizon rich in ejecta constituents. Grajales-Nishimura et al., (2000) assigned a K-T boundary age for the oil-producing breccias of offshore Campeche and the outcrop analogs from the Sierra de Chiapas, based on detailed biostratigraphy of core and outcrop samples, and linked its deposition to the Chicxulub impact event.
The purpose of this paper is to provide geological information about the K/T boundary carbonate breccias in southeastern Mexico, and from oil fields from the Campeche Sound. This paper deals with stratigraphic and sedimentological observations and petrographic analysis of the representative carbonate microfacies of the K-T boundary calcareous breccia. We have examined samples of the K-T boundary calcareous breccias from outcrops at El Guayal, Tabasco, Bochil, Chilil,
Soyalό, Chiapas, and some cores from oil fields. We studied excellent outcrop sections from the Sierra de Chiapas, whereas the oil fields are located offshore Campeche (Figs. 1 and 2). We used fossil content, inorganic carbonate grain constituents, depositional texture, and major diagenetic features to characterize carbonate microfacies within the lithoclasts. These data point toward diverse depositional settings, including both shallow- and deep-water environments.
Diverse carbonate microfacies were recognized within the lithoclasts of the K/T boundary calcareous breccias from El Guayal, Bochil, Chilil, Soyalό, State of Tabasco, Chiapas, and oil fields in Campeche Sound. Identification of the major microfacies types in the carbonate breccias is based mainly on their fossil assemblage, non-skeletal carbonate constituents, sedimentary structures, and major diagenetic features.
We grouped the descriptions of the carbonate microfacies according to their depositional environments, including inner-platform (e.g., lagoon and tidal flats), platform-margin, and deep-water settings. We observed no systematic vertical trend regarding the distribution and abundance of lithoclast types in the carbonate breccias at any location.
Inner-platform microfacies include: (1) miliolidpeloid wackestone and packstone; (2) alveolinid packstone and wackestone; (3) orbitolinid peloidal-skeletal packstone; (4) macroforaminifer-algal packstone; (5) lime mudstone and wackestone containing fenestrae and cryptmicrobial laminites; (6) Microcodium-bearing lime mudstone and packstone; (7) dolostone, and; (8) dolomitized evaporites.
Platform-margin carbonate microfacies types include: (1) skeletal-peloidal grainstone; (2) macroforaminifer grainstone, and; (3) rudist fragment microfacies.
Deep-water carbonate microfacies are represented by lime mudstone and wackestone with planktonic foraminifers.
Impact related materials
Within the upper units of the K-T boundary sequence, we found several grains of feldspar and quartz and other objects that clearly show impact related characteristics. The uppermost subunit contains impact material which has been affected by shock metamorphism, such as quartz with planar deformation features and ballen structure. Clay minerals represent altered melt glass components, although in some cores pristine glass was also identified. Additionally, we found impact related accretionary lapilli and calcified glass microspherules (altered microtektites). Accretionary lapilli are round bodies formed by the accretion of mineral and lithic fragments, resulting in a very finegrained outer crust and a coarser grained core. The glass spherules have been altered to calcite; they are spherical and normally vesicular.
Age of the K-T boundary sedimentary succession
Both in outcrops and subsurface, the K-T boundary graded sedimentary succession is sandwiched between Late Cretaceous limestone below and lower Paleocene marl and limestone above (Fig. 2). In the subsurface of Campeche Bay, the Late Cretaceous and lower Paleocene facies are pelagic and contain planktonic foraminifers, indicating a deep-water deposition environment. In the Sierra de Chiapas and Tabasco, the K-T boundary sequence has been deposited within different settings, such as terrestrial, inner-platform, outerplatform, slope, and basin environments. The biostratigraphy of deep-marine deposits of the K-T boundary successions has been published elsewhere and they have been correlated with sequences of the same age from La Ceiba in Veracruz, México, and El Kef (Tunisia) (e.g., Arenillas et al., 2000, 2002, and 2006). However, other authors claim that those deposits are not K-T boundary in age and that they are not related to the Chicxulub impact (e.g. Stinnesbeck et al., 2001; Keller et al., 2003).
In summary, based on the stratigraphy published by Grajales-Nishimura et al., 2000; Murillo-Muñetόn et al., 2002; Grajales-Nishimura et al., 2003, and the biostratigraphic work of Arenillas et al., 2000; 2002 and 2006, we conclude that the graded carbonate sequence that rests on Late Cretaceous limestone and underlies lower Paleocene marine sediments is of K-T boundary age (65 Ma).
Mechanism of sedimentation
The K-T boundary calcareous breccias in the Bay of Campeche have previously been interpreted as slope deposits accumulated along the western margin of the Yucatan platform (Meneses de Gyves, 1980; Santiago-Acevedo et al., 1984). Several lines of stratigraphic, paleogeographic, and petrologic evidence support this interpretation. For instance, the sedimentary succession that includes the calcareous breccias under discussion lies between deep-water Maastrichtian and Paleocene sediments. Late Cretaceous paleogeographic reconstructions of southeastern Mexico and offshore Campeche are consistent with an outer platform setting during that time (Meneses de Gyves, 1980). The finingupward trend of the K-T boundary carbonate sedimentary succession resulted from gravity-controlled sedimentation. Additional supporting evidence is the similar stratigraphic architecture displayed by the K-T boundary sedimentary succession in the Sierra de Chiapas at El Guayal and Bochil (Grajales-Nishimura et al., 2000) as well as in western Cuba (Kiyokawa et al., 1999; Takayama et al., 2000).
Gravitational deposition of thick calcareous breccia successions in slope or base-of-slope settings can be induced by several mechanisms. Metastable conditions at the platform margin can result from thick, rapidly deposited sedimentary accumulations (Cook, 1983; Cook and Mullins, 1983; Mullins, 1983; Read, 1985; Coniglio and Dix, 1992). Storm surges, tsunami activity, and seismic shocks can trigger carbonate debris flows into deep-water settings (Cook, 1983; Cook and Mullins, 1983; Mullins, 1983; Coniglio and Dix, 1992). Deposition of debris flows and carbonate turbidites in deep-water environments occurs more commonly during relative sea-level lowstands when the platform margin is eroded by massive slope failure (Sarg, 1989). Spence and Tucker (1997) have proposed that overpressure by pore water confined aquifers, porous and permeable facies, within lime mudstone facies can induce slope failure, leading to thick carbonate breccia deposition in deep-water environments during relative sea-level lowering. On the other hand, high carbonate sedimentation rates associated with platform flooding during rising of relative sea level apparently provide more suitable conditions to transport large amounts of carbonate sediments to deep-water settings (e.g. the “highstand shedding” principle; Mullins, 1983; Schlager, 1992).
We interpret the K-T carbonate sedimentary succession found in southeastern Mexico and in the offshore Campeche oil fields as the result of the collapse of the western Yucatan platform margin and the carbonate platform sequence of the Sierra de Chiapas area. Extraordinary severe seismic activity resulting from the Chicxulub meteorite impact in northern Yucatan at the K-T boundary is postulated as the trigger mechanism for this deposition event (Bralower et al., 1998; Grajales-Nishimura et al., 2000). A similar origin has also been proposed for the carbonate breccias of K-T boundary age in western Cuba (Takayama et al., 2000). Within this geologic framework, the K-T boundary sedimentary succession represents a complex interaction of gravity-driven deposition associated with ballistic sedimentation. The basal coarse-grained and fine-grained carbonate breccias of the succession, units 1 and 2, represent, respectively, the main base-of-slope carbonate apron and its waning portion. It is likely that a significant proportion of lithoclasts have been transported and deposited in the offshore area by a ballistic mechanism. The ejecta layer, unit 3, is also interpreted as the result of ballistic sedimentation (Grajales-Nishimura et al., 2000). Additionally, the calcareous breccia (3a) interbedded within unit 3 at some fields (see Fig. 2A) indicates unstable conditions at the platform margin, perhaps due to the enormous amount of carbonate material instantaneously removed from different parts of the platform margin. Therefore, the carbonate breccia (3a) within unit 3 is interpreted as a gravity-driven flow, likely induced by tsunami-generated currents from the unusual seismic event. Unit 4 is a dark clay bed deposited during a global decrease in ocean productivity after the meteorite impact (Hsü, 1985). An Iridium anomaly is reported at the base of this dark clay bed (Montanari et al., 1994).
Diversity in lithoclast composition in the K-T boundary calcareous breccias varies from one place to another. It is clear that lithoclast distribution in unit 1 and unit 2 calcareous breccias is not homogeneous. For instance, lithoclasts from some oil fields are predominantly inner-platform facies, such as lime mudstone, wackestone, and packstone containing abundant benthic foraminifer, peloids, and other skeletal shallow-water components. Platform-margin microfacies are subordinate and deep-water microfacies, such as lime mudstone to wackestone containing planktonic foraminifers, are minor. It is important to note that documentation of microfacies types in the oil fields depends on available core samples, and it is quite possible that some carbonate microfacies have not been sampled in cores.
In the stratigraphic section exposed at El Guayal, most lithoclasts are from inner-platform and platformmargin settings, comprising microfacies such as packstone and grainstone containing abundant benthic foraminifers, rudist fragments, and other shallow-water bioclasts. However, significant amounts of deep-water lithoclasts are present, mainly in the middle part of the section. In the Bochil section, shallow-water lithoclasts and abundant rudist fragments dominate the calcareous breccias. However, in contrast to the other two studied locations, the calcareous breccias in Bochil have significant matrix content that includes numerous planktonic microfossils.
It is clear that the diversity of the microfacies documented in K-T boundary calcareous breccias of southeastern Mexico and the Campeche oil fields is extensive and provides insights into the platform architecture. The abundance of lithoclasts derived from platform margin settings suggests the presence of a high-energy facies tract dominated by skeletal and nonskeletal sand shoals and possibly isolated rudist buildups. Behind the high-energy facies tract, a wide lagoon existed where a variety of inner-platform facies were accumulated. The predominant abundance of lithoclasts derived from shallow-water facies in the K-T boundary calcareous breccias is an important factor in determining the reservoir quality of the main producing stratigraphic interval in the offshore oil fields. The diagenetic potential of these shallow-water lithoclasts has led to the development of excellent reservoirs during subsequent severe and extensive dissolution and dolomitization under deep-burial conditions in some oil fields.
The stratigraphic, sedimentological, mineralogical, and biostratigraphic data indicate that the carbonate sequence that rests on Late Cretaceous limestone and underlies lower Paleocene marine sediments is of K-T boundary age. It represents a single graded gravitational deposit triggered by the Chicxulub impact. The provenience of the carbonate lithoclast is mainly from innerand margin-platform facies, although outerplatform facies are also present as lithoclasts, or as lime mud sediment in the matrix of the carbonate breccia.
Supported by Instituto Mexicano del Petrόleo project YNF D.01003 and project CGL2004-00738 of the Spanish Ministerio de Ciencia y Tecnologia.