The Sierra Madre Formation is a

geologic formation A geological formation, or simply formation, is a body of rock having a consistent set of physical characteristics ( lithology) that distinguishes it from adjacent bodies of rock, and which occupies a particular position in the layers of rock exp ...

Chiapas Chiapas (; Tzotzil and Tzeltal: ''Chyapas'' ), officially the Free and Sovereign State of Chiapas ( es, Estado Libre y Soberano de Chiapas), is one of the states that make up the 32 federal entities of Mexico. It comprises 124 municipalities ...

state, southern

Mexico Mexico ( Spanish: México), officially the United Mexican States, is a country in the southern portion of North America. It is bordered to the north by the United States; to the south and west by the Pacific Ocean; to the southeast by Guate ...

. It consists of marine

dolomite Dolomite may refer to: *Dolomite (mineral), a carbonate mineral *Dolomite (rock), also known as dolostone, a sedimentary carbonate rock *Dolomite, Alabama, United States, an unincorporated community *Dolomite, California, United States, an unincor ...

s and

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms wh ...

s. The formation dates to the Middle Cretaceous, spanning from the

Aptian The Aptian is an age in the geologic timescale or a stage in the stratigraphic column. It is a subdivision of the Early or Lower Cretaceous Epoch or Series and encompasses the time from 121.4 ± 1.0 Ma to 113.0 ± 1.0 Ma (million years ag ...

of the

Early Early may refer to: History * The beginning or oldest part of a defined historical period, as opposed to middle or late periods, e.g.: ** Early Christianity ** Early modern Europe Places in the United States * Early, Iowa * Early, Texas * Ea ...

to the Cenomanian of the

Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the younger of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''creta'', ...

. The dolomites, dolomitic breccias and limestones of the formation have been deposited in a

lagoon A lagoon is a shallow body of water separated from a larger body of water by a narrow landform, such as reefs, barrier islands, barrier peninsulas, or isthmuses. Lagoons are commonly divided into '' coastal lagoons'' (or ''barrier lagoons' ...

al to

estuarine An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime environmen ...

depositional environment, environment and contain many fossil fish, flora and rudists, typical reef-building organisms of the Cretaceous. The formation rests on top of the Santiago Formation, Chiapas, Santiago and San Ricardo Formations, and is overlain by the Campanian to Maastrichtian Ocozocoautla Formation. The thickness of a composite section of the formation amounts to .

Description

The Sierra Madre Formation was first formally described by Gutiérrez Gil in 1956, but previously reported by other authors (Böse, 1905; Ver Wiebe, 1925; Müllerried, 1936; Imlay, 1944). The formation was studied in more detail and subdivided in several members (Chubb, 1959; Sánchez-Montes de Oca, 1969; Zavala-Moreno, 1971; Castro-Mora et al., 1975; Michaud, 1987; Quezada-Muñetón, 1987). A composed thickness of was suggested by Steele and Waite (1986) for the Sierra Madre Formation, subdivided into 21 lithofacies. The lowermost lithofacies corresponds to the stratigraphic level of El Espinal quarry, defined by Steele and Waite (1986) as dolomite (rock), dolomite and dolomite (mineral), dolomitic breccia, located between from the base of the formation, which rests conformably on top of the :Upper Jurassic Series of North America, Late Jurassic to Early Cretaceous San Ricardo Formation. The top is covered unconformity, unconformably by the :Campanian Stage, Campanian to :Maastrichtian Stage of North America, Maastrichtian Ocozocoautla Formation. The depositional environment is described as an environment with high oxygen concentration and high primary productivity with sporadic influence of strong waves and/or currents in a brackish marginal marine environment; shallow lagoon or estuary.

Fossil content

The formation preserves fossil fish, flora and marine invertebrates dating back to the Cretaceous Period (geology), period, ranging from the :Aptian Stage, Aptian of the :Lower Cretaceous Series of North America, Early Cretaceous to the :Cenomanian Stage, Cenomanian of the :Upper Cretaceous Series of North America, Late Cretaceous.Sierra Madre Formation, Chiapas
at Fossilworks.orgEl Espinal Quarry
at Fossilworks.org The fossils in El Espinal quarry were found in finely laminated orange clay layers interbedded with dolomitic limestone and interbedded with relatively thick layers of cream limestone that range from , with some layers showing ripples, desiccation cracks, algal mats, and flat-pebble conglomerates. Similar fossils have been found in the Tlayúa Formation, of :Albian Stage, Albian age in Puebla State, Puebla, southern Mexico, as fishes, one odonate nymph and isopods.Vega et al., 2006, p.324

Fish

Fossils of ''Archaeochiapasa, Archaeochiapasa mardoqueoi'' and ''Pepemkay'', a Cenomanian prehistoric ray-finned fish, were found in the formation. Other prehistoric fish fossils, found in quarries near the Municipality of Ocozocoautla de Espinosa, include Macrosemiidae, Macrosemiids (''Macrosemiocotzus'' species), Clupeomorpha, Clupeomorphs (''Triplomystus, Triplomystus applegatei'' and ''Paraclupea''-like species), and Alepisauriformes (''Saurorhamphus'' and ''Enchodus'' species).Alvarado Ortega et al., 2009

Rudists

These rudists are reported from the formation: * ''Caprinuloidea, Caprinuloidea sp.'' * ''Coalcomana, Coalcomana sp.'' * ''Kimbleia, Kimbleia sp.'' * ''Radiolites, Radiolites sp.'' * ''Requienia (bivalve), Requienia sp.'' * ''Sauvagesia, Sauvagesia sp.'' * ''Texicaprina, Texicaprina sp.'' * ''Toucasia, Toucasia sp.''

Flora

The following flora were found in the formation: * ''Ephedra (plant), Ephedra'' * ''Arecipites, Arecipites sp.'' * ''Osmunda, Osmunda sp.'' * ''Selaginella, Selaginella sp.'' * ''Sphagnum, Sphagnum sp.'' * Bombacaceae, Bombacaceae indet. * Lauraceae, Lauraceae indet. * Moraceae, Moraceae indet.

References

Bibliography

* * {{citation , first1=F.J. , last1=Vega , first2=P. , last2=García Barrera , first3=M. Carmen , last3=Perrilliat , first4=M.A. , last4=Coutiño , first5=R. , last5=Mariño Pérez , year=2006 , title=El Espinal, a new plattenkalk facies locality from the Lower Cretaceous Sierra Madre Formation, Chiapas, southeastern Mexico , url=https://www.researchgate.net/publication/28131471 , journal=Revista Mexicana de Ciencias Geológicas , volume=23 , pages=323–333 , accessdate=2020-03-16