Boletín de la Sociedad Geológica Mexicana

Volumen 70, núm. 1, 2018, p. 241 ‒ 280

 Concentraciones fosilíferas del Kimmeridgiano Superior en Mazapil, Zacatecas, México: Tafonomía e interpretación paleoambiental

Iriliana López-Caballero1,*, Federico Olóriz2, Ana Bertha Villaseñor3

1 Posgrado en Ciencias de la Tierra, Sede Instituto de Geología, UNAM, Ciudad Universitaria, C.P. 04510, CDMX, México.
2 Departamento de Estratigrafía y Paleontología, Facultad de Ciencias, Universidad de Granada, Av. Fuentenueva s/n, C.P. 18002, Granada, España.
3 Departamento de Paleontología, Instituto de Geología, UNAM, Ciudad Universitaria, C.P. 04510, CDMX, México.

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A 410 cm thick succession of late Kimmeridgian deposits belonging to the La Caja Fm. has been analyzed in the San Matias Canyon as preliminary approach to the study of fossil concentrations in the Upper Jurassic of Mexico.

Lithostratigraphic, sedimentologic, taxonomic, biostratigraphic and taphonomic observations based on a bed-by-bed sampling with centimeter scale resolution. Taxonomic and biostratigraphic analyses focused on macroinvertebrate assemblages, mainly ammonites. The taphonomic analysis applied to 1175 fossil remains of neritic communities (ammonites 53 %, aptychi 27 %, bivalves 18 % and gastropods 2 %), accumulated in depths lesser than those estimated for implosion of ammonite carcasses.

Within a shelf-area without evidence of high-irregular seabeds, an eco-sedimentary context of low energy agrees with depositional conditions resulting in lithofacies of floatstone-wackestone of bioclasts and wackestone of radiolaria, together with local patches of packstone and the intercalation of calcareous siltstone. In addition, low energy conditions are in accordance with the absence of expressive erosion surfaces, synsedimentary sliding, exotic clasts, and sedimentary structures typically related to wave action, as well as with the common occurrence of parallel and/or subtle oblique laminations. Ecological conditions included disaerobic-poikiloaerobic seabeds with rare record of benthos in situ, common ferruginization related to organic matter-rich sediments, and water column instabilities affecting pelagic and benthic communities resulting in increased accumulation of skeletals remains on the seabed.

The recorded fossiliferous concentrations are more evident to the bottom and top surfaces of beds, they are discontinuous, and their macroscopic biofabric corresponds to pavements and shell beds. These fossil-rich horizons are interpreted as distal eventites resulting from turbulence events of variable effective energy (hurricanes, storms, storm-forced bottom currents, and winnowing), the influence of which reached proximal sectors of the outer shelf at depths close to the storm wave base. Incomplete sequences related to reworking are interpreted as resulting from events of higher-than-background effective energy and over-imposition of depositional horizons. The pattern of the type sequence of re-deposition related to the succession of higher-than-background energy events, progressive but rapid dissipation of energy, and the return to background eco-sedimentary conditions.

The eco-sedimentary dynamics proposed for the succession analyzed at the San Matias Canyon, Mazapil, Zacatecas, indicates pulses of paleoenvironmental instability within a neritic shelf, and with recurrence periods shorter than the range of biostratigraphic resolution. No evidence of high-irregular seabeds has been found in the surrounding area nor the occurrence of the shelf-break and slope in the area during the investigated stratigraphic interval of late Kimmeridgian age.

Keywords: Shell concentrations, macroinvertebrates, eventites, Upper Jurassic, Kimmeridgian, Mexico.