APON, Formación
VALIDO
CRETACICO (Aptiense-Albiense)
Estado Zulia
Consideraciones Históricas: Kiser (1989-a) interpreta la presencia de Apón en el pozo Milagros Sur-1X del área de Burgua en el intervalo 3265 m-3451 m (10708'-11322' P:F:). Heybroek (1953), en la depresión de Táchira, cartografió los Miembros Tibú y Guaimaros (Formación Apón). Fierro y Useche (1985) también describieron a la formación el área Nula de la Depresión Táchira. Esto sería la primera mención publicado de la presencia de Apón en el subsuelo de la cuenca de Barinas-Apure.
Descripción litológica: La secuencia del pozo Milagros Sur-1X se compone, en los 80 m superiores, de areniscas grises a blancas de grano fino, friables, calcáreas, glauconíticas, con algunas calizas delgadas micríticas de color marrón a crema; los 107 m inferiores, hasta la profundidad final del pozo, consiste en una secuencia de calizas delgadas a gruesas (hasta 6 m de espesor), cremas y marrones, microcristalinas, calcareníticas, glauconíticas y piríticas, con intercalaciones de lutita gris a marrón oscura, fisible, astillosa, calcárea, micácea, pirítica y con vetillas de calcita. La sección de Heybroek (op. cit.) se compone de 80 m de calizas grises, arenosas, glauconíticas y fosilíferas, seguido arriba por 60 m de lutita arenosa gris. Fierro y Useche (op. cit.) la describieron como "calizas biostrómicas, masivas, cristalinas, nodulares, coquinosas y margosas; lutitas y limolitas negras, fosilíferas, localmente micáceas, con concreciones elipsoidales de caliza negra, comúnmente, varían entre "wackstone", "packstone" y grainstone".
Espesor: La formación tiene un espesor mínimo de 187 m en el Milagros Sur-1X, unos 140 m en área de Heybroek (op. cit.) y unos 150 m de espesor en el área de Fierro y Useche (op. cit.).
Fósiles: En muestras de canal de Milagros Sur-1X, intervalo 3115 m-3404 m, se identificaron a Corallina sp., Apiculatisporites sp., Gemmamonocolpites sp. Spiriterites sp., Gnetaceapollenites sp., Foveotrilete sp. y dinoflagelados (Monroy, comunicación verbal). Heybroek (1953) menciona a Ostrea sp. y Chofatella sp.
Edad: La edad indicada por la flora y la Chofatella es Aptiense-Albiense.
Paleoambiente: Plataforma somera con aguas marinas cálidas de moderada circulación.
G. D. Kiser, 1997
Anexo una comunicación de mi querido profesor y tutor, Laurence L. Sloss, fallecido en Evanston el 2 de noviembre de 1996, porque la misma enfoca uno de los grandes problemas estratigráficos del Cretácico de Venezuela y porque la nota mantiene su vigencia hoy en día.
W. Scherer, 5 de mayo 1997.
NORTHWESTERN UNIVERSITY October 10, 1994
Dr. Wolfgang Scherer
INTEVEP
Friend Wolfgang
Sincere apologies for being absent-without-leave when you visited Locy Hall on October 6! ( Older people have difficulties with short-term memory retention.)
I was looking forward to talking with you about the Venezuelan Cretaceous; the outcrop succesion has become a cause célébre among sequence-stratigraphers -- specifically, between the Exxonians and Pindel1 & associates. At the AAPG meeting (Denver, June) there was an SEPM go-around featuring the latest Vail +CRSN + ELF, etc., fIndings from the Cretaceous of the Vercours (sp.?) Plateau (S.E.'n France; many "third-order" sequences) and parallel studies by Pindell et alia from the Colombian border across most of Venezuela ( many fewer recognizable sequences). It is an act of faith that such sequences are created by eustatic events and are therefore of global range - - certainly, during the opening of the Atlantic, southern Europe and northern South America were intercommunicating regions of ancient Tethys, both lay adjacent to the inboard (or proximal) flank of a passive margin, and both should have shared the same history of sea-level oscillation.
The stratigraphic community is thus presented with a choice among difficult alternatives:- a) the groups use different criteria in identifying intersequence unconformities (downlap surfaces, flooding surfaces, etc. ) or , b) there are more unconformities in France as a result of a higher frequency of regional tectonic uplift/downwarp or, c) there are gaps in the reported Venezuelan stratigraphic record because of inadequate observation or because the record is truncated by post-Cretaceous deformation.
During the discussion session following the formal papers it was noted that there were new data from recent deep drilling in the Maracalbo Basin and that these findings tended to corroborate the Pindellian view from the outcrop ( I think). In any case, you and INTEVEP could be of great service by letting us in on what you have learned from outcrop and subsurface studies that may have a very serious bearing on this continuing controversy. How about a contribution to the SEPM sessions to be held in Houston in March? I would be happy to cooperate in any useful manner ( i.e., by leaning on the program committee, or whatever).
Let me know what you think, preferably by an old-fashioned letter or, if you must, by Email. Messages via Bob Speed (speed@ earth nwu.edu) reach the box in our joint outer office.
Cordially, L.L. Sloss
Department of Geological Sciences Locy Hall Evanston, Illinois 60208-2150 Telephone (708) 491-3238 Facsimile (708) 491-8060
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