Madrid 2014
New data on Lower Ordovician graptolites from Colombia and their correlation around the Gondwanan margin of South America M. Moreno-Sánchez1, A. de J. Gómez-Cruz1 and J.C. Gutiérrez-Marco2 Departamento de Ciencias Geológicas, Universidad de Caldas, Calle 65 # 26-10 Manizales, Caldas, Colombia. mario.
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[email protected] 2 Instituto de Geociencias (CSIC, UCM), José Antonio Novais 12, 28040 Madrid, Spain.
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North meets South
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Lower Ordovician graptolites from South America are mostly known from the Central Andean Basin, where a nearly complete biozonation covering most of the Tremadocian and Floian stages has been established for Argentina and Bolivia. North of this basin, a single record of the Tremadocian graptolite Rhabdinopora flabelliformis (Eichwald) was recognized from a deep borehole in Los Llanos (plains) of Colombia, west of the Guyana Shield. From the same country a number of occurrences of poorly preserved “Arenig” graptolites have been listed from the Central Cordillera (La Cristalina Formation), the Eastern Cordillera (La Uribe and Venado Fms.), the Serranía de La Macarena and the boreholes Negritos-1 and Trinidad-1 from eastern Llanos. However, many of these occurrences have not been published in detail and the cited graptolites display a wide chronostratigraphic range from the Floian to the lower Darriwilian stages (= Arenig s.l.), and thus could be of either Dapingian or early Darriwilian age (Middle Ordovician). Undisputable Floian graptolites from the northern Andean domain in Colombia are recorded here from the Venado Formation northeast of Baraya, possibly not far from the “La Uribe trail” section mentioned by Trumpy (1943) and later authors. The first Ordovician graptolite collected from this formation was identified as “cf. Didymograptus murchisoni (Beck)” by Villarroel et al. (1997, fig. 3), but based on their illustration it looks more like a Floian species such as Acrograptus filiformis (Tullberg). A more recent find is reported by Moreno-Sánchez et al. (2008), who identified several specimens of Phyllograptus sp. in association with an unidentified graptolite (a stipe of dichograptoid) from typical outcrops of the green shales belonging to the Venado Formation and the Vereda El Totumo of the Río Venado. Further collection at the same locality bears nicely preserved specimens of Baltograptus kurcki (Törnquist), Phyllograptus cf. ilicifolius (Hall) and Expansograptus cf. extensus (Hall). This assemblage can be adscribed to the late Floian Baltograptus minutus Biozone. Two other localities with Colombian graptolites have been examined from the Serranía de La Macarena area, where Trumpy (1943) reported the occurrence of Arenig graptolites studied by Turner (1960). Our new localities are in green shales of the Zanza Formation near the type section (the Río Zanza, a tributary of the Río Güéjar). There we have collected Acrograptus filiformis (Tullberg) and abundant deflexed graptolites (presently under study) of the Baltograptus varicosus (Wang) and B. turgidus (Lee) groups, indicative of the Floian Baltograptus jacksoni to Baltograptus minutus biozones and their equivalents in Argentina and Bolivia. Fom a paleogeographic perspective, the new records of Lower Ordovician graptolites from Colombia deserve special interest. First, because they can be easily correlated with similar Floian assemblages described from the Central Andean Basin in Argentina and Bolivia, so far completely unknown in the northern Andean-west Guyana Domain. Second, because they constitute the first “intermediate” peri-Gondwanan occurrence of some robust deflexed baltograptids, originally defined from SW China but also reported from Argentina, Bolivia and southern Britain. This research is a contribution to the projects CGL2012-39471 of the Spanish MINECO and IGCP 591.
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