Formación Venado, Nueva Unidad Litoestratigráfica del Ordovícico Colombiano

Se propone una nueva unidad litoestratigráfica para el Ordovícico de Colombia: Formación Venado. EI estratotipo se expone en el flanco occidental de la Cordillera Oriental, a lo largo de la carretera que bordea el Rio Venado, afluente del Cabrera (Departamento del Huila). EI total de la sección se divide en tres unidades: la infrayacente se caracteriza por el predominio de shales gris oscuros micáceos; la intermedia, aquí designada como Formación Venado, esta constituida por una espesa secuencia (670 rn) donde se interestratifican areniscas feldespáticas y lodolitas verde claras, micáceas: la superior, compuesta por areniscas feldespáticas, areniscas conglomeráticas líticas y conglomerados polimícticos, presenta estratificación cruzada "hummocky".  La Litología y las estructuras sedimentarias indican que la secuencia ordovícica (edad determinada con base en la presencia de graptolites) se depositó en un ambiente de plataforma siliciclástica, bajo la influencia de oleaje normal y de tormentas, estas últimas probable mente debidas a eventos meteorológicos, pues son episódicos repetitivos, no periódicos.  EI total de la columna del Rio Venado constituye una secuencia progradante; si se tiene en cuenta su edad y su posición geográfica, podría marcar el retiro del mar ordovícico durante el Llanvirniano, es decir en la época previa al gran hiato pos-Llandeiliano/ pre-Emsiano.
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A new lithostratigraphic unit, Venado Formation, Ordovician in age, is proposed. The stratotype is exposed on the western flank of the Colombian Eastern Cordillera, along the highway bordering the Venado river, a tributary of the larger Cabrera river (Huila Department). The exposed section has been divided into three units: the lowermost one, with predominantly gray micaceous shales; the middle unit, or Venado Formation, as we design it, is a thick sequence (670 rn) of interbedded feldespathic and light green micaceous sandstones; the uppermost unit contains feldespathic sandstones, conglomeratic lithic sandstones and polymictic conglomerates, with hummocky beds. Lithology and sedimentary structures point out to the deposition of the Ordovician sequence on a siliciclastic platform under fine and bad weather. The storm waves, episodic, non-periodic events, were produced probably by meteorological phenomena. The stratigraphic column of Rio Venado shows a prograding sequence; its age and geographic position may indicate the retreat of the Ordovician sea during the Llanvirnian time, or the previous epoch to the post-Llandeilian / pre-Emsian hiatus.  A new lithostratigraphic unit, Venado Formation, Ordovician in age, is proposed. The stratotype is exposed on the western flank of the Colombian Eastern Cordillera, along the highway bordering the Venado river, a tributary of the larger Cabrera river (Huila Department). The exposed section has been divided into three units: the lowermost one, with predominantly gray micaceous shales; the middle unit, or Venado Formation, as we design it, is a thick sequence (670 rn) of interbedded feldespathic and light green micaceous sandstones; the uppermost unit contains feldespathic sandstones, conglomeratic lithic sandstones and polymictic conglomerates, with hummocky beds. Lithology and sedimentary structures point out to the deposition of the Ordovician sequence on a siliciclastic platform under fine and bad weather. The storm waves, episodic, non-periodic events, were produced probably by meteorological phenomena. The stratigraphic column of Rio Venado shows a prograding sequence; its age and geographic position may indicate the retreat of the Ordovician sea during the Llanvirnian time, or the previous epoch to the post-Llandeilian / pre-Emsian hiatus.