The district of Colombia overlaps the boundary between the Upper and Middle Magdalena Valley regions, and sedimentary facies characteristic of the two adjoining regions must be traced into each other through it. Stratigraphic nomenclatures, descriptions, and correlations, and geological interpretations published during the past are varied and confusing. The literature is widely scattered and accessible only with difficulty. The purpose of this article is to assemble and discuss earlier information and to present new data in order to provide a clarified basis from which further more detailed and extensive studies may be undertaken. The geomorphology is described. The post-upper Oligocene stratigraphy is discussed and type localities are indicated. The oldest strata described are correlated with the upper Oligocene Colorado series of the Middle Valley. These sediments contain a fossil zone, associated with lignite beds, that is believed to be correlative with the La Cira formation of the Middle Valley. The occurrence is 120 kilometers south of the type locality, and outcrops on the Cambao road extend its occurrence about 80 kilometers farther south. The apparently conformably overlying Miocene series comprises more than 4,000 meters of continental sediments the sandstones of which, except near the very base of the series, are characterized by abundant amphibole and magnetite in the heavy-mineral content of the sandstones. The lower (approximately 1,600 meters thick) is differentiated from the upper part (more than 2,400 meters thick) by the absence of andesitic and dacitic pebbles in the lower part and the generally finer texture of its sediments. The series is believed to have once extended eastward, in places, far into the area now occupied by the Cordillera Oriental. Correlation with the Miocene Real series of the Middle Valley is substantiated on the basis of identical heavy-mineral content, lithologic character, nd stratigraphic position. An interval of intense folding, faulting, and erosion followed time and probably lasted throughout the Pliocene. Then the Mesa formation of late Pliocene, or more probably early Pleistocene, age was deposited unconformably on the and older strata. It is a piedmont plain deposit composed of coalescing alluvial fans that were deposited in the Magdalena River Valley by tributary streams heading in the Cordillera Central. The formation is characterized by the presence of abundant volcanic material. Maximum known thickness is 350 meters. The fan of the Mesa formation deranged the drainage by ponding the ancient Magdalena River to form Lake Hettner, which was drained when overflow incised the water gap at Honda. Contemporaneous lake beds and small deltas of local extent formed in the now extinct Lake Hettner. The Pleistocene Guali formation, named here for the first time, occupies erosional valleys cut in the Mesa formation. Its composition is similar to that of the Mesa beds, but is generally less indurated, and ranges from 21 to 33 meters thick. Recent alluvium is not as widespread in the district as it is on the north and south. Important periods of volcanism left traces in the district in the upper Miocene and Pleistocene sediments. The topographic depression of the post-Honda and pre-Mesa Magdalena Valley is believed to be largely due to erosion by the ancient river system. The presence of the Magdalena Valley graben structure in the district is questioned. The Cambras and other thrust faults east of the district exhibit great displacements, but the west side of the Magdalena Valley seems to be an east-dipping uncovered peneplain on crystalline rocks. No conclusive evidence proving or disproving the existence of the Honda fault was observed in the district. Several local features favor the inference that no Honda fault exists, but studies in adjacent regions are necessary to arrive at a final conclusion. End_Page 793------------------------------ The important diastrophic movements that developed the Andean system of Colombia and its orographic relief took place during the Pliocene. The chief structural features that contributed toward outlining the Magdalena Valley are believed to date from that time, rather than late Cretaceous; thus the interandean depression is assigned an age much younger than previously considered. The existence of a supposed long narrow Magdalena embayment since the close of the Cretaceous is questioned and attention is called to the necessity of obtaining more field information. The problem is significant because results confirming the writer's hypothesis will require revamping the present concept of Tertiary paleogeography in Colombia.
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Jan 1, 1970
Fred A Frey 
