ABSTRACT: Climatic Influence in Controlling the Authigenic Mineralogy of Volcanolithic Sandstones

SOLANO-ACOSTA, W., Indiana State University, Terre Haute, IN

The Cretaceous (Aptian?) Yavi Formation in the Upper Magdalena Valley basin consists of continental red beds deposited within a back-arc basin on the eastern margin of the Central Cordillera in the Colombian Andes. A systematic stratigraphic variation in sandstone composition within the sequence reveals a climatic change during deposition, ranging from semiarid to semihumid-humid. In addition, this climatic shift has an effect on the authigenic mineralogy, at least in the type section where the samples for this work were collected.

The diagenetic changes recorded in the Yavi Formation can be described by six main episodes of cementation coupled with compaction as seen by the presence of pseudomatrix (or squeezed grains) throughout the sequence. From bottom to top, these six episodes are characterized primarily by: 1) microcrystalline quartz and chalcedony; 2) carbonates; 3) phyllosilicates; 4) phyllosilicates and opal; 5) phyllosilicates and carbonates; and 6) phyllosilicates, microcrystalline quartz and chalcedony. This diagenetic pattern seems to be independent of detrital mineral composition and mostly controlled by climatically influenced pore-water chemistry. The presence of quartz and chalcedony is interpreted to have formed during relative semi-humid conditions whereas the carbonate cement and phyllosilicates formed during more arid conditions.

The Yavi Formation is characterized by its compositional immaturity and poor sorting that resulted in low initial porosity. Further reduction of porosity was accomplished by pore filling cement, as described above. The lack of porosity in these sandstones precludes them as hydrocarbon reservoirs in the Upper Magdalena Valley, unlike the more mature quartzose sandstones overlying this sequence.

Search and Discovery Article #90907©2000 AAPG Eastern Section Meeting, London, Ontario, Canada