Controles estratigráficos en la preservación de restos de vertebrados en la formación Cerro Barcino (Cenomaniano), provincia del Chubut

the factors that controlled the differential concentration of vertebrates remains in specific stratigraphic intervals of cerro casta？o member are considered in this paper (fig. 1). the finding of vertebrates is only limited to pebbly sandstone beds that form fluvial channels with non erosive bases and tabular geometry. the closely associated alluvial plains, and other types of channels intercalated in the sequence, have not yielded significant paleontological remains. in order to analyze the origin of the differential concentration of vertebrates, the sequence was divided in six sedimentary facies (figs. 2 and 4, table 1), including different kinds of channel (chmt, chsl y chsw) and flood plain (pd, pp y pc) deposits. chmt facies is composed of cross-bedded sandstones and conglomerates forming tabular multistorey channel complex bounded by non erosive bases (fig. 4). sandstones comprise lithoarenites and feldslithoarenites with low matrix percentage (less than 4 %), high quantities of cement (up to 21 %), open fabric, scarce diagenetic alteration and almost absent porosity. three different types of cements have been distinguished in these rocks: 1) rim-type zeolite cement, 2) poikilitic calcite and 3) microcristalline and poikilitic analcime (fig. 5d,e). chsw facies forms simple lenticular channels with erosive bases deeply incised into floodplain deposits (fig. 4). the lowermost part of each channel shows lenses of intraformational conglomerates covered by massive and cross-bedded medium- and fine-grained sandstones. chsw facies is dominated by lithoarenites showing moderate to high matrix content (up to 20 %), mainly zeolitic cement and frequent formation of pseudomatrix (fig. 5f). the last type of channel deposits correspond to chsl facies that form thin lentiform channels (up to 0.5 m thick) encapsulated into floodplain facies (fig. 4). these channels comprise coarse-grained sandstones showing low proportions of matrix, relatively open fabric and scarce diagenet