Abstract

CLADERA, G; LIMARINO, C. O; ALONSO, M. S  and  RAUHUT, O. Stratigraphic control in the preservation of vertebrates remains in the Cerro Barcino Fm (Cenomanian), Chubut Province. Rev. Asoc. Argent. Sedimentol. [online]. 2004, vol.11, n.2, pp.39-55. ISSN 1853-6360.

The factors that controlled the differential concentration of vertebrates remains in specific stratigraphic intervals of Cerro Castaño 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 diagenetic transformations (Fig. 5a,b). Zeolites forming both rim and granular cements predominate (Fig. 6). Pd facies are the coarsest deposits identified in the alluvial plain area. They comprise pyroclastic-rich silty sandstones stratified in centimetric to decimetric massive tabular beds. Frequently, Pd facies occurs either interstratified with vitric tuff belonging to the Pc facies or associated to mudstones and chonites forming paleosoils (Pp facies). The analyzed section of the Cerro Barcino Fm was deposited in different kinds of allociclycally controlled fluvial environments. Even though the area was located rather far away from the volcanic segment, the recurrent ash falls exerted a hard control on sedimentation. Moreover, as the eruptivity was changing throughout time, periods dominated by high amounts of pyroclastic sedimentation can be distinguished from others where ash falls were almost totally absent. Figure 8 shows the three types of fluvial systems recognized in this paper: 1) sineruptive, 2) early posteruptive and 3) late posteruptive. Following this model, it is very likely that during high eruptivity periods large amounts of ash falls had produced rapid agraddation of the alluvial plains. These sineruptive fluvial system would have favored the formation of shallow braided channels (CHsl facies) laterally associated to fine sandstones (Pd facies) and vitric tuffs (Pc facies). When eruptivity ceased, and consequently the pyroclastic input, channels began to evolve towards its equilibrium profile eroding earlier flood plain deposits. This stage is represented in the here named early posteruptive fluvial system (Fig. 8), characterized by deeply incised lenticular channels (CHsw facies) that laterally pass to bioturbated chonites and mudstones (Pp facies). After some time, the fluvial system reached quasi-equilibrium conditions (late posteruptive stage) forming multistorey tabular channels (CHmt) that laterally pinched out into muddy fine sandstones (Pd facies) and bioturbated mudstones (Pp facies). Both depositional (environmental) and postdepositional (diagenetic) factors are likely to have controlled the high concentration of vertebrates in specific stratigraphic levels of the Cerro Barcino Fm. The fact that vertebrate remains only appear in CHmt channels can be explained by the favorable conditions that prevailed during the late posteruptive stage when pyroclastic falls ceased and a flood plain was stabilized (Fig. 9). Moreover the early carbonate cementation affecting channel sandstones closed the diagenetic system and favored bone preservation. On the contrary, early carbonate cementation was not found in the associated alluvial plains deposits; therefore, compactation and chemical diagenesis processes were not inhibited and the transformation-dissolution of bones was favored in these rocks. The lack of vertebrates in syneruptive deposits (only microscopic bones have been found) could be explained as the consequence of recurrent pyroclastic falls that limited the formation of vegetated soils and consequently the number of living vertebrates. Finally, neither environmental nor diagenetic conditions would have been the best for vertebrates preservation during the early posteruptive stage. The extremely low agradational rate of the alluvial plain promoted the chemical, physical and biological alteration of bones. Moreover, early diagenetic cementation has not been identified in this interval; on the contrary, closed fabrics as well as pseudomatrix formation and autigenesis of clay minerals features were frequently found in these sandstones. In conclusion, the high bone concentration in some stratigraphic levels of the Cerro Barcino Fm results from the combination of favorable environmental conditions for life (reached during late posteruptive stage) coupled with early carbonatic cementation that closed the diagenetic system.

Keywords : Diagenesis; Taphonomy; Cretaceous.

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