Ammonite facies in the Lower Campanian of Antarctica: onshore-offshore trends in stratinomy and faunal composition

OLIVERO, E. B.

Mendoza, Argentina

Congreso; 18th International Sedimentological Congress; 2010

International Association of Sedimentologists

Recent biostratigraphic and sedimentologic studies in the Lower Campanian ammonite-rich shelf deposits of the James Ross Basin, Antarctica made possible a precise reconstruction of facies tracts along an onshore-offshore transect about 70 km in length. On this basis, the characterization of proximal vs. distal trends in ammonite facies and their corresponding environmental controls are analyzed. In proximal, inner-shelf settings the Lower Campanian part of the Santa Marta Formation (including the Ammonite Assemblage 6: Karapadites aff.centinelaensis-Natalites spp. Group 2), consists of about 150 m of coarse and thick tempestite beds, trigoniid-rich coquinas, and cross-bedded sandstones. In distal, mid- to outer-shelf settings the age-equivalent Rabot Formation consists of more than 300 m of bioturbated mudstones and interbedded tempestites that bear abundant articulated brachiopod and inoceramid shells. Each of these sedimentary facies is characterized by a very distinctive ammonite facies. In proximal settings the ammonite facies is dominated by relatively large and ornate kossmaticeratids (relative frequency c. 50%), with subordinated heteromorphs (10%), desmoceratids (12%), and gaudryceratids and tetragonitids (28%). Vertically embedded kossmaticeratids associated with abundant plant material are relatively common. Conversely, in distal settings the ammonite facies is dominated by gaudryceratids and tetragonitids (60%) and heteromorphs (20%), with subordinated small, juvenile kossmaticeratids (15%) and large pachydiscids (5%). The shell size of some gaudryceratids, particularly Anagaudryceras subsacya and similar morphotypes, are in excess of 100 mm. Most of the ammonite shells are horizontally embedded and concentrated by currents on the lee sides of large inoceramid shells. In both facies, preservation of the ammonite shells is similar; planispiral shells are relatively complete, with calcite-filled phragmocones and sediment-filled body chambers whereas heteromorph shells are commonly fragmented. These data are consistent with the idea that ornate ammonite morphotypes dominate inner shelf settings; however the relative abundance of juvenile, ornate kossmaticeratids in mid- to outer-shelf deposits could reflect either selective sorting during post-mortem transport or a wider and deeper life habitat than mature specimens. Similarly, the restriction in midto outer-shelf settings of large Anagaudryceras shells, with coarse ornamentation in the body chamber, seems to reflect a life habitat limited to these settings. Dominance of vertically embedded shells in shallow-water and horizontally embedded ones in deep-water settings not necessary reflects an absolute bathymetry, but results from a combination of bathymetry (water pressure), turbidity of the water mass, and sedimentation rate.