Sequence stratigraphic significance of skeletal concentrations: an example from the Lower Cretaceous of the Neuquén Basin (Northern Patagonia, Argentina)

ARCHUBY, FERNANDO M

Aachen, Aquisgrán (Aachen), Alemania

Congreso; 19th Sedimentological Conference 2004 at the Rheinish Westphalian University of Technology; 2004

Rheinish Westphalian University of Technology

Sequence stratigraphic significance of skeletal concentrations: an example from the Lower Cretaceous of the Neuquén Basin (Northern Patagonia, Argentina) Fernando Archuby (1) 1 - Institut für Paläontologie der Universität, Pleicherwall 1, D-97070 Würzburg The upper Lower Hauterivian-Lower Barremian Upper Member of the Agrio Formation (Neuquén Basin, Argentina) is an about 600 m thick sedimentary package, deposited in an open marine ramp environment. It accumulated at the eastern passive margin of a back arc basin. This unit and the aeolian and fluvial sandstones of the Middle Member of the Agrio Formation are regarded as the deposits of a 2nd order sea-level cycle. Three third-order depositional sequences were formerly published for the two units, and a hierarchy of a stacked higher frequency cycles (4th and 5th order) can be recognized. The 5th (?) order elements, the parasequences, are usually composed of two units, a basal thin shell rich transgressive bed or set of beds, and a much thicker upper silicilastic, shell poor facies succession. Skeletal concentrations in shell poor facies are rare but, when present, are composed of low density, very well preserved fauna. Basal transgressive portions of parasequences contain one or more densely packed shell concentrations, showing always evidences of low sedimentation rates. During highstands of 3rd order sequences, these deposits are rather thin (5 to 40 cm) bioclastic sandstones, embracing a fossil concentration dominated by thick shelled, commonly disarticulated and broken fossils. Highly fossiliferous, thicker (up to 2 m), more calcareous and complex basal transgressive deposits are present in parasequences deposited during 3rd order transgressions: a basal (early) transgressive lag deposit (ETD) is usually followed by a densely packed concentration of very well preserved fossils (middle TD), grading into the less densely packed late TD. These transgressive beds show features of low sedimentation rates such as strong bioturbation, presence of autigenic glauconite, high degree of alteration of bioclasts (breaking, borings, micritization), high degree of encrustation by serpulids, oysters and foraminifera, as well as amalgamation and truncation of parasequences. The onset of oyster-serpulid and coral patch reef development occurred with the MTDs. The highest fossil density, the high rates of encrustation and boring, the growing of patch reefs and the recurrent presence of glauconite points to MTDs as the intervals of lowest terrigenous input during parasequence development.