Sequence stratigraphy and skeletal concentrations in the Lower Cretaceous of the Neuquén Basin (Northern Patagonia, Argentina)

ARCHUBY, FERNANDO M

Göttingen, Alemania

Congreso; 74. Jahrestagung der Paläontologischen Gesellschaft Gesellschaft „GEOBIOLOGIE“ (74o Congreso Anual de la Sociedad Alemana de Paleontología “GEOBIOLOGÍA”); 2004

Deutsche Paläontologischen Gesellschaft (Sociedad Alemana de Paleontología)

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 550 m thick sedimentary package, deposited in an open marine ramp environment 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. 151 transgressive-regressive (T-R) cycles or parasequences were recognized along the studied section. From their stacking pattern, the three third-order depositional sequences, formerly published for the two units, and a hierarchy of a stacked higher frequency cycles (4th, 5th and 6th order) were identified. The 6th order elements, the T-R cycles or parasequences, are usually composed of two units, a basal thin shell-rich transgressive bed or set of beds, and a much thicker upper siliciclastic, shell-poor facies succession. Skeletal concentrations in shell poor facies are rare but, when present, are composed of low density, very well preserved faunas in muddy sediments. Basal portions of parasequences contain one or more densely packed shell concentrations, showing evidence of low rates of sedimentation. Parasequences of middle to late HSTs of 3rd and 4th order sequences contain a relatively thick regressive sedimentary package, and a thin transgressive one, the later with a usually poorly preserved, low diversity fossil assemblage with evidence of physical reworking. TSTs, MFZ and early HSTs of 3rd and 4th order sequences are characterized by the presence of truncated and amalgamated parasequences, and the development of thick (up to 2 meters) and complex shell concentrations in the basal part of the parasequences with high preservation quality of the macrofauna. These highly fossiliferous levels consist of a basal (early) transgressive lag deposit (ETD) usually followed by a densely packed concentration of very well preserved fossils (late TD), grading into deposits of the less densely packed muddier early highstand (early HST). The time represented by each T-R cycle can be estimated around 23000 years. This figure is close to the ca. 20 ky precession cycle of the Milankovitch band. Considering that each TST and early HST represent approximately one-third of the duration of a complete cycle, each single basal skeletal concentration would represent around 7000 years. With few exceptions, base-of-cycle skeletal concentrations are considered here as comparable, isotaphonomic, within-habitat assemblages. A detailed bed-by-bed quantitative study of a 5th order sequence shows that shell concentrations in the early TST and late HST have the lowest preservational quality, while the opposite can be stated for the shell beds bracketing the maximum flooding (i.e. late stages of TST, MFZ and early HST). These results coincide with those of Fürsich & Pandey (2003). References: Fürsich, F.T. & Pandey, D.K., 2003. Sequence stratigraphic significance of sedimentary cycles and shell concentrations in the Upper Jurassic-Lower Cretaceous of Kachchh, western India. Palaeogeogr. Palaeoclimatol. Palaeoecol. 193,149-159.