High Frequence Cyclicity in a Fining-Upwards Lowstand Continental Deposit, Upper Jurassic, Neuquen Basin, Argentina

Arregui, Carlos¹, Santiago Grosso² (1) Petrobras Energia SA, Neuquen, Argentina (2) Petrobras Energia S.A, Neuquen, Argentina

It is presented a surface and subsurface analysis of the Tordillo formation along the Neuquen Basin, Argentina. It is a continental backstepping unit subdivided in cycles deposited as a consequence of a tectonic reactivation during Middle-Upper Kimmeridgian times. In the subsurface eastern border basal contact is discordant above synrift or prerift Triassic-Jurassic volcanics. Its thickness distribution is controlled by paleotopographic narrow valleys with 100 mts thick or no deposition areas in the eastern border and up to 800 mts in the west.

Facies recognized are conglomerates or sandstones of alluvial fans, fluvial braided and eolian environments. The paleowind direction is to the east and fluvial paleocurrents to the basin center. Andean outcrops along the western border between 32°-35° South Latitude is composed by red sandstones with an eolian upper section of green sandstones between 35°-39° South Latitude.

The overall fining-upwards arrangement, belong to a Low Frequence Cycle of 2nd order LST (Vail et. al .1991), that was affected by smaller periodic variations in its sedimentation conditions (High Frequency Cycles). Upper eolian system (Catriel formation) was expanded in successive stages producing the retraction of the fluvial–alluvial environment (Sierras Blancas formation) anticipating a Tithonian transgression. In some places of the center of the basin eolian sandstones are only present. Climatic changes associated with up-lifts producing geographic barriers in the west could be the explanation for the recognized ciclicity interpreted as alternations of humid and arid conditions.

The adequate correlation has strong impact for the petroleum geology changing the classical lithoestratigraphic correlation for a chronoestratigraphic one.