High Frequence Cyclicity in a
Fining-Upwards Lowstand Continental Deposit, Upper
Jurassic,
Arregui, Carlos1,
Santiago Grosso2 (1) Petrobras Energia SA,
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.
AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California