Base level control on the development of continental sequences: the Middle Jurassic Challacó Formation (Neuquén Basin, Argentina).

VEIGA, G.D.

Congreso; 15th International Sedimentological Congress; 1998

At the southwestern margin of the Neuquén Basin, the Lower-Middle Jurassic Cuyo Group ends with continental deposits of the Challacó Formation. This formation is a 150 m-thick red bed unit composed of sandstones and conglomerates intercalated with mudstones. Five sedimentological logs were measured in the westernmost outcrops of this unit in order to identify sedimentary environments and to investigate the main controls on sedimentation. Facies and sedimentary-body analysis allowed us to recognize four main facies associations, each one related to a different continental environment. Facies Association 1 represents the accumulation of a mixed-load high sinuosity river system, with sandy lenticular channel deposits intercalated with fine-grained floodplain facies. This association can be further divided on the basis of channel-fill amalgamation and ?coarse member?/?fine member? ratio. Association 1A is dominated by floodplain facies with isolated sand bodies, whilst association 1B is characterized by a high ?coarse member?/?fine member? ratio and important amalgamation of channel bodies. Facies Association 2 is mostly composed of conglomerates and coarse grained sandstone facies, formed in a braidplain system, characterized by coarse bedload deposits of longitudinal and transverse bars. Facies Association 3 is composed of aeolian sandy deposits and Facies Association 4 by fine-grained suspension deposits attributed to floodplain to lacustrine environments. Palaeoenvironmental and architectural changes in fluvial deposits suggest temporal variations in accommodation space. Thus, in the Challacó Formation two depositional sequences are defined. These sequences are controlled by changes in river base level, induced in turn by relative sea level oscillations. Therefore, sections with distinctive accommodation conditions are correlated with the systems tracts of the classic sequence stratigraphic scheme. Because of the nature of deposition in continental environments and the major influence of local factors in the aggradation of the alluvial plain, it is impossible to recognize the development of parasequences and flooding or transgressive surfaces. In this case, zones of maximum marine influence and the beginning of the transgression are interpreted by recognition of changes in the rate of accommodation space development. The basal sequence begins with an interfluve sequence boundary, separating the studied continental deposits from shelf sandstones of the underlying Lajas Formation. Above this boundary, a transgressive systems tract is developed. It is characterized by a high sinuosity, mixed-load, river system with isolated channel deposits (Facies Association 1A). This TST suggests a rapid rate of accommodation development related to a rapid base level rise. The upper part of this sequence is characterized by another high-sinuosity fluvial unit, but with more amalgamated and coarser-grained channel deposits (Facies Association 1B). The decrease in accommodation rate showed by these deposits suggests the development of a highstand systems tract that resulted from a decrease in base level rise and coastal progradation. The upper sequence is, in some aspects, different from the lower and begins with the accumulation of low sinuosity, braided river conglomerates (Facies Association 2) deposited upon an important unconformity that locally includes incision of the lower sequence. The lower erosional surface was produced by a rapid fall in base profile, and the subsequent coarse-grained fluvial deposits results from the steepening of the base profile. Both the erosional surface and the conglomerates represent a lowstand systems tract and the lowermost part of a transgressive systems tract. Overlying these coarse-grained sediments, aeolian deposits (Facies Association 3) and floodplain-lacustrine deposits (Facies Association 4) are recognized, showing a decrease in sediment supply associated with the development of a transgressive systems tract. The transition from the transgressive systems tract to the highstand systems tract (maximum flooding zone), is shown by the presence of small channels with local marine influence.