Architectural analysis of upper-shoreface deposits at intra-parasequence scale: different morphological configurations and possible controls.

ISLA, M.; SCHWARZ, E.; VEIGA, G.D.

Toulouse

Congreso; 33rd International Meeting of Sedimentology; 2017

IAS

It is commonly assumed that parasequences are the expression of a unique depositional system, prograding trough time. This would imply that internal bedsets within a parasequence should be composed of similar facies associations. However, careful inspection of upper-shoreface (and foreshore?) deposits at intraparasequence scale in Cretaceous deposits of the Neuquén Basin revealed significant changes in the configuration of the nearshore system. The aim of this contribution is to describe different types of uppershoreface deposits within successive bedsets (of a single parasequence) in order to interpret changes in the morphology of the costal setting and to discuss the mid-term evolution of shallow marine systems.The 30-m-thick PS400 parasequence of the Pilmatué Member (Late Valanginian-Early Hauterivian) has been investigated along a 12-km, dip-oriented, transect. This parasequence is composed of twelve bedsets, and facies associations suggest the long-standing development of a storm-dominated, shoreface-to-offshore system. All deposits, but those interpreted as reflecting upper-shoreface conditions, remain relatively constant through the unit. For the architectural analysis of the upper-shoreface deposits, outcrop sections 1-3 km wide and up to 5 m thick were selected. Detail sedimentary logs (15-30 m apart) were combined with mapping of internal surfaces. On the basis of the architectural analysis, three types of upper-shoreface deposits were identified. Type 1 deposits, the most common through the unit, consists of one facies association composed of fine-to mediumgrained siliciclastic or mixed (siliciclastic-carbonate) sandstones, with trough cross-stratification. Type 1 deposits, do not present internal surfaces. In Type 2 deposits, the previous facies association regularly alternates with ripple cross-laminated, unburrowed, fine-grained sandstones, forming packages up to 30 m wide. These packages are laterally bounded by tangential seaward-dipping surfaces (5-10°). Type 3 deposits are characterized by closely spaced, large-scale, concave-up to tangential foreset beds. These dip seaward up to 10° and represent the alternation of sandstones having parallel planar lamination or ripple cross-lamination, with pebbly, bioclastic sandstones showing trough cross-lamination. The vertical and lateral relationships of upper-shoreface types suggests short-term accretion and seaward migration of a shoreline under different morphological conditions. Types 1 and 2 would likely reflect barred coasts, with a bar-trough system morphology. However, steep seaward-dipping surfaces and drastic lateral changes in Type 2 could be a response to a higher gradient. Foreset beds of Type 3 would also reflect high gradients, but in a non-barred coast. The presence of three types of upper-shoreface deposits reflect an important variability in coastal morphology and dynamics at intra-parasequence scale frequency (possibly 10 kys) that might not be anticipated by looking at distal facies alone or without a detailed architectural analysis. Moreover, it suggests that controlling factors such as sediment supply, depth of receiving basin and/or wave climate changed trough successive bedsets. Specifically, high-gradient morphology, departing from the most common low-gradient, barred system (Type 1), could be related to an increase in sediment supply in Type 2, but to an increase in depth of receiving basin (and wave regime?) in Type 3.