Sedimentology and facies architecture of the tide-influenced, river-dominated delta-mouth bars in the lower Lajas Formation (Jurassic), Argentina

Abstract

The lower Lajas Formation (Jurassic), western Argentina, has been interpreted to represent a progradational tide-dominated deltaic succession with tidal dominance throughout complete base-level cycles. Two sequences have been identified in this study containing different suites of facies associations. Sequence 1 is composed of a river-dominated suite, with prodelta to distal delta front, river-dominated mouth bars, and terminal distributary channels. Sequence 2 is composed of both river-dominated and tide-influenced mouth bars, channels and interdistributary bays. Both sequences contain distinct event bedding (mainly in the mouth bars and interdistributary bays) that is expressed as alternating layers of finer and coarser sediment, representing interflood and river-flood layers, respectively. Variations exist in the character of event beds between river-dominated settings and tide-influenced settings. In river-dominated settings, almost no tidal indicators are found in flood or interflood layers. This is in contrast to tide-influenced settings, which contain tidal indicators such as bi-directional ripples and tidal rhythmites in interflood layers. Tidal indicators such as tidal rhythmites are rare in the flood layers. Many organic-matter drapes that were previously interpreted as tidal in origin lack a regular cyclicity and are inferred to have been deposited through fluvial processes with no tidal influence. The event beds are interpreted to have been formed on an annual cycle, rather than by individual storms, due to commonalities with modern seasonal bedding and the large size of the drainage basin. The recognition of heterolithic event beds provides another criterion for recognizing river influence and differentiating these from heterolithic tidal sediments. Because of the overwhelming presence of a seasonal signature, the Lajas is re-interpreted to be overall river-dominated, with tide-influence found mainly in Sequence 2. The extensive exposure of the river-dominated mouth bars in Sequence 1 allowed for the architecture to be described in detail. A compensational architecture was recognized between individual bars and was delineated by slight changes in dip direction. Individual bars are comparable in size to the modern Wax Lake delta and the ancient Ferron Sandstone. Mouth bars amalgamate to form mouth-bar complexes, within which slight changes in relative sea-level can be deciphered.