Recognition criteria, characteristics and implications of the fluvial to marine transition zone in ancient deltaic deposits (Lajas Formation, Argentina)

Abstract

The seaward end of modern rivers is characterized by the interactions of marine and fluvial processes, a tract known as the fluvial to marine transition zone, which varies between systems due to the relative strength of these processes. To understand how fluvial and tidal process interactions and the fluvial to marine transition zone are preserved in the rock record, large‐scale outcrops of deltaic deposits of the Middle Jurassic Lajas Formation (Neuquén Basin, Argentina) have been investigated. Fluvial–tidal indicators consist of cyclically distributed carbonaceous drapes in unidirectional, seaward‐oriented cross‐stratifications, which are interpreted as the result of tidal modulation of the fluvial current in the inner part of the fluvial to marine transition zone. Heterolithic deposits with decimetre‐scale interbedding of coarser‐grained and finer‐grained facies with mixed fluvial and tidal affinities are interpreted to indicate fluvial discharge fluctuations (seasonality) and subordinate tidal influence. Many other potential tidal indicators are argued to be the result of fluvial–tidal interactions with overall fluvial dominance or of purely fluvial processes. No purely tidal or tide‐dominated facies were recognized in the studied deposits. Moreover, fluvial–tidal features are found mainly in deposits interpreted as interflood (forming during low river stage) in distal (delta front) or off‐axis (interdistributary) parts of the system. Along major channel axes, the interpreted fluvial to marine transition zone is mainly represented by the fluvial‐dominated section, whereas little or no tide‐dominated section is identified. The system is interpreted to have been hyposynchronous with a poorly developed turbidity maximum. These conditions and the architectural elements described, including major and minor distributary channels, terminal distributary channels, mouth bars and crevasse mouth bars, are consistent with an interpretation of a fluvial‐dominated, tide‐influenced delta system and with an estimated short backwater length and inferred microtidal conditions. The improved identification of process interactions, and their preservation in ancient fluvial to marine transition zones, is fundamental to refining interpretations of ancient deltaic successions.

Integrated sedimentology and ichnology of Late Jurassic fluvial point‐bars – facies architecture and colonization styles (Lourinhã Formation, Lusitanian Basin, western Portugal)

“…Pearson & Gingras, 2006;Rebata et al, 2006;Hovikoski et al, 2008;D ıez-Canseco et al, 2015). As a result, there is now a better understanding of spatial and temporal changes in both sedimentary processes and trace fossil colonization styles over the fluvial-tidal transition zone (Dalrymple & Choi, 2007;Gugliotta et al, 2016). In contrast, far fewer integrated studies have focused on meandering river systems located landward of the fluvial-tidal transition.…”

“…Although the broad sedimentological aspects of modern and ancient meandering river systems are fairly well-understood (e.g. Allen, 1964Allen, , 1965aNijman & Puigdef abregas, 1978;Plint, 1983;Miall, 1987Miall, , 1988Miall, , 1992Smith, 1987;Collinson, 1996;Bridge, 2003;Pranter et al, 2007;Donselaar & Overeem, 2008), understanding of the precise effects of tidal modulation on river flow, including the style (and preservation) of bedforms produced, is a more recent phenomenon (Martinius & Gowland, 2011;Dalrymple et al, 2015;Gugliotta et al, 2016). Accordingly, it is important to ensure that any ancient meander belt chosen to 'anchor' the purely fluvial part of a meandering system in any integrated sedimentological and ichnological study is demonstrably clear of the effects of tidal modulation.…”

Overprinted allocyclic processes by tidal resonance in an epicontinental basin: The Upper Jurassic Curtis Formation, east‐central Utah, USA

“…It is important to note that similar bedforms to the ones observed in the Curtis Formation (Figure 7) do occur in other depositional systems in which tidal currents act only as a modulating factor rather than a dominant control upon sedimentary character (Baas et al, 2016;Gugliotta, Flint, Hodgson, 2016;Martinius & Gowland, 2011). However, because of the lack of any major fluvial systems within the neighbouring and contemporaneous Entrada Sandstone and Summerville F I G U R E 1 1 (a) Photogrammetric model of Cedar Mountain showing the earthy facies of the Entrada Sandstone, the erosive relief developed at its top, as well as the lower and middle Curtis strata overlying the J-3 Unconformity.…”

Sediment distribution and depositional processes along the fluvial to marine transition zone of the Mekong River delta, Vietnam

“…Tidal influence is recorded by heterolithic rhythmites (S3 and S4). Additional evidence of tidal processes could not be identified in the non‐cohesive sands because their sedimentary structures were disrupted during sampling; however, tide‐related cyclical patterns in cross‐stratifications have been reported from the corresponding part of the FMTZ in the modern Fraser River delta (Hendershot et al ., ) and in ancient systems (Martinius & Gowland, ; Gugliotta et al ., ).…”

“…The FMTZ extends from the landward limit of any tidal effect (upstream of which the system is purely fluvial) to the river-mouth area (Dalrymple & Choi, 2007;Jablonski & Dalrymple, 2016); however, in systems with relatively high river discharge, the interaction of fluvial and marine processes extends up to hundreds of kilometres outward of the river mouth (Ayranci et al, 2012;Xue et al, 2012;Loisel et al, 2014). In distributive deltaic systems, the FMTZ of each distributary channel shows minor differences in extension, location and depositional processes, depending on the configuration of the channel (for example, gradient and cross-section area), local factors (for example, the presence of bedrock) and the relative strength of river and tidal processes therein (Gugliotta et al, 2016(Gugliotta et al, , 2017. The FMTZ also experiences temporal variations in extension, location and depositional processes, which are driven by changes in fluvial (for example, seasonal), tidal (for example, ebb-flood or neap-spring) and wave processes (Dalrymple et al, 2015;La Croix & Dashtgard, 2015;Gugliotta et al, 2016Gugliotta et al, , 2017Jablonski & Dalrymple, 2016).…”