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

Abstract

Integrated sedimentological and ichnological case studies of ancient meandering river systems have, for the most part, focused on the deposits of the fluvial–tidal transition zone; much less emphasis has been placed on the purely fluvial realm above the landward limit of tidal effects. This problem needs to be addressed so that in future the defining sedimentological and ichnological criteria assigned to ancient fluvial reaches are sufficiently well‐established to enable their separation from the down‐dip fluvial–tidal transition zone. Accordingly, a case study has been carried out on a well‐exposed meander belt deposit from the Late Jurassic Lourinhã Formation of the Lusitanian Basin, western Portugal (Praia Do Valmitão, Ribamar). Analysis indicates that the meander belt here comprised mixed‐load fluvial channels traversing a vegetated floodplain subject to a seasonal winter wet/summer dry palaeoclimate. This setting facilitated the development of both calcic palaeosols and shallow lakes on the adjacent floodplain. Critically, there is no evidence of the effects of tidal modulation on bedding structures, thereby establishing purely fluvial conditions. Heterolithic point‐bar deposits generated in this setting are bioturbated extensively by a trace fossil assemblage dominated by the meniscate trace Taenidium barretti, with Skolithos linearis, Planolites beverleyensis and Cylindricum isp. also recognized. A number of factors suggest that the Taenidium barretti producer in this case was a subaquatic organism living in an active fluvial channel setting (i.e. not colonizing subaerially exposed channel‐margin/floodplain deposits). Accordingly, there are some implications for current ichnofabric/ichnofacies models in the continental realm. Firstly, Taenidium‐dominated ichnofabrics need not necessarily be confined to colonization beneath subaerially exposed surfaces; they might also be produced within submerged substrates. Secondly, there is scope to extend the range of the Scoyenia ichnofacies to include active fluvial channels and not simply those channels that were inactive or abandoned.

Evolution of an ancient (Lower Cretaceous) marginal‐marine system from tide‐dominated to wave‐dominated deposition, McMurray Formation

“…Facies 6 and F7 deposits that show depositional dip are interpreted as inclined heterolithic stratification (IHS) deposited in fluvio‐tidal (brackish‐water) channels. Although IHS also occurs in fluvial environments, the muds and sands in fluvial IHS are devoid of extensive bioturbation (La Croix et al ., ; Gowland et al ., ) and fluvial IHS are mainly described from counter point bars and eddy accretion deposits (Smith et al ., , ; Gowland et al ., ). Interbedded mud and sand near the top of FA3 successions (commonly F7 deposits) appear to have consistent, shallow dips, and are interpreted as mud‐dominated IHS.…”

Evolution of a gradually abandoned Oligocene fluvial palaeovalley fill: Abu‐Roash area, north Western Desert, Egypt

“…Buatois & Mangano, 2004, 2007) and it is linked to subaerial exposure and palaeosol (cf. Gowland et al ., 2018).…”

Sedimentology and the facies architecture of the Ghaggar‐Hakra Formation, Barmer Basin, India: Implications for early Cretaceous deposition on the north‐western Indian Plate margin

“…Sets of both facies are bound by first‐ or second‐order surfaces and sporadically truncated by third‐order surfaces. Capping the elements is cross‐laminated sandstone (Scl) forming individual sets up to 5 cm thick with internal structures disrupted by meniscate trace fossils of Taenidium or Beaconites (Gowland, Taylor, & Martinius, ).…”

“…Overlying these are sets (10 cm) of rippled sandstone (Srha), climbing super‐critically up to 30°, and stacked 30 cm cosets of cross‐laminated sandstone (Scl) climbing sub‐critically at 5°. Pedogenic sandstone and siltstone (Sp, Ip) display mottled textures, and contain fossil leaf imprints along with meniscate trace fossils typical of Taenidium or Beaconites (Gowland et al., ).…”