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In a sedimentological sense a fluvial to tidal transition zone can be defined in rivers as a zone that separates the upstream fluvial from the downstream estuarine zone. Characteristic sedimentary structures within this zone are notoriously difficult to recognize. This study demonstrates the influence of tidal modulation within the most proximal part of the fluvial-tidal transition zone (i.e. the 'backwater zone') of an ancient fluvial system. Criteria have been established to differentiate between purely fluvial facies and those modulated by tidal energy. The stratigraphic interval from which the data were derived is the Lourinhã Formation (Late Jurassic) of the Lusitanian Basin, WesternPortugal. An analysis of sedimentary features at four key localities has identified a temporal spectrum of tidal influence ranging from the daily modulation of fluvial flows to the effects of tidal bore passage. A combination of quantitative and qualitative data reveals systematic changes in the internal architecture of dune-scale bedforms deposited in a channel-floor setting. The key co-occurring features are: (i) increasing-decreasing organic particle concentration; (ii) increasing-decreasing bottomset thickness; (iii) increasingdecreasing foreset dip and shape (from convex to concave); and (iv) increasingdecreasing brinkpoint height. Collectively, these features are interpreted as having been produced by successive fluctuations in flow regime conditions from lower (during flood tidal retardation) to higher (during ebb tidal drawdown) current velocities. Bedforms showing these features occur in both meandering fluvial channels and straighter distributary systems. In addition, several examples of a specific type of stepped erosion surface and draping sediment have been recognized, the interpretation of which strongly suggests generation by the passage of tidal bores. If this interpretation is correct, then it represents one of the first published examples of tidal bore propagation in ancient fluvial systems. Palaeoclimatic evidence (cellular analysis of woody tissue, palaeosol character and plate reconstruction) indicates a warm, seasonal, winter wet to summer dry climate during deposition of the Lourinhã Formation. From this evidence it is suggested that tidal modulation and tidal bore effects are more likely to develop in the 'dry season', when fluvial flow in the main river channels was reduced.

The c. 700 m thick succession of continental–brackish‐marine deposits forming the Lourinhã Formation, cropping out along the coast of western Portugal between Baleal and Santa Cruz, has been correlated using laterally persistent shelly marker beds. Three shelly units record the episodic establishment of relatively short‐lived, brackish‐marine embayments, transgressing from the southwest, onto a low‐lying coastal plain. The succession displays systematic changes in facies types and stacking patterns reflecting differences in fluvial style, bedload character and palaeontological content. Based on these observations, four new members for the Lourinhã Formation are proposed: the Sáo Bernardino, Porto de Barças, Areia Branca and Ferrel members. New biostratigraphical data indicate that the Lourinhã Formation is Late Kimmeridgian to earliest Early Tithonian in age. This age has also been obtained from the underlying mixed carbonate and clastic deposits of the Abadia Formation at Consolação. As a result, these latter sediments are now re‐assigned to the Alcobaça Formation, a lithostratigraphical term currently in use in other areas of the Lusitanian Basin. Improved regional mapping of the Lourinhã Formation has established a new sub‐basin within the western parts of the Lusitanian Basin. This sub‐basin, now named the Consolação Sub‐basin, is bounded to the east by the Lourinhã–Caldas de Rainha (L–C) fault zone and to the west by the Berlengas Horst. Copyright © 2013 John Wiley & Sons, Ltd.

An extensive core database from the Late Jurassic Fulmar Formation illustrates the practicalities of facies analysis and depositional modelling in highly bioturbated, heterolithic, shallow marine siliciclastic strata. Twelve major sedimentary facies (1-12) are defined on the basis of lithology, grain size, visually estimated sand-silt content and the presence or absence of primary sedimentary structures. Ichnofabric characteristics are also defined and have proved to be not always facies specific. For purposes of depositional modelling the 12 facies types are resolved into six facies associations (A-F), each representative of distinct shoreline/shallow marine environments. From these associations three broad depositional models have been constructed: storm-influenced shoreface (model 1), bioturbated shoreface (model 2) and a speculative bioturbated shelf model (model 3). Stratigraphical and palaeontological criteria suggest that models 1 and 2 are shorelineattached; sedimentological data indicate that they occur as end-members to a spectrum of shoreline settings constrained by incident wave energy 'bands'. For shorefaces subject to significant storm influence, well-preserved event beds occur seaward of the upper shoreface. In the bioturbated shoreface model, however, the lower shoreface and adjacent shelf deposits are characterized by intense infaunal reworking. The bioturbated shelf model displays no clear evidence of shoreline connection, with the resultant facies often exhibiting highly abundant siliceous sponge spicules, together with other open marine indicators such as belemnites and rare ammonites. Unlike the shoreface sand bodies, which received sediment from basin-margin fluvial systems, shelf sand bodies in the Fulmar Formation are more likely to have been intrabasinally sourced along relay ramps or other forms of transfer zone. This sediment may have undergone a long transit/residence period on the shelf prior to eventual burial in hanging-wall depocentres strongly influenced by Zechstein salt withdrawal.This study demonstrates that an ichnofabric approach (as opposed to one of simple ichnodiversity) is highly significant with respect to the description and modelling of highly bioturbated shallow marine siliciclastic strata. In the Fulmar formation, ichnofabric analysis is capable of providing sensitive information on fluctuations in depositional energy levels, water depth, sedimentation rates, erosion rates, substrate consistency and dissolved oxygen levels. Evidence of these fluctuations is often apparent at intra-facies scale, but would largely go unnoticed using a conventional sedimentological approach to data gathering, based on ichnodiversity at best. Depositionally significant changes in ichnofabric may occur without any significant change in ichnodiversity. Ichnofabric analysis also plays an important role in the hierarchical assessment of key stratal surfaces for purposes of sequence stratigraphic modelling.

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