A depositional model for a wave‐dominated open‐coast tidal flat, based on analyses of the Cambrian–Ordovician Lagarto and Palmares formations, north‐eastern Brazil

Abstract

Open‐coast tidal flats are hybrid depositional systems resulting from the interaction of waves and tides. Modern examples have been recognized, but few cases have been described in ancient rock successions. An example of an ancient open‐coast tidal flat, the depositional architecture of the Lagarto and Palmares formations (Cambrian–Ordovician of the Sergipano Belt, north‐eastern Brazil) is presented here. Detailed field analyses of outcrops allowed the development of a conceptual architectural model for a coastal depositional environment that is substantially different from classical wave‐dominated or tide‐dominated coastal models. This architectural model is dominated by storm wave, low orbital velocity wave and tidal current beds, which vary in their characteristics and distribution. In a landward direction, the storm deposits decrease in abundance, dimension (thickness and spacing) and grain size, and vary from accretionary through scour and drape to anisotropic hummocky cross‐stratification beds. Low orbital wave deposits are more common in the medium and upper portion of the tidal flat. Tidal deposits, which are characterized by mudstone interbedded with sandstone strata, are dominant in the landward portion of the tidal flat. Hummocky cross‐stratification beds in the rock record are believed, in general, to represent storm deposits in palaeoenvironments below the fair‐weather wave base. However, in this model of an open‐coast tidal flat, hummocky cross‐stratification beds were found in very shallow waters above the fair‐weather wave base. Indeed, this depositional environment was characterized by: (i) fair‐weather waves and tides that lacked sufficient energy to rework the storm deposits; (ii) an absence of biological communities that could disrupt the storm deposits; and (iii) high aggradation rates linked to an active foreland basin, which contributed definitively to the rapid burial and preservation of these hummocky cross‐stratification deposits.

Depositional evolution of a progradational to aggradational, mixed-influenced deltaic succession: Jurassic Tofte and Ile formations, southern Halten Terrace, offshore Norway

“…While such a mixed-energy coastline is common in the present-day, it is less commonly interpreted in the geological record (Johnson, 1975;Fan, 2012;Eide et al, 2016). The combination of HCS and heterolithic facies reworked by tidal currents has been observed in both modern linear coastlines with tidal flats (Yang et al, 2005;Fan, 2012) and their ancient equivalents (Basilici et al, 2012).…”

“…Non-deltaic successions comprise alternating tidal channels (FA5) and tidal shorelines (FA6), which display high bioturbation intensity. In combination, these facies associations represent tide-dominated lower delta plains and associated coastlines, which were distant from active fluvio-deltaic sediment input points ( Figure 11C,D) and subject to periodic storms and wave reworking (Yang et al, 2005;Basilici et al, 2012;Fan, 2012). In ancient and modern analogues of this type of non-deltaic coastline episodic HCSbearing storm deposits are interbedded with tidal current ripples (Yang et al, 2005;Basilici et al, 2012).…”

Facies evolution and depositional model of an arid microtidal coast: example from the coastal plain at the mouth of Wadi Al-Hamd, Red Sea, Saudi Arabia

“…Well-known storm-generated structures such as hummocky and swaley cross-stratifications are absent. This is explained by the low gradient shallow depth of the intertidal flat and the landward decrease in energy due to attrition by the sea bottom (Bassoullet et al, 2000;Yang et al, 2006;Basilici et al, 2012).…”

“…The deposition of sand with mud interbeds (facies S11) in the upper part suggests deposition under alternated storm and fairweather conditions in a generally bedload dominated intertidal flat. The absence of internal structure, the scarcity of bioturbation, and the mud interbeds indicate poststorm rapid deposition (Yang et al, 2005;Basilici et al, 2012).…”

“…The scours and fills structures (facies S12) were formed by storms. During storms, the open coast intertidal flat is lowered and sediments become sandy, and an upward fining succession is deposited at and after the storm waning phase (Basilici et al, 2012;Daidu, 2013). These scours experienced a brief period of stagnation and probable low oxygen conditions as shown by gray-colored sediments and rare horizontal burrows.…”