Asymmetrical wave-dominated siliciclastic shorelines with evidence of along-strike variability of sedimentary processes: A revised interpretation for the Toarcian Marrat red beds, central Arabia

Marine siliciclastic red beds are common in the geological record; however, distinguishing them from continental red beds can be challenging without detailed sedimentological and stratigraphic analyses. Owing to the common tendency in the geological literature to relate the reddish color to the oxidization processes within the vadose zone, siliciclastic red beds of marine genesis can be misinterpreted as continental deposits. Unlike the other Toarcian successions in the Tethyan basins surrounding Arabia, the sedimentological and stratigraphic characteristics of the Marrat Formation of Arabia in general and its middle unit's red siliciclastic beds, in particular, have not been well constrained. For more than 50 years, the red siliciclastic beds of the Toarcian Marrat Formation have been interpreted as continental to marginal marine deposits based on their reddish color and the absence of fossils. The lack of supporting sedimentological and stratigraphic evidence to this interpretation raises critical concerns regarding its validity. Along the Jurassic outcrop belt in central Arabia, three outcrop localities of the middle Marrat reddish siliciclastic deposits (with a total thickness of more than 120 m) demonstrate sedimentological and ichnological features that are contrary to the non-marine to marginal marine interpretation. The integration between sedimentology and ichnology reveals the existence of asymmetrical wave-dominated shorelines with evidence of river hyperpycnal flows and tidal processes during the middle Marrat time. A total of seventeen facies are identified and grouped into four facies associations: mixed wave and river-dominated delta front to prodelta mudstone (FA1), shoreface (strandplain) and underlying shelf mudstone (FA2), tidally-influenced distributary channel (FA3), and offshore carbonates (FA4). Stratigraphically, six major parasequences (each with a thickness of similar to 3-13 m) were mapped and correlated between the three measured sections with clear overall progradational to retrogradational stacking patterns. To the best of our knowledge, this study is the first to document the presence of Toarcian shallow-marine siliciclastic red beds on the Arabian Plate. It displays an example of a case study where ichnology and process-based sedimentology were integrated to constrain the depositional environment of these red beds. This study provides new insights into the depositional settings of the Toarcian red siliciclastic beds of the Marrat Formation, with implications to the stratigraphy of Arabia and the Early Jurassic paleogeography of the western Tethys and, in particular, the Arabian Plate.

Automated summary

Marine siliciclastic red beds are commonly found in the geological record, but distinguishing them from continental red beds can be challenging. This study presents the first documentation of Toarcian shallow-marine siliciclastic red beds on the Arabian Plate, providing new insights into their depositional environment.