Undulating stratigraphy of a carbonate inner ramp lagoon and associated architectural facies heterogeneity: An example from the Late Jurassic Hanifa Formation, Central Saudi Arabia

This study offers new insights into the internal architecture of shallow marine carbonate sediments deposited in an epeiric sea formed on a broadly flooded passive margin continental shelf. The Ullayah Member of the Late Jurassic Hanifa Formation is exposed along the Tuwaiq Mountain Escarpment in Central Saudi Arabia and serves as an example. High-resolution drone imagery over a 4 × 4 km² wadi system, measured-sections and thin-section petrography are utilized to develop the three-dimensional architecture of the lagoonal sediments deposited in an inner-ramp setting. Parasequence sets have been mapped on the digital outcrop models that demonstrate a high degree of lateral and vertical thickness variability associated with stromatoporoid/coral buildups and inter-buildup sediments. The stromatoporoid/coral buildups have been interpreted to be deposited during the late Transgressive Systems Tract and Highstand System Tract. The positive relief developed by the stromatoporoid/coral buildups resulted in hydrodynamic acceleration of currents in the inter-buildup areas, which limited sediment deposition. The incomplete filling of the inter-buildup areas led to an undulating topography, which significantly influenced depositional architecture in the next parasequence set. This theme of sediment filling, also referred to as compensational stacking, was repeated over the next two parasequence sets, with an overall decreasing accommodation towards the late highstand, eventually levelling the uneven topography at the top of the Ullayah Member. The study offers insights into the processes of sediment accumulation in an inner ramp lagoon and/or on a flat-top shallow marine epeiric carbonate shelf. Although accumulation is cyclic, neither strict facies patterns nor uniform cycle thicknesses can be discerned on a kilometre scale. This insight potentially provides a reason for the frequently debated correlation of cycles and facies patterns in carbonate sequences in shallow epeiric seas.