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Sedimentology and Stratigraphy of the Middle Jurassic Dhruma Siliciclastic Deposits: Northern Central Arabia, Saudi Arabia


The Middle Jurassic Dhruma Formation in Saudi Arabia has long been known as part of Saudi Arabia’s carbonate, oil-producing succession. During the 1980-2000’s, it was discovered that the carbonate rocks of the Dhruma are replaced laterally by siliciclastic deposits in northern and southern central Arabia. As yet, the sedimentology and stratigraphy of the Dhruma siliciclastic deposits are not well known.

Recently drilled shallow stratigraphic wells on and near the outcrops of the Dhruma Formation, 250 km northwest of Riyadh, provide a unique opportunity to analyze the sedimentological and stratigraphic characteristics of these siliciclastic sediments. In this study, around 4500 feet of cores from six shallow stratigraphic wells were described to understand the transgressive and regressive siliciclastic deposits of the Dhruma Formation, and how these fit with their better-known carbonate counterparts.

The transgressive facies associations comprise: (I) fluvial-tidal channel deposits in upward-fining units with erosional bases, dune/ripple cross stratification, besides tidal indicators such as double mud/carbonaceous drapes and inclined heterolithic stratification; (II) bioturbated intertidal and supratidal deposits; (III) estuarine central basin muddy deposits with intersecting tidal channels; (IV) barrier island/tidal inlet deposits; (V) shelf ridge deposits; and (VI) offshore deposits. The regressive facies associations are; (VII) fluvial distributary-channel deposits, (VIII) coastal embayment and floodplain deposits, (IX) tide-dominated delta-front to pro-delta deposits, and (X) shoreface with muddy shelf deposits.

Palynology and XRD analyses were integral part to this study. The palynology results assigned Aalenian-E. Bathonian age to the Dhruma siliciclastic deposits based on the preserved terrestrial and marine palynomorphs. The XRD results contributed to the paleoenvironmental construction by indicating varying contents of kaolinite and chlorite across the identified facies associations.

This study is the first to document the sedimentology and stratigraphy of the Dhruma siliciclastic succession in northern central Arabia, and highlights a complex mixed system of deltaic and estuarine deposits that are interfingered with carbonate deposits. It contributes to the reconstruction of the existing Middle Jurassic paleogeographic map in Saudi Arabia, and correlation with the recently discovered age-equivalent siliciclastic reservoir in the Rub Al-Khali basin.