Minjur Fm

Column: Saudi Arabia Outcrop, Saudi Arabia Subsurface, Kuwait, Bahrain, Oman Subsurface. The type section extends from the back slope formed by the top of the Jilh Formation up to the base of the Marrat Formation in the face of Khasm al Khalta, Saudi Arabia (Powers, 1968). The Oman subsurface reference section is Lekhwair-70 in North Oman.

Synonym:

Buff, massive, commonly cross bedded, fine to coarse-grained quartz sandstone, a few layers of which contain abundant small quartz pebbles, locally calcareous, weathering to give small spherical concretionary masses; several irregular zones of red, purple, and, blue-gray varicolored shale, sandy shale and shaly sand; at several levels much black to brown ironstones as thin platy layers and concretionary masses, locally containing molds of fossil wood.

"The Minjur Fm in Oman is represented by a coastal-plain siliciclastic unit in the northwest (Lekhwair area) and by lateritic paleosols to the southeast. In the Al Huqf outcrops it is a relatively thin unit comprising deep red-colored lateritic paleosols, shales and sands of Triassic age, called the ‘Oman Soil’. The ‘Oman Soil’ has been penetrated by wells, such as Mafraq-1. The main depocentre of the Minjur Fm in Oman is in the Lekhwair area of northwestern Oman, where it can reach up to ca. 100 m thickness. In the subsurface it is characterized at the base by a thin transgressive sandstone, overlain by an overall shallowing coastal plain succession. Lagoonal shales are overlain by coastal-plain estuarine and fluvial sandstones. The estuarine sandstones, 10–20 m thick, are cross-bedded and contain gravels with darcy-range permeabilities. They have a characteristic blocky low Gamma signature, which is correlatable as a distinct belt in all the wells of the Lekhwair area. They are overlain by more heterolithic, tidally influenced fluvial channel sands and floodplain/ marginal-marine shales. The Formation is capped by a distinctive set of three paleosols (Figure 8.13). As the Minjur thins onto the high towards the southeast, these seem to coalesce, potentially together with younger paleosols, to ultimately form the ‘Oman Soil’." (Forbes et al., 2010)

"In Oman, the uppermost soil, dominantly red claystone section has yielded Palynozone 2242 miospore assemblages, with significant fungal debris, but these are now considered to be facies dependent and not age specific in this context, as they also occur higher up within the Lower Mafraq section. The Late Triassic Palynozone 2255 has been recorded in the shales towards the base of this ‘red’ shale package and is particularly well developed (as Palyno-subzone 2241) in the more marine shaly section below the gravely sand.

Kharusi (1989) assigned a Carnian – Norian age (Palyno-subzone 2241) to the lowermost Mafraq (Minjur) in the Lekhwair-Dhulaima regions, but Jacovides et al. (1998) were unable to replicate this degree of interpretation, even when working on the same wells. They assigned only general Late Triassic age. Wells Lekhwair-85 and Dhulaima-7 seem of key importance in reconciling the two studies. Kharusi (1989) again forwards a case for a Carnian – Norian miospore assemblage in the lowermost Mafraq (Minjur) whereas dinocyst data from Jacovides et al. (1998) provides a compelling case for an uppermost Jilh Formation that is likely to be no older than Norian (influx of Rhaetogonyaulax wigginsii and presence of rare Wanneria listeri). Significantly, in well Lekhwair-85, Jacovides et al. (1998) also record the dinocyst Dapcodinium priscum in the basal Mafraq (Minjur), which restricts the age to no older than Rhaetian, thereby suggesting that Palyno-subzone 2241 should be extended up into the Rhaetian." (Forbes et al., 2010)

"In Oman, the Minjur Formation starts with a thin transgressive sand at the base, which disappears in a seaward direction towards the northwest. The basal sand is overlain by lagoonal shales, with a maximum flooding surface near the base. This flooding is correlatable throughout the Lekhwair area. From there, the Formation shallows upward into estuarine and fluvial sandstones, capped with palaeosols. The estuarine deposits contain gravelly cross-bedded sandstones with high-permeabilities. These sandstones are present in all the wells that currently penetrated the Minjur Fm in the Lekhwair area. They consist of two sub-units, with a distinct low Gamma spike in between that corresponds to a thin layer of coal or coal clasts. The lateral continuity of these sands suggests deposition as coastal-plain bars, rather than local channels. The presence of the coal layer in all the wells is probably related to sea- level fluctuation at parasequence scale, depositing swamp deposits on top of the bars during relative sea-level fall, followed by another sand during subsequent sea-level rise. The unit is overlain by more heterolithic fluvial sandstones and flood-plain shales. Tidal influence is indicated by alternating high and low energy deposits, represented by sands with clay drapes (sometimes stylolitised) and flaser bedding. Mud clasts are present at the high-energy bases of the cross-bedded sands. Small rootlets and plant remains indicate a swampy environment. The succession culminates in three layers of paleosols." (Forbes et al., 2010)