Umm Bab Dolomite and Limestone Member Fm

See Dammam Fm

Synonym: Umm Bab Dolomite and Limestone Member;

Reference Section:

Generally, the Umm Bab Dolomite and Limestone Member is extremely variable lithologically (both laterally and vertically) as it has undergone various degrees of alteration and weathering. The unit can vary from a moderately strong crystalline dolomitic limestone to a very weak calcareous siltstone over very short distances and can contain various proportions of the weaker material as vugs and pockets. In and around Doha, particularly towards the coast, the upper layers are generally highly weathered and frequently very difficult to core. The base is commonly marly (attapulgite) and very fossiliferous from 0 to 6.6 ft. (2m) or 9.8 ft. (3 m). Its overall composition is a compact crystalline white limestone, with local layers of chert and red attapulgite; with some hard brownish dolomites irregularly developed in the upper half in the absence of a more recent cover. The total thickness is variable. The Umm Bab Dolomite and Limestone Member is not fully exposed in Qatar. Chatton (1948) evaluated the thickness of the Umm Bab Limestone Member to be 93.2 ft. (28.4m) on the northern part of the Dukhan anticline. Cavelier (1970a) concludes, with no explanation provided, that the Umm Bab Member could vary from a thin 98.4 ft. (30 m^[2]) to a thick of 164.0 ft. (50 m) throughout the whole of Qatar.

In addition to the above, a communication with Gulf Laboratories^[3] dated March 31^st 2015, states that the thickest sequences so far discovered in boreholes are along the east coast of Qatar between Doha and Messaid. A sequence up to around 29 metres thick in southern Doha near the coast and 33.9 metres (the thickest in Qatar) north of Messaid at coordinates 25º01'58.49" and 51º36'30.93" were penetrated in 2009. Unfortunately, no log suites were run in this latter well. Pictures of the core were taken (Appendix A) and its detailed description made (Appendix B) before the core itself was destroyed. Lastly, the boreholes that completely penetrated the Umm Bab Member at the locality of two important projects in Doha encountered only thicknesses of 42.9 ft and 65.6 ft. (13 m and 20 m).

One naturally occurring partial, but thick (~11m), exposure of this member (upper portion) was measured in Naslat Umm Hadidah area (western Qatar) and is shown in Appendix C. In addition, a measured section at the QNCC limestone quarry^[4] east of Umm Bab town (Figs. 44 & 45) shows a continuous portion of the Umm Bab dolomite and limestone member at the level and below the Naslat Umm Hadidah section, down to the Rus Formation.
[Figure 1: Reddish granular limestone near the top of the Umm Bab Dolomite and Limestone Member in a sector of Naslat Umm Hadidah area at Lat. 25°23'53.08"N, Long. 50°52'59.78"E]

[Figure 2: Area on Ras Abarug Peninsula (Lat. 25°34'52.99"N, Long. 50°49'56.70"E) displaying few metres of the top portion of the Umm Bab Dolomite and Limestone Member. Echinoderms are frequent. Immediately above it, and in the background, is the Abarug Member (the Bir Zekreet Shale member is not present/visible at that particular locality).

[Figure 3: Lithological section at the QNCC Limestone quarry east of Umm Bab town. See Fig. 4 and Fig. 1 in Dammam Fm.]

[Figure 4: Umm Bab Dolomite and Limestone Member seen from its base at the Umm Bab QNCC limestone quarry (25°11'18.49"N, 50°50'9.83"E). Picture also shows a 3 to 4° dip in the Dammam & Rus Formations in this area resulting from the uplift of the nearby Dukhan anticline [The Dukhan Alveolina Limestone and the Midra (and Saila) Shales outcrop less than a kilometre to the west]

At the bottom (over a 5m maximum thickness), the Umm Bab limestone locally includes marls and even thin stringers of attapulgite shales quite rich in fossils (Pycnodonte sp., Ampullospira sp., Gisortia sp., Alveolina and especially Nummulites) overlain by an often-reddish granular limestone (Fig. 1). The overlying fairly calcareous layers carry less Nummulites; on the contrary, Echinoderms (Figs. 5, 6, 7, 8, 9) are abundant (Scutellina, Echinolampas, ...) as well as some large Molluscs/gastropods (Campanile sp.. Velates cf. schmiedeli) (Fig. 10) - with their respective trace fossils / burrows (Fig. 12) - bivalves, large Foraminifera, rare nautiloids (Fig. 11), fossil mangrove root system (Fig. 13) and corals (Fig. 14). The Echinoderms are common in the upper half: Echynocyamus polymorpha (Duncan et Sladen), Porocidaris aff. Schmiedeli (Munster), and especially at the top, where they make up large deposits on the western coast of Ras Abarug (Fig. 2), with: Porosoma aff. Lamberti Checchia-Rispoli, Echinocyamus polymorpha (Duncan et Sladen), Echinolampas perrieri de Loriol, Oppisaster derasmoi Checchia-Rispoli, Schizaster beloutchistanensis (d’Archiac), Eupatagus formosus de Loriol. Cavelier, 1975; Gelin (2020).

Burrowing by gastropods (Fig. 12) is essentially a continuation of surface locomotion obliquely into soft substrata. Burrows are nearly as large as the borers (Fig. 15). An important difference between bivalves and gastropods in respect to their burrowing habits is that bivalves commonly burrow vertically into the substratum, whereas gastropods generally enter the sand or mud at an oblique angle and only burrow to a limited depth so as to maintain access to the water above the surface of the substratum by means of the siphon (Fig. 16). Deeper burrowing may, however, occur under certain circumstances, as when females are carrying eggs or in response to rough seas. (Trueman et al., 1992).

[Figure 5: Echinolampas. Front, Back & Side views (left, centre & right)]

[Figure 6: Opissaster.]

[Figure 7: Clypeaster.]

[Figure 8: Eupatagus.]

[Figure 9: Schizaster. Single specimen (left); various specimens (right).]

[Figure 10: Various specimens of gastropods.]

[Figure 11: A rare Nautiloid (species unknown) from the Umm Bab Dolomite and Limestone Member. Left = Natural position; right = partially extracted.]

[Figure 12: Large gastropod/echinoderm burrows/trace fossils. Profile view (top; see watch as scale) at Lat. 25°16'20.04"N, Long. 50°51'50.34"E, and aerial view (bottom) at Lat. 25°22'56.94"N, Long. 50°52'43.72"E. Refer as well to Appendices C & D.]

[Figure 13: Left = Fossil mangrove root system at Lat. 25°41'33.41"N, Long. 51°32'58.67"E; Sadooni, Al-Saad (2012). Right = Present-day mangrove habitat]

[Figure 14: Fossil coral specimens.]

[Figure 15: A fossil gastropod still inside its lithified burrow.]

[Figure 16: Burrowing activities of a gastropod. The siphon is kept at the surface of the substrata. (Trueman et al., 1992).]

Paleo-fluid escape structures: Paleo-fluid escape structures, such as those described in the Rus Formation, are also common in the Umm Bab Dolomite and Limestone Member (Figs. 17 & 19). When overlaid by a younger limestone, the latter is normally highly silicified (Figs. 17 & 18).

Caves: The three bottom members of the Dammam Formation (Midra, Dukhan & Umm Bab) are often observed in the caves/sinkholes of Qatar. Other than at Dahl Al-Misfir (Fig. 17), stratigraphic measurements were also taken in Dahl Al-Hammam near Landmark Mall in Doha (Fig. 20).

The thicknesses measured in the Dahl Al-Hammam were:

Top

• 10m (32.81ft) - Umm Bab Dolomite & Limestone Member of the Dammam Formation
• 01m (03.28ft) - Dukhan Alveolina Limestone Member of the Dammam Formation
• 04m (13.12ft) - Midra (and Saila) Shales Member of the Dammam Formation (including the submerged portion)

Base

Total depth: 15m (49.21ft) from the top metallic stair

[Figure 17: Paleo-fluid escape structures (gryphons) at locality 25°29'30.68"N and 50°54'50.62"E, just southeast of the Ras Abarug Peninsula. While these structures are not silicified, compared to those found in the Rus Formation over the Dukhan area (Fig. 4), the layer just overlaying them demonstrates a high degree of silicification.]

[Figure 18: The silicification of the younger limestone affected by the paleo-fluid structures is better appreciated on a road cut. At this Naslat Umm Hadidah locality 25°22’11.85”N and 50°52’46.2”E, multiple horizontal layers of silica are observed.]

[Figure 19: Paleo-fluid structures (gryphons) found in a locality of the Inland sea in SE Qatar (24º 43’ 40.92”N and 51º 24’ 10.31”E) at the level of a sabkha. This is the stratigraphically highest level of the Dammam Formation in this area; thus, no younger silicified horizon is present. Left - Large gryphons protruding from the ground. Right - Some gryphons with their tubular core infilled with recent sediments.]

[Figure 20: Dahl Al-Hammam near Landmark Mall (25°20'4.19"N, 51°28'49.30"E). Only Umm Bab Member is visible. Bottom of the cave was too dark to photograph the stratigraphy.]