YAMAMA FORMATION-LOWER CRETACEOUS

DEFINITION

The Yamama Formation is named for the town of Al Yamamah (lat 24^o11'N.) about 20 km north of the type section and so on line with the strike of outcrop. Development of Yamama nomenclature from an un-named part of the Tuwaiq Formation to a formation within the Thamama Group is described in detail in the definition of the Sulaiy Formation. 

The Yamama type section was pieced together from a number of short exposures on the Al 'Qusay's' upland surface between lat 24^o00'24"N., long 47^o15'42"E. and lat 24^o00'24"N., long 47^o20'54"E. 

As defined by Steineke, Bramkamp, and Sander (1958), 58 m of section between the dense Sulaiy limestone below and pre-Buwaib unconformity above was assigned to the Yamama. As already mentioned in connection with the Sulaiy, recent work has shown that the basal 4 m of the Yamama type sequence belong logically with the Sulaiy. The Lower limit of the Yamama is herewith redefined to account for this change. 

The picture is further complicated by the fact that many beds assigned to the lower part of Buwaib at its type locality further north were also included in the Yamama type sequence. In addition, it is certain that the "pre-Buwaib unconformity" was somewhat misplaced in both areas. To straighten out the relationships between the Yamama- and Buwaib- type sequences and adhere as far as is feasible to older concept, the upper limit of the Yamama has been adjusted in the outcrop and subsurface to coincide with the now accurately placed "pre-Buwaib" surface of unconformity. At the Yamama type locality this involves lowering the Yamama-Buwaib contact about 8 m. The effect on the Buwaib-type sequence has been to include the lower 23 m in the Yamama and restrict the Buwaib to approximately the upper 10 m. Evidence for these changes is as follows:

1. Analysis of the type Buwaib microfaunas by means of recently developed species keys shows that Cyclammina greigi sensu strictu does not range below the top 10 to 12 m of the formation. Also, forms which appear in samples immediately below that level are those which are known elsewhere only from the Yamama Formation. Furthermore, the echinoids from the type Buwaib are associated with these lower forms and appear to fall below the lower limit of Cyclammina greigi. 

2. Specimens of Pygurus cf. P. rostratus from the type Buwaib came from a position high in the formation as originally defined. This species can now be accurately placed within the type Yamama microfaunal sequence and can be demonstrated to fall below the range of Cyclammina greigi here as well. 

There is no possible doubt that least the lower 23 m of the type Buwaib is equivalent to a roughly comparable interval in the lower part of the type Yamama and that the echinoid occurrences in the two areas fall very nearly at the same level in time. 

Redefinition of the Yamama now places 45.5 m of strata in this unit at the type locality. The base is at the contact of chalky aphanitic and calcarenitic limestone of the Sulaiy below with golden-brown calcarenite of the Yamama above. The top is placed at the unconformable break from golden-brown pellet-calcarenite below to molluscan calcirudite of the Buwaib above.

OCCURRENCE AS THICKNESS

Outcrops of the Yamama form a very thin, curved band from lat 23°04'N. to the northern end of Jal Buwayb (lat 25°24'N.), a distance of nearly 300 km. In the central area the band reaches a maximum width of about 9 km. Yamama rocks are continuously exposed from their southern limit to Wadi Nisah where the beds are covered by younger deposits. The formation reappears north of the wadi, then persists to the northern edge of the Al Jubayl plateau where it passes under Quaternary cover. It is again exposed for about 25 km at Jal Buwayb in the steep face of the Al 'Aramah escarpment. 

Except for the clifflike exposures at Jal Buwayb, Yamama rocks usually weather to a gently tilted, steplike series of dip slopes and low parallel benches or hills. These are in turn superimposed on the flat upland surface dipping east from the Hit escarpment. 

Complete measurement of the Yamama sequence has been made a two localities. At the reference section (near lat 24°00'N.) the formation is about 45 m thick; far to the north near Khashm Buwayb (lat 25°15'N.) it is about 25 m thick. Intraformational correlations show that most of the loss of section north from the reference area is due to pre-Buwaib truncation. 

About 10 km south if the reference section (near lat 23°54'N.) another full Yamama sequence (40 m) can be pieced together from a measured section and two shallow air-drilled holes. As these holes were spudded directly on the outcrop, their stratigraphic position in terms of the surface sequence is precisely known. At this locality, loss of beds relative to the reference section can be attributed to pre-Buwaib erosion. Beveling of the Yamama continues south-ward nearly to lat 23°00' No. where the entire unit is eliminated.

LITHOLOGIC CHARACTER

Basically the Yamama Formation, like the Sulaiy, is a nearly pure carbonate unit. Calcarenite, calcarenitic limestone, and aphanitic limestone occur in varying proportions along strike. Clean-washed calcarenite and calcirudite are the main rock types at the type locality; away from this area progressively muddier sediments-aphanitic and calcarenitic limestone and even very thin layers of shale-come in. 

R. W. Powers and H. A. McClure in 1962 measured in detail a full Yamama sequence not far from where R. A. Bramkamp originally pieced together the type section. Samples collected at closely spaced intervals along the traverse provide the most complete record of Yamama outcrop lithology now available. As a result, the section is herewith designated a reference locality. 

The reference section (fig. 9) is a composite of three isolated but overlapping measurements. The lower 26.5 m was measured along the back slope of the Hit escarpment between lat 24°00'37"N. long 47°15'40"E. and lat 24°00'13"N., long 47°16'54"E. Another 11 m was picked up in the side of a low hill from lat 24°01'04"N., long 47°19'08"E. to lat 24°00'48"N., long 47°19'08"E. The upper 8 m was described at another hill between lat 24°02'47"N., long 47°20'20" E. and lat 24°02'42"N., long 47°20'27" E. 

Rocks found in the reference section are typical of Yamama exposures elsewhere; hence a complete description is given in section 46 (p. 140).

NATURE OF CONTACT WITH ADJACENT FORMATIONS

The Yamama Formation rests conformably on the Sulaiy, and thus, throughout the length of outcrop, limestone lies on limestone. Generally, however, the break is marked by a change from tan, moderately porous aphanitic limestone below to brown, tightly cemented pellet calcarenite above. In addition, Sulaiy bedding tends to be thicker and more resistant to erosion, and, on weathering, these beds form a steplike series of 2- to 10-m sheer cliffs separated by short intervals of dip slope. In the Yamama, on the other hand, short intervals of soft beds form low-angle slopes each capped by a thin 0.5- to 2.0-m resistant ledge. 

The contact between the Yamama and overlying Buwaib Formation is uncomformable. In spite of this, the break-limestone on limestone-is inconspicuous, and exact placement must often must often await detailed faunal control. Beveling of the Yamama involves only a few meters per kilometer, and the unconformity can only be recognized by the systematic disappearance of units over a long distance. For example, from the reference section south, the Buwaib rests on progressively older units of the Yamama, cuts across the Sulaiy, and then rests on Arab beds. 

Although some pre-Buwaib truncation of the Yamama is apparent north of the reference section, surficial deposits cover the units before the Yamama is fully cut out. In fact, it is quite possible that some lower Yamama strata persist beneath the cover far enough north to intersect the pre-Wasia unconformity near Wadi al 'Atk.

PALEONTOLOGY AND AGE

Occurrences of Pygurus rostratus Agassiz and Trematopygus cf. T. grasi d'Orbigny (identified by J. Roger fide Sander in Steineke, Brampkamp, and Sander, 1958) within the Yamama Formation are indicative of a Valanginian age. 

A partial list of Foraminifera from the Yamama Formation is as follows: Everticylammina eccentrica Redmond, Everticylammina elegans Redmond, Pseudocyclammina cylindrica Redmond (Redmond, 1964b), Pseudocyclammina spp., Trocholina sp. (a large, low-spired form with the base more convex than the spiral side). As nothing is known of the occurrence of the above Foraminifera outside of Saudi Arabia, they can have not special significance, in themselves, in determining the age of the formation. 

A comprehensive list of Yamama fossils includes: Astrocaenia ? sp., Milleporidium cf. sp., Pygurus rostratus Agassiz, Trematopygus cf. T. grasi d'Orbigny, cidaroid radioles, Cyclammina spp., Evertiyclammina eccentrica Redmond, Everticylammina elegans Redmond, Pseudocyclammina cylindrica Redmond, Pseudocyclammina sp., Trocholina spp., Haplophragmium sp., Haplophragmoides sp., Trochammina sp., Anisocardia sp., Aporrhais sp., Cerithium (s.l.) sp., Corbis sp., Corbula sp., Chione cf. sp., Cuculaea? sp., Exogyra spp., Gyrodes? Sp., Gryphaea balli (Stefanini), Homomya sp. Lima sp., Modiolus?, Natica sp., Nuculana sp., Ostrea sp., Paphia sp., Pholadomya cf. P. decussata, Pleurotomaria? sp., Siliqua sp., Straparolus? Sp., Tellina sp., Trigonia sp., Trochus sp., Venus sp., unidentified ammonite genus showing affinities with Knemiceras and Hypengonoceras, and unidentified small molluscs.

ECONOMIC ASPECTS

The offshore field of Manifa contains productive oil in the Yamama Formation (Upper Ratawi reservoir). This is the only occurrence in this formation within Saudi Arabia, but other Middle East field presumably have accumulations at this level as well. At Manifa, the effective thickness is small and limited to calcarenite development in a dominantly aphanitic interval.

STEINEKE M. & BRAMKAMP R.A. [1952].- Geology of the Arabian Peninsula. Sedimentary Geology of Saudi Arabia. A review of the sedimentary geology of Saudi Arabia as shown on USGS Miscellaneous Geologic Investigations Map I-270 A, “Geologic Map of the Arabian Peninsula,” 1963. Geological Survey Professional Paper 560-D, Washington, 147 p.
From pages D70 to D72.
Reprinted by courtesy of the U.S. Geological Survey, U.S. Department of the Interior.