SULAIY FORMATION - UPPER JURASSIC(?) AND LOWER CRETACEOUS(?)
DEFINITION
The Sulaiy Formation is named for Wadi as Sulayy, a gravel-filled channel at the foot of the Hit escarpment.
As originally defined by H.L. Burchfiel and K.W. Hoover in 1935, all beds between the post-Marrat and pre-Wasia unconformities were assigned to the Tuwaiq Formation. In the northern sequence studied by them, the Jubaila is the highest unit present below the Wasia.
Max Steineke in 1937, following the early definition, included in the Tuwaiq Formation a number of younger beds that underlie the "Nubian Sandstone" (that is, Biyadh plus Wasia) in the central area but that cut out to the north. The additional beds above the Jubaila were referred to simply as "Riyadh chalks and limestone, including Yamama detrital member at top."
In 1938, R.A. Bramkamp and T.C. Barger, after studying the upper part of the Tuwaiq Formation, separated out the Riyadh chalks and limestone, raised this sequence to formational rank, and defined members within it. At this stage, the Riyadh Formation included from the top down, the Yamama detrital, Sulaiy limestone, Hith Anhydrite and Lower Riyadh members.
On the discovery of what was then believed to be Lower Cretaceous fossils in the Sulaiy, the Sulaiy and Yamama detrital members were broken out of the Riyadh Formation and placed in a new formation, the Thamama. In addition to the Sulaiy and Yamama Sulaiy and Yamama detrital members were broken out of the Riyadh Formation and placed in a new formation, the Thamama. In addition to the Sulaiy and Yamama members, a group of stratigraphically higher carbonate beds was included within the Thamama formation and recognized as constituting a third, as yet unnamed member.
The first formal publication (Steineke and Bramkamp 1952b) still included these three units within the Thamama, but, by that time, the members had been formally named and raised to formational rank and the Thamama was elevated to group status. From top to bottom the three formations were designated as the Buwaib, Yamama, and Sulaiy. Later, the upper limit of the Thamama Group was redefined to include a fourth formation, the Biyadh Sandstone.
The type section of the Sulaiy Formation (fig. 8) is in the cliff above Dahl Hit lat 24o29'18"N.; long 47o00'06"E. The upper limit is placed at the change from chalky aphanitic limestone of the Sulaiy to the more detrital calcarenitic limestone and calcarenite of the Yamama Formation. The base is at the contact of the basal calcarenite of the Sulaiy with the limestone breccia of the Hith. As pointed out in the discussion of the Hith contacts, the limestone breccia (included in the Sulaiy by Steineke and others, 1958), is now considered a part of the Hith.
OCCURRENCE AS THICKNESS
The Sulaiy crops out in a gently curving are from lat 22o38'N. to Al Khatilah (lat 25o26'N.). Only the Nisah graben and a few thin banks of recent cover break the continuity of the Sulaiy over this 350 km distance. Width of the outcrop belt varies little between Khashm Khartum (lat 23o24'N.) and Khashm al An (lat 24o44'N.).
The most prominent feature of the Sulaiy landscape is the 50- to 100-m thick, west-facing slump escarpment in the upper part of the formation. Readily recognizable for over 200 km, the escarpment (herein termed the Hit escarpment throughout its length) marks the boundary between two areas of quite different topographic expression. To the east is a gently tilted, steplike, upland surface formed by the weathering of tight, well-bedded limestone. To the west is an undulating surface of contorted limestone beds and low, complexly slumped hills. These slump features characterize lower Sulaiy outcrops even in areas where upper Sulaiy beds and the escarpment are no longer exposed-that is, generally north of Khashm al An and south of Khashm Khartum.
A detailed analysis of the origin of the escarpment has already been presented in the discussion of the Hith Anhydrite. Suffice to say, the main cause was removal of underlying Hith evaporite by solution and subsequent draping of Sulaiy beds over the front edge of the remaining thick anhydrite sequence.
The only full measurement of the Sulaiy is at the type section where about 170 m are exposed. An Al Khari water well, about 40 km southeast, penetrated 168 m of Sulaiy. Lack of accurate thickness may be attributed to several factors including: (1) the basal Sulaiy is exposed only Dahl Hit, (2) the lower beds are complexly slumped, and (3) middle Sulaiy beds occurring at the foot of the escarpment are almost everywhere covered by recent fill.
LITHOLOGIC CHARACTER
Both surface data and near-outcrop wells show the lithology of the Sulaiy to be remarkably uniform. The main rock type is tan, chalky, massively bedded, aphanitic limestone. Some thin beds of skeletal and pellet calcarenitic limestone and calcarenite occur throughout but become particularly prominent in the lower 40 m. The basal beds also include moderate amounts of fine quartz sand.
The sequence in and above Dahl Hit was studied in 1938 by R. A. Bramkamp and T. C. Barger and was designated the type locality. About 110 m is exposed in the escarpment and approximately 60 m on the back slope.
In 1952 S.J. Roach reworked the same outcrop; his description of the sequence is given in section 44 (p. 139).
The few locations studied south of the structural complications of Wadi Nisah show marked lithologic constancy along strike. In 1962 R.W. Powers and H.A. McClure measured the upper 44.5 m and lower 30.6 m of the Sulaiy respectively from lat 23o54'14"N., long 47o13'47"E. to lat 23o54'09"N., long 47o14'17"E. and from lat 23o46'31"N., long 47o01'22"E. to lat 23o46'16"N., long 47o01'23"E. Recent sand and gravel cover the middle part of the formation. The accessible parts of the succession are summarized in section 45 (p. 140).
NATURE OF CONTACT WITH ADJACENT FORMATIONS
The Hith-Sulaiy contact is visible only at Dahl Hit. Here, bedding planes in massive anhydrite show low angular discordance below a plane of truncation. Until recently, this discordance was cited as evidence for a disconformity at this level. Now, however, it is believed that the contact is gradational and conformable and that the angular discordance is the result of subsurface solution rather than subaerial erosion. Elsewhere, along strike, Hith strata are missing, and the Sulaiy has been down dropped against contorted Arab limestones.
From lat 23o04'N. nearly to the Darb al Hijaz (lat 24o50'N.) the Sulaiy is overlain conformable by the Yamama Formation. Throughout this distance he contact is marked by a change from tan, chalky aphanitic limestone below to massive beds of clean-washed Yamama calcarenite above. Although the contact is covered north of the road, conformable relations between the units probably extend nearly to Al Khatilah. At about this latitude, Middle and Upper Cretaceous rocks swing across Yamama and Buwaib beds, truncate the Sulaiy, and come in contact with the underlying Arab Formation.
On the southern end of the outcrop, below lat 23o04'N., pre-Buwaib erosion cuts progressively deeper into Sulaiy strata. The formation is completely cut out near lat 22o35'N., where Buwaib rocks rest directly on Arab.
Recent sampling of the type Yamama and bracketing strata by R. W. Powers and H. A. McClure in 1962 permits accurate placement within the type sequence of faunal horizons recognized in adjacent shallow air-drilled holes. Placement of these horizons affects not only Sulaiy-Yamama-Buwaib surface correlations but sheds considerable light on their subsurface relationships as well.
C. D. Redmond also in 1962 demonstrated that the top of the Sulaiy in the area of the type Yamama should be revised upward approximately 4 m so as to include the basal member of the type Yamama. This member represents a thinned-down equivalent of the uppermost 9 to 18 m of the "Yamama detrital limestone" of deep wells. Concurrently with this the designation "Yamama detrital limestone" as used in deep-well terminology is changed to Sulaiy Formation. The advantages of these two related changes are as follows:
1. The top of the Sulaiy Formation coincides with the top of a major faunal division. Pseudocyclammina sulaiyana Redmond (Redmond, 1964b), which appears in the Sulaiy Formation and ranges upward to the top of the present "Yamama detrital" of the deep-well subsurface section, is thus confined to the Sulaiy Formation. The same conditions apply at outcrop where Pseudocyclammina sulaiyana ranges to the top of the basal member of the type Yamama, now redefined as uppermost Sulaiy.
2. In most deep-well sections, the tip of the revised Sulaiy Formation coincides with a major lithologic break accompanied by a marked electric-log signature pattern; a point corresponding to the older Sulaiy top would have to be picked on faunal evidence along.
3. The proposed modification of existing stratigraphic terminology reconciles surface and deep-well usage. Prior to the recent findings, Lower Cretaceous stratigraphic relationships between outcrop and subsurface were uncertain. Deep-well nomenclature, originally based on tentative comparison with the surface beds, can now be shown to be inexact and misleading. For example. "Yamama detrital" of deep wells in reality almost entirely falls within the Sulaiy of outcrop. Other correlations permit accurate subsurface definition of the Buwaib and Yamama, and these names will be carried into the subsurface as well.
PALEONTOLOGY AND AGE
A partial list of Foraminifera occurring in the Sulaiy Formation is given below. All the forms listed range to within a few feet of the top of the formation. Their lower limits are not so clear, however, with the exception of that of the Everticyclammina sp., which is known to range through all but the uppermost beds of the formation.
Foraminifera of the Sulaiy Formation include: Bramkampella arabica Redmond (Redmond, 1964b), Everticyclammina sp., Iberina Iusitanica (Egger), Nautiloculina sp., Pseudocyclammina sulaiyana Redmond, Trocholina sp. 1 (a large, high-spired form with a strongly protruding base and blunt apex), Trocholina sp. 2 (a medium-sized, low-spired form with radial fluting in the inner side of the marginal band).
Presence of a common Nautiloculina sp. gives the Sulaiy faunas a Jurassic aspect although this genus is known to extend into the Lower Cretaceous.
Recent work by Maync (1959) has demonstrated that Iberina Iusitanica has a somewhat extended vertical range, first appearing in the lower Kimmeridgian and extending upward across the Jurassic-Lower Cretaceous boundary into the lower Valanginian. Its presence in the Sulaiy Formation of Saudi Arabia is helpful only insofar as it limits the possibilities for age assignment to either Late Jurassic or earliest Certaceous.
Pseudocyclammina sulaiyana Redmond was discussed and figured by Henson (1948) as Pseudocyclammina aff. litus (Yokoyama) from Qatar Dukhan well 2, presumably in beds equivalent to the Sulaiy Formation. Henson listed a stromatoporoid, Cladocoropsis sp., as occurring at the same level and considered the age of the beds to be Late Jurassic.
Until such time as more defined evidence is obtained it seems best to leave the age assignment of the Sulaiy Formation open, listing it as Tithonian to early Valanginian.
A comprehensive list of fossils found in the Sulaiy Formation includes: Chaetetes sp., Milleporidium sp., Aporrhais sp., Diceras ? sp., Exogyra couloni d'Orbigny Nerinea aff. N. blancheti Pictet and Campiche, Nerinea sp., Ostrea sp., and miscellaneous cidaroid radioles.
In addition, regional considerations suggest that the overlying Sulaiy Formation is of Tithonian to Berriasian (?) age. If these relationships are valid, the Hith-Arab sequence can be dated as early Kimmeridgian and Tithonian with some confidence.
ECONOMIC ASPECTS
GROUND WATER
Because of their generally aphanitic and tight character, Sulaiy rocks appear unfavorable as water reservoirs. This impermeability does serve a useful purpose, however, in that water falling on the back slope of the Hit escarpment drains into and recharges both the Wadi Nasah-Wadi as Sahba channel system and the Biyadh-Wasia sandstone aquifer.
PETROLEUM
Porous Sulaiy rocks contain oil in one Saudi Arabian field-that is, the offshore field of Manifa. Here, the producing interval for Lower Ratawi reservoir is about 250 ft of calcarenite and calcarenitic limestone. As is true of most Arabian carbonate reservoirs, oil accumulation is directly related to development of clean washed lime sand (calcarenite). Other Middle East field outside of Saudi Arabia also presumably have discovered productive oil in the Sulaiy.
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 D66 to D70.
Reprinted by courtesy of the
U.S. Geological Survey, U.S. Department of the Interior.