Al Watyah Fm

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about change in nomenclature! “In modern stratigraphy for Libya, the Permian-age rocks that were informally or historically associated with the name "Bir Hakeim" (or "Bir Hacheim") are generally referred to as the Al Watyah Formation (or included within the broader Bir al Jaja Fm depending on the specific basin boundary). Al Watyah Fm: Used across the subsurface to define the clastic and carbonate beds of Early to Middle Permian age that rest directly on the Hercynian unconformity. … Because there are no Permian outcrops in Libya (the nearest equivalent surface exposures are across the Tunisian border at Jabal Tebaga), knowledge of the Al Watyah Formation comes entirely from exploratory borehole wireline logs and core samples.” (Google Gemini overview)

Synonym: P-1 & P-II (“P” for Permian – informal designation in boreholes prior to formation-name system?). Note that the underlying Carboniferous units are then designated as “C-I” through “C-V” in this basin, although there are only 3 formations (Assedjefar Fm, Dembaba Fm, Tiguentourine Fm) in the Carboniferous stratigraphy for this basin.

References: SEPM Strata overview of Khufra Basin region, and Google Gemina (Aug’26)

Lithology and Thickness: “Primarily comprised of siliciclastics, including thick intervals of quartzose sandstones, siltstones, and interbedded carbonaceous shales.”

Lithology-pattern: Sandstone

Relationships and Distribution:

Lower contact: “It rests above the regional Hercynian (Variscan) unconformity, which separates it from older Paleozoic (Carboniferous or older basement) rocks below.” However, the SEPM summary diagram does not indicate a hiatus between it and the underlying Tiguentourine Fm (as used here)

Upper contact: Major unconformity (entire upper Permian and Triassic), then the Sirual Fm. Or, it can “Grade upward into marine Permian carbonates or Permo-Triassic clastic sequences (such as the Bir al Jaja Fm or Ouled Chebbi Fm equivalents)” (Google Gemini overview)

Regional extent: Cyrenaica Basin and adjacent platform areas in northeastern Libya. “Geologically, these Permian siliciclastics are regional equivalents to major prolific Paleozoic reservoir intervals found across North Africa and the Middle East (such as the Unayzah Formation in Arabia),” (Google Gemini overview)

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Fossils:

Age: “Early to Middle Permian (typically bracketed within the Sakmarian to Roadian stages).” (Google Gemini overview). Therefore, assigned mid-way within each of those stages.

Age span:

Beginning stage: Sakmarian

Fraction up in beginning stage: 0.5

Beginning date (Ma):

Ending stage: Roadian

Fraction up in ending stage: 0.5

Ending date (Ma):

Depositional setting: “It records a transition upward from continental/fluvial braided-river sands into coastal deltaic, intertidal, and shallow-marine shelf environments as the Permian Tethyan Seaway expanded westward across the North African margin..” (Google Gemini overview)

Depositional-pattern:

Additional Information: From Google Gemini overview:

“The Bir Hacheim Fm [Bir Hakeim Fm - alternate spelling] was a subsurface geological unit of Early to Middle Permian age located within the Cyrenaica Basin and adjacent platform areas in northeastern Libya. Because Permian-age rocks do not crop out at the surface in the Bir Hacheim area—where surface geology is dominated by much younger Paleogene and Neogene marine strata—this unit is defined and studied almost exclusively through deep oil and gas exploration wells (such as exploratory boreholes A1-19, C1-2, and C1-33) and seismic profiles.” (Google Gemini overview).

HOWEVER: “It is a classic example of stratigraphic rank shifting and regional vs. basin-wide naming practices in North African geology. In Libyan subsurface stratigraphy, the name "Bir Hacheim" (or Bir Hakeim) has been applied in two slightly different contexts, depending on whether a study focuses on the localized platform edges or deep basin-wide sequences:

1. Lower Cretaceous (Aptian–Albian) Shale Unit: In modern, high-resolution subsurface studies (particularly along the southern and eastern margins of the Cyrenaica Basin and adjacent platform blocks), the term Bir Hacheim Formation (or Bir Hacheim Shale) is formally assigned to an Aptian–Albian (Lower Cretaceous) sequence.

2. Why the earlier Chart (SEPM Strata) listed Permian: Deep exploratory boreholes drilled around the Bir Hacheim area encounter both Permian siliciclastics and Aptian–Albian shales sitting in relatively close structural proximity above and below major unconformities. Therefore, older regional charts or broad cross-sections sometimes grouped the deeply buried pre-Cretaceous/Permian section under informal local names (or used "Bir Hacheim" as a structural/geographic descriptor for the entire underlying Permian–Carboniferous column encountered beneath the Cretaceous cover).

Shift in Terminology: Because the name Bir Hacheim Formation was formally standardized by the Industrial Research Centre (IRC) of Libya to designate an Aptian–Albian (Lower Cretaceous) shale sequence in the subsurface, stratigraphers replaced its informal application to Permian strata to resolve the ambiguity.

Al Watyah Fm: Used across the subsurface to define the clastic and carbonate beds of Early to Middle Permian age that rest directly on the Hercynian unconformity.

Bir al Jaja Fm: In areas where the Upper Permian to marine Permo-Triassic transition is present, these rocks are assigned to or correlated with the Bir al Jaja Fm.

In deep borehole charts for the Cyrenaica Basin, if the Permian sequence is not assigned to the Al Watyah Fm, it is often left under informal terms like "Permian Clastics" or mapped as regional equivalents to the Amal Sandstone Fm / Amal Fm or the Unayzah Fm equivalents.