Sedimentology of Sandstone Formation in Al-Chouf During the Lower Cretaceous Period
Article 4: Sedimentology of Sandstone Formation in Al-Chouf During the Lower Cretaceous Period
By Hadi Issa
The oldest observed rocks in Lebanon date back to the early Jurassic Period, nearly 200 million years old. The relatively thick Jurassic sequence (nearly 2 km in thickness), known as Keserwan Formation, can be observed in three different locations:
- Matn and Keserwan, Mount Lebanon.
- 2. Chouf and Jebel Barouk, Mount Lebanon.
- The Anti-Lebanon Mountains that extends along the Syrian-Lebanese border.
Mostly, the Jurassic time was dominated by the deposition of marine limestone. The scientific evidence tends to point to a quite tectonic activity during that period, and it was only until the late Jurassic when tectonic disturbances effectively began to take place.
By the beginning of the Cretaceous Era, the region experienced more faulting, giving rise to erosion, and the result was the formation of the Lower-Cretaceous sandstones known as the Chouf Formation.
Figure 1: Section from Bqaatouta, Keserwan shows the formation during the upper-Jurassic and lower-Cretaceous periods in the geologic time scale. Credit: Granier B., 2016
Figure 2: Stages of Cretaceous Period
Figure 3 & 4: Basal Cretaceous Sandstone outcrop map of Lebanon. Green: Chouf Formation Outcrop, Yellow: Lower-Cretaceous Bed Exposure (Dubertret et al., 1955, 1956; Kanaan, 1966; Wakim, 1968, Tixier, 1971-1972, Touma, 1985; Khalaf, 1986)
The Neocomian-Barremian sandstones are characterized by several compositional features:
Enrichment in quartz.
Lignite content.
Iron content.
Traces of amber.
Rare presence of fossilized material.
The Chouf Formation is mainly cross-bedded and orange-brown in color. Alternating sand and clay beds have been observed at the base, and argillaceous carbonate beds at the mid section.
Figure 5: Orangey cross-bedded sandstone in Bsalim, Matn District. Credit: George Bellos, 2008
The lower and mid parts of the formation indicate that it was deposited both in fluvial (aquatic) and eolian (terrigenous) environments. The lower parts indicate an aquatic dominance, whereas the mid parts indicate a continental dominance. The heavy mineral content of the Chouf Formation indicates that it had been derived from older sandstone formations in the region, and starting from that point, studies have pointed to the origin being the Silurian sandstones in Jordan. Similarities between the Chouf Formation and the Silurian sandstones have been observed to be in garnet compositions as well as apatite, zircon, and other heavy granitic minerals. The evidence points to deposition spanning between the Hauterivian and the Barremian.
The permeability of the rock layers indicates the presence of numerous springs containing iron-rich water, which in turn, suggests the presence of iron-rich cementation phases in the formation process. Porosity has been observed to be mostly depositional, in addition to that obtained through dissolution. Petrographic studies have shown that the lower layers, with aquatic dominance, are of organic origin, with enrichment in bitumen. This resulted in the corrosion and dissolution of quartz grains, increasing the porosity and permeability in the rocks. A study in 1974 showed that some beds range from non-porous to impermeable; limiting the process of infiltration.
Figure 6: Marine sandstone in Toumatt-Jezzine. Credit: George Bellos, 2008
Figure 7: Eolian sandstone in Toumatt-Jezzine. Credit: George Bellos, 2008
The Chouf Formation is divided into 86 beds which are grouped into three main zones:
The lower (base) zone: includes about 10 meters of coal, in addition to aquatic sand and clay.
The middle zone: mainly consists of sand.
The upper zone: nearly similar to the lower part.
Figure 8: Sandstone facies of the middle part of the Chouf Formation in Homsiye-Jezzine. The dark coal layer, in contrast with the strata, can also be observed in the photo. Credit: George Bellos, 2008
The Mesozoic Basalts of Lebanon (MBL) cover a surface area of about 150 km² in Central Lebanon. Evidence points to the presence of Mesozoic Basalts in Cretaceous beds, including the Chouf Formation. As a result of tremendous volcanic activity that took place in the Early Cretaceous, interstratified deposits have been observed in the base of the Chouf Formation, in addition to Abeih and Mdairej Formations.
Milankovitch-cycle influences can be clearly observed in the strata, indicating high sea-level depositional environments. The fact that sea level was highest (330m) between the late Berriasian and early Valanginian explains aquatic dominance in the lower part of the strata. On the other hand, the lowest recorded sea levels (90m) were during the Early Hauterivian, which corresponds to eolian dominance in the middle part of the strata.
Sandstone formations in the Cretaceous Period has been addressed by multiple studies in the Middle East. However, the lack of available studies in Lebanon indicates that the Neocomian-Barreminian rocks have not been well-explored to date. While petrographic analysis of sandstones in Lebanon has been carried out, and contributed to petroleum exploration studies, the boundaries of the Chouf Formation are still not well-defined.
