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LithologyEdit

Sandstone , shale , conglomerate

Stratigraphic relationshipsEdit

Unconformably overlies the Masuhi Formation ; unconformably overlain by the Mawab Formation

DistributionEdit

Lasang and Davao Rivers; Mawab and Asuncion, Davao del Norte; type locality is at Mandog, Davao City

AgeEdit

Early Pleistocene (NN19) to Late Pleistocene (NN20-21)

ThicknessEdit

200-250 m

Named byEdit

Casasola (1956)

DescriptionEdit

Unconformably overlying the Masuhi Formation is the Mandog Sandstone (Casasola, 1956) whose type locality is at Bgy. Mandog, Davao City. Exposures of the Mandog may be encountered along Lasang and Davao Rivers on the western flank of the Davao Basin and the Mawab and Makgum anticlines. The first fold is located along km 58 to 68 of the national highway west of Mawab while the second fold is located north of Asuncion.

The Mandog Sandstone consists of a poorly consolidated, thin sequence of interbedded sandstone and shale with conglomeratic portions. The latter are crossbedded, poorly sorted and polymictic, having igneous, sedimentary and metamorphic clasts. Casasola (1956) gives a thickness of 600 to 800 meters. Elsewhere, a sequence of bluish gray, fine grained argillaceous sandstone outcrops at the core of the Makgum Anticline in Asuncion.

The Mandog occupies the same stratigraphic position as the Mawab Formation , but where Casasola (1956) also describes the Mandog Sandstone west of Mawab between km 58 and 68, what was mapped by Quebral (1994) are coarse conglomerates with dacitic clasts in a sandy matrix and not polymictic conglomerates as described by Casasola (1956). If correlated with the Agusan Basin, the Mandog Sandstone probably corresponds to the lower clastic member of the Pleistocene Wawa Formation .

Casasola (1956) assigns a continental environment of deposition although he found it to be locally fossiliferous. At the Makgum Anticline, in Asuncion, the nannofossil content and the numerous megafossils, such as pelecypods and gastropods in fine sediments, as well as the presence of limestone, indicate sedimentation within a shallow marine environment.

Quebral (1994) revised the age of this formation to a range of Early (NN19) to Late (NN20-21) Pleistocene based on the presence of nannofossils in the sandstones and marls at the core of the Makgum Anticline. This Pleistocene age is confirmed by foraminifera from the limestone.

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