MGB-7
MINES AND GEOSCIENCES

 

 
BUREAU, REGION VII
MGB-7
PROFILE - GENERAL GEOLOGY
CEBU
  • The oldest rock unit in Central Cebu is the Tunlob Schist which is probably of Jurassic age. This consists of chloritic orthoschists and micaceous paraschists belonging to the albite-epidote-amphibolite facies of moderate grade metamorphism.
  • The Mananga Group of Cretaceous to Paleocene age, overlies the Tunlob Schist. This consists of limestone, clastic sedimentary rocks, andesitic to basaltic pyroclastics and lava, calcareous mudstone, conglomerate and sandstone. The group includes the Pandan Formation (Corby et al.), Cansi Volcanics and Tuburan Limestone(Santos Yñigo, 1951).
  • Unconformably overlying the Mananga Group are the late(?) Eocene to early Miocene sedimentary formations consisting of the Lutak Hill Formation, Cebu Formation and Malubog Formation. The Lutak Hill formation is mainly limestone with basal sandstone containing Lepidocyclina and Nummulites. An upper Orbitoidal limestone, a lower clastic unit with coal measures and a basal conglomerate (Guindaruhan) comprise the Cebu Formation. The Malubog Formation is composed of mudstone, shale and occasional beds of conglomerate, limestone and coal.
  • Middle Miocene rock formations unconformably overlie the older rocks. First in the sequence is the Luka Formation consisting of interbedded sandstone and mudstone with conglomerate and limestone lenses. The Uling Limestone comes next and is a generally hard, massive limestone but partly porous and coralline. A pinkish to reddish colored facie occurs at places. At the top of the sequence is the Toledo Formation consisting of thin to thick bedded sandstone and shale with occasional lenses of conglomeratic limestone and calcarenite.
  • The Late Miocene Maingit Formation unconformably overlies the older rocks. The formation is composed of granule to cobble conglomerate with interbedded shale, sandstone, limestone and conglomeratic limestone in the lower part.
  • Unconformably overlying the older formations is the Barili Formation of Late Miocene to Early Pliocene age. This consists of a Lower Limestone member which is generally light brown in color, hard, coralline and porous and an Upper Marl member which is poorly bedded and slightly sandy.
  • The Plio-Pleistocene Carcar Formation occupies the flanks of the island. It is a dominantly coralline limestone, poorly bedded to massive and partly dolomitic.
  • The Quaternary Alluvium occupies the low lying areas and the river banks. This consists of detrital materials made-up of silt, sand and gravel.
  • The Lutopan Diorite intrudes the Mananga Group at places which have been postulated to have brought the mineralization of economic quantity and quality in the Central Cebu area.
  • The Bulacao Andesite of Late Miocence age also occur at places. This consists of porphyritic andesite and partly intrusive breccia.
  • Serpentinized ultramafic and mafic rocks occurs as diapiric intrusions along the major faults.
BOHOL
  • The Alicia Schist which appears to be Cretaceous, is the lowest formation in Bohol area and the Ubay Volcanics which appear to be Paleocene are exposed in the eastern to northern Bohol. Both units were accreted by the Cretaceous Boctol Serpentinite and they were intruded by the Paleocene Talibon Diorite.
  • Formations deposited after Miocene occur extensively in southeastern to western portions and is overlain by the Pliocene to Pleistocene Maribojoc Limestone.
    The Alicia Schist occurs as elongated shape in N-S trend in eastern flank of Bohol Island and is accompanied by the Boctol serpentinite. The unit consists of medium grade green schist and mica schist.
  • The unit generally trends N10-60 E, and dips northwest or southwest. Fractures parallel to the schistosity are observed at exposures with less deformation.
    The Ubay Volcanics occurs at the northeastern part of Bohol Island, and consists of Paleocene dacite, pyroxene andesite and basalt. Various kinds of alteration and various grades of weathering are observed. The relationship to other formations are not clear.
    The Calape Limestone Corby, et al, (1951) and Arco (1962) described Late Eocene Cameria bearing limestones which occurs the southeast of Tubigon, northwestern Bohol Island. It is now known that these limestones are not exposed but floated.
  • The Ilihan shale is described as steeply dipping unit with interbeds of sandy tuffs and calcareous volcanic rubble beds, and occurs at Ilihan Sur. Tubigon, the present study gives an Oligocene age to this unit based on paleontrogical interpretation. (Yolanda, 1988).
  • The Wahig Limestone is white to light gray and massive to bedded limestone rich in small orbitoids. This unit overlies unconformably the Ubay Volcanics which is exposed widely in the central and northern parts of Bohol and covers the untruded Talibon Diorite, Arco (1962) reported this unit as Upper Oligocene to Miocene. However, more recent dating of samples of this unit placed the age as ranging from Early to Middle Miocene (possible Early Miocene) (E.G. Sajona, et al. 1986).
  • The Carmen Formation is distributed mainly in the eastern part of Bohol occupying 30 to 40 percent of the total island area. This low dipping thick sedimentary sequence is essentially composed of shale, sandstone, slabby to massive limestone, conglomerate, siltstone, marl and some Tuffaceous and siliceous facies. Arco (1962) reported this formation to be Middle to Upper Miocene. However, samples obtained during this survey yielded fossils from Early to Middle Miocene. The conglomerate of this formation occurs around Dimiao, the southern coast and along Tanguhay River.
  • At the northeastern part of the island, occasional coal lenses in association with Tuffaceous sandstone are observed.
    The Sierra Bullones Limestone is massive limestone mainly exposed in the southeastern part of
  • Bohol. The upper part of the formation consists of white tuffaces-calcareous siltstone, shale and mark. The limestone is mainly composed of medium to thick coralline beds, biocalcarenite beds and reefal limestone. This formation overlies the Carmen Formation unconformably. Almost 95 percent of the whole Anda Peninsula, in the southeastern part of Bohol is covered by this limestone and it’s absence on the western part suggests the tilting and subsequent submerging of the southeastern part, during the Late Miocene.
  • The Maribojoc Limestone occurs extensively in the western part of Bohol. It is mainly massive or lagoonal limestone and creamy to brownish yellow. It is also coralline, porous and in places marly or agglomeratic. The rocks resembles the Carcar Formation of Cebu. Bedding planes are generally flat though poorly bedded. It is more likely that it was formed either after the deposition of the Sierra Bullones Limestone during Late Miocene, or after the Jagna Andesite activity (E.G. Sajona, et al. 1986).
  • The Maribojoc Limestone overlies unconformably the Sierra Bullones Limestone. It was dated Pliocene through fossil studies, but it is inferred that the deposition continued to Pleistocene (E.G. Sajona, et al. 1986) and this long deposition period suggests that the environment of this area was stable during this time.
  • Alluvium is composed of silts, sands and gravel and is confined in alluvial plains and coastal parts.
NEGROS ORIENTAL
  • Negros Island is made up of volcanic and Sedimentary rocks of Cretaceous to Pleistocene age.
  • The Basak Formation is the oldest rock formation in the island. This consists of mainly of basalt with intercalated meta-sediments and grayish volcanic wackes (GOP, 1982). The Pagatban Formation intrudes into the Basak formation. It consists of quztrz diorite and diorite occurring in medium to coarse grained granular porphyries and very coarse grained gabbro.
  • Unconformably overlying the above formations is the Dacong Cogon Limestone of Early Miocene to Middle Miocene in age. Overlying this formation is the Cantoray Formation of middle to late Miocene age. It is a thick sequence of well bedded sandstone and shale with unsorted grains and shell fragments with carbonaceous composition.
  • The middle Miocene Paghumayan Formation overlies the older rocks. This is composed of andesite and clastic andesitic rocks and tuffaceous rocks.
  • The Talave Formation of late Pliocene age unconformably overlies the older formations. It consits of an upper limestone and related facies and a lower interbedded sandstone, siltstone and shale.
    The Caliling Limestone Formation unconformably overlies the older formations. It is of upper Pliocene to Pleistocene limestone sequence. This limestone is usually massive to thin bedded, coralline to conglomeratic and sometimes dolomitic with occasional lenses of tuff and bentonitic tuff (Salvado and Buenavista, 1984).
  • Quaternary Volcanics and Quternary Alluvium overlie the older rocks. The former occupies large areas in the southeastern portion of the island and along the northwest most section of the province. The latter is found along floodplains, beaches, rivers and riverbanks.
SIQUIJOR
  • The geology of Siquijor Area is composed of four formations namely, Kanglasog Volcanics, Basac Formation, Siquijor Limestone and Alluvium (Sorem, et al., 1851)
  • The lowest Kanglasog Volcanics is composed of volcanics breccia, agglomerates and a small number of volcanic flows and occurs as independent peaks of rugged, sharp crested ranges. It is overlain unconformably by the Basac Formation which consists of the Middle to Upper Miocene sequence of tuffaceous, calcareous and clastic materials. The formation constitutes undulating foothills elongated towards the coast. The youngest formation is the gently dipping Siquijor Limestone dated Upper Pliocene to Pleistocene and occurs as topographically flat-flying coastal plane, sea eroded cliffs and various terraces.
  • The Kanglasog Volcanics occurs at higher parts of the island and the type locality of the unit is Mt. Kanglasog. The unit is composed of volcanic breccias, agglomerates and a small amount of volcanic flows. The breccias and agglomerates consist of basaltic to andesitic and angular to subangular fragments which are amygdaloidal, vesicular and irregular in size. They are cemented with Tuffaceous sandy matrix. In weathered parts of the unit, pale colored fragments are scattered in reddish yellow clayey matrix. Steeply dipping tuff is partly observed in the unit. Dark gray to black tuff exhibits ripple marks suggesting marine deposition. However, most of the tuff appears to have been deposited subareally. A small amount of volcanic flow occurs localized and it consist of amygdaloidal basalt filled with zeolite and calcite in the cavities. Secondary manganese minerals which occur in fractures and at boundaries of the flows and quartz-calcite veinlets are also seen. This formation is assigned to Oligocene and covered by the Basac Formation unconformably.
  • The Basac Formation is composed of two intertonguing members of limestone members of limestone and shall. The limestone member which lies above the shale member is well exposed at the type locality of Basac. It is hard, cavernous and creamy to buff colored when fresh and gray to almost black on weathered surface. At the basal part of the limestone member, brecciated zones contain pieces of volcanic or pyroclastic rocks, it’s diameter are no longer than one centimeter. Lepoidocyclina (Nephrolepidina) smatrensis is present in the lower portion of the interbedded member and it indicates Middle Miocene (R.B. Rovillos, Jr., et al. 1985).
  • The Siquijor Formation is considered to be the youngest formation in the island, which has gently dipping beds.
  • This formation is considered with massive, hard cavernous and fossiliferous limestone and shows creamy to pinkish color on fresh outcrops and pale gray to black on weathered surface. Clastic limestones, particularly calcarenites, contain large amount of foraminiferas. Calcirudite consisting of limestone clasts in a sandy matrix are noted in various localities.
    Alluvium consists of mud, clay, silt, sand and gravel and is confined to the coastal parts and lower stream valleys.
Free Web Hosting