By SIENG Sotham Deputy Director General

General Department of Mineral Resources Ministry of Mines and Energy,

The Kingdom of Cambodia

Due to the degree of study, the Jurassic of Cambodia is divided only into 02 divisions: 1) The Lower-Middle Jurassic(Lias-Middle Jurassic or Upper Triassic-Middle Jurassic in old literatures) and 2) The Upper Jurassic-Lower Cretaceous (Late Jurassic-Cretaceous in old literatures).

The first study on these formation is dated back to 1930s’ during the compilation of the first 1:500,000 geological map of Indochina. Observation on Upper Jurassic-Lower Cretaceous formation was furtherly and separately studied by different 1/200,000 geological map authors. Different studied areas and different rock type description make hard to correlate these different Jurassic outcrops.

J

T

K

J3-K1

J1-2

The latest study of the Jurassic of Cambodia were done by Vietnamese geologists during the compilation of 1:1M geological map of Cambodia, Lao and Vietnam in late 1980s. The J1-2 finally replaced the T3-J2 in the 1/200,000.

Despite the study, correlation can be made in some parts, but in general, the boundary between J3 and K1 was not detailly studied and, thus, it can not be drawn.

The age of the formation age is based mainly on stratigraphic position and thus it is not very clear. Study of microfossils and pollens enable to give more precise date to some levels of this Jurassic formation

There is no further effort to study and date the series made by local geological institution or foreign geologists; and thus, information given in this presentation is mainly based on data provided in old literatures and on some recent observation as well.

2/3 of Cambodian territory covered by flood plain composed of Quaternary sediments and occupying the Central part of the country. To the Northern and Eastern plateau, thin cover of Quaternary make older Jurassic formation appearing on the surface in quite a large area.

The Jurassic formation also occupies a large area in western and northern Cambodia, where it forms high land along the border with Thailand.

Isolated Jurassic can also be seen in the Central Plain, where it outcrops in form of isolated hills and mesas.

As the Jurassic of Cambodia is divided into J1-2 and J3-K1, it is clearly seen that the J1-2 occupies a low relief, where it appears in flood plain below a thin alluvial cover in central and eastern Cambodia.

In difference from the J1-2, the J3-K1 forms high mountain ranges in W and N Cambodia and outcrops as isolated hills within the Central Plain. In E Cambodia, its thickness is insignificant and, in some areas, blocks of J3-K1 sandstone make seen underlying bedrock.

J1-2, or T3-J2 in old literature, known as “The Terrain Rouge” in french or “Red Terrane” in English, is equivalent to the “Undetermined Indosinian- (It)”in the 1/500K geological map of Indochina.

The name of “Red Terrane” came from purplish red strata of siltstone, claysotne, carbonaceous sandstone and marl.

In E Cambodia, to E of Mekong River, the “Red Terrane”is seen in river valleys, where it occupies a large area and covered by thin Quaternary overburden..

The thickness of J1-2 can varies from hundreds to thousands meters.

At Prek Krieng, E Cambodia, the sensu stricto red series, which composed solely of purplish red strata including fine grain sds, slt, carbonaceous sds, claystone have a thickness reaching to 2000meters.

A cross section of the Red Terrane in E Cambodia (1/200K Ratanakiri Sheet )shows from bottom to top, series of strata as following:

1.Purplish red rhyolitic breccias 2.Fine to coarse, conglomeratic carbonaceous sds(25m) 3.Purplish red strongly carbonaceous sds, slt with conglomeratic

interbeds.(400m) 4.Purplish red poorly carbonaceous sds, slt with several claystone

beds.(500m) 5.Purplish red fine grain sds, slt with conglomeratic beds and

marl.(200m) 6.Purplish red cly. and slt with sds beds and delerite veins. (200m) 7.Purplish red and greyishgreen claystone.(400m) 8.Purplish red claystone, sandstone with few green middle grain

sds beds(250m) 9.f. to m. grain bluish greywith purplish red clay interbeds.(150m)

In Central Cambodia, outcrops of purplish red sandstone, siltstone and claystone covers a large area and easily determined by its weathering product (red color soil) that covers it.

At a site to the southwestern of Preah Vihear town the “Red Terrane” is overlain by a bed of younger coarse grain bluish grey very hard quartzitic sandstone, which is similar to sandstone materials that is used to built towers of Takeo Temple in Siem Reap.

In W Cambodia, the J1-2 unconformably underlies the J3-K1 sandstone of Cardamom range.

To the N, in Mongkul Borey river bed, the J1-2 consists of purplish red sds and slt can be seen in lower water season.

1

2

The “Red Terrane” is considered (R.P Fontaine) as of sub-continental lacustrine, lagunal or brackish.

The “Red Terrane” contains, in its whole thickness , intercalations of more marine greenish blue and greyish green sandstone.

By microfauna fossils, the “Red Terrane” is dated from Lias (in N of Siempang district, to Upper Lias.

If not affected by nearby fault, the dip of the Red Terrane is from 20 to 250.

In old 1/500,000 geological map of Indochina, a thin layer of lumachelle bearing greenish yellow sandstone is reported found in NE Cambodia, to the N and E of Siempang town.(N-NE of Stung Treng). The layer extends with more importance to the E of Cambodia

The discovery was not verified by later mapping surveys in the 1960s.

The name of “Upper Sandstone” is used in the old 1/500K geological map of Indochina due to that its position occupies the upper part in the “Indosinian” sandstone formation deposited from Triassic upward to Upper Cretaceous.

This “Upper Sandstone” is part of the Khorat formation found in Cambodia, where its age is supposedly from Upper Jurassic to Lower Cretaceous.

In all observed areas of Cambodia, the “Upper Sandstone” overlies the J1-2 “Red Terrane”sub-continental formation.

The “Upper Sandstone”covers a large area in W Cambodia, where it forms mountain ranges (Cardamom range (1) in the west, Dangrek range(2)in the north, group of isolated hills(3) in central north of Cambodia. In E Cambodia(4), its outcrop is very few with thin thickness.

1

2

3

4

The series of “Upper Sandstone”is a bit different from one to another areas and can be divided in three including the SW , the N and the Central N Cambodia, where it outcrops in form of isolated sandstone hills.

1

2

3

The cross section of Bokor mountain (1) and Kirirom plateau(2) are identical and can be used as the cross section type of “Upper Sandstone” in SW Cambodia. At Kirirom plateau P. Fleuriot de Langle and B. Martin de Vivies shows a typical cross section that consists of, from bottom to top, the following successive strata: 1. At the base, white to pink coarse grain

siliceous quartz fine gravels bearing, sometime conglomeratic sandstone.

2. Grey green and purplish red claystone and siltstone,

3. White fine grain siliceous sandstone with coarser interbeds,

4. Greyish green well bedded fine and very fine sandstone with beds of sedimentary breccias,

5. Many colored (greyish green, purplish red or yellow)claystone and silty claystone typical of Pich Nil.

6. Saccharoidal siliceous white massive sandstone on the top.

Only the 4th strata is seen in Kg. Som-Ream.

1

2

The cross section of Northern Cardamom range , south west of Moung Russey town shows a typical cross section that consists of, from bottom to top, the following successive strata: 1. 100m Red claystone with sds intercalation (J1-2). 2. 100m various continental sds. 3. 60m of red claystone overlaid by 100m purplish red

sds with red claystone intercalation. 4. 100m of red claystone (continental) 5. 150m of pure sub-quartz sds with cross

beddings 6. 200m of red claystone (continental) 7. 100m of pure sub -quartz sds same as the 5

level 8. 500m of fine to middle grained white, grey,

yellow, pink sds with cross bedding and several conglomeratic interbeds.

The Dangrek range in N of Cambodia(1) is just the southernmost clip of the Khorat Plateau.

Based on poor dating due to lack of fossils as well as its position of overlying the “Red Terrane” at its base of the series, the age of Dangrek series was supposed as J3-K1. However, recent litterature dates the series only Upper Jurassic.

1

A cross section at Preah Vihear cliff shows, from bottom to top, the following strata(O. Dottin): 1. Purplish red sds and siltstone of T3-J1(J1-2) age. 2. 130 m Yellowish white sds with thin Mn stained

black layers. 3. 15m Thin layers of middle grained white sds

with dark manganese stains and contains thin interbeds of black clay

4. 35m Crossed strata of fine grained yellowish white sds. 5. 250m Massive layers of white sandstone. 6. 2-3m of onglomeratic sds 7. 15-20m Fine grained yellowish sds with very thin

reddish brown beds. 8. Red sds. Only exist in Khorat, Thailand. This

strata is observed in Thailand territory.

Phu Phan Member

Prah Vihan M.

Phu Kadung M.

The J3-K1 of Phnom Kulen and Phnom Tbengshows similar strata to those of the Dangrek range.

However, there exist different facies of pyroclastic sandstone, which is absent from the Dangrek cross section.

It should also note the presence of bluish grey sandstone, which is the materials to build antic temples in Cambodia, at the bottom of the series.

3

Small isolated outcrops of J series are found in Preah Neth Preah district of Banteay Mean Chey Province.

Strata of yellow sandstone with conglomeratic interbeds and multicolor claysotne layers are found.

Red sandstone, which look similar to the building materials of Banteay Srey temple, occupies the observable lowest part of the series of about 4 meters below the surface.

In Ratanak Mondul district of Battambang province, dinosaur footprints are found in a yellow fine grained sandstone.

1

In NE Cambodia(Kratie province), the J3-K1 is found as scattered blocks overlying the “Red Terrane”.

The age of this NE J3-K1 series in given by its stratigraphic position.

1

This formation, which is dated also to Jurassic-Cretaceous forms several hills in the plain of Siemreap as well as in area to the N of Phnom Penh including Phnom Kraom, Phnom Dey, the base of Phnom Morech at the northwesternmost of W flank of Kulen, Phnom Batheay etc.. . At Phnom Kraom, this formation composed of alternate succession of compact cinerite, scoria, rhyolite lava, grey rhyolitic tuff, grey cinerite sandstone and volcanic conglomerate. At Phnom Morech, these tuffs intercalate in “Upper Sandstone” serie. This tuff intercalation and tuff fragment found in “Upper Sandstone” indicates the the 2 formations forms contemporary

Rhoylite and dacite are commonly found in W Cambodia.

The age of these rock in this area is contemporary to that of the Upper Sandstone . Presence of acid pyroclastic elements in some layers of Upper sandstone seems to prove the above datation.