La Nga Fm
Type Locality and Naming
Dat Lat Basin. Named by Vu Khuc et al., 1983.
Synonym: Ma Da Fm (for lower part); and Song Phan Fm (for upper part)
Lithology and Thickness
Lower member ("Ma Da Fm): about 410 m thick in type section at Ma Da: (1) Black-grey shale, dark-grey thin-bedded siltstone, fine-banded siltstone; 230 m thick, containing Aalenian ammonites. (2) Black-grey shale with pyrite crystals; interbedded with siltstone; 110 m thick. (3) Fine-banded siltstone and shale, interbedded with some medium-grained sandstone; 120 m thick, containing Bajocian ammonites. Upper member ("Song Phan Fm") is 750-850 m of a regular rhythmic intercalation of sandstone and siltstone.
Relationships and Distribution
Lower contact
Based on the attitude of the beds, the La Nga Fm rests conformably on the Dray Linh Fm, but the direct contact is not observed.
Upper contact
Upper boundary not clear. Regionally, an disconformity (absence of Bathonian-Callovian?) is indicated to the next younger unit of the volcanics of the Late Jurassic Deo Bao Loc Fm.
Regional extent
Upper member occurs only in coastal areas, indicating that the Da Lat Basin is only the marginal part of a large Jurassic ocean, whose center is situated in the present Indonesia Archepelago.
GeoJSON
Fossils
Lowest unit of first member (Ma Da Fm) contains Aalenian ammonites of Planammatoceras planinsigney Pleydellia aalensis. Highest unit of first member contains Bajocian ammonites of Euhoploceras cf. crescenticostata, and other genera. Bivalves of Bositra species are also present.
Age
Depositional setting
Upper member occurs only in coastal areas, indicating that the Da Lat Basin is only the marginal part of a large Jurassic ocean, whose center is situated in the present Indonesia Archepelago.
Additional Information