Guitar Fm
Type Locality and Naming
Named after Guitar Island. The name ‘Guitar Limestone and Calcareous Sandstone Formation’ was proposed informally by Chandra and Guha (1963) and. Chatterjee (1967) defined Guitar Limestone Formation and proposed Guitar Island as its type area. The name Guitar Formation as proposed by Srinivasan and Azmi (1976b) is amendment of the term ‘Guitar Limestone Formation’, used by Chatterjee (1967) for the same formation (Source: Sharma and Srinivasan, 2007).
[Original Publication: Chandra, P.K. and Guha, D.K. (1963) The Neogene rocks from Andamans. Sci. and Cult., 29(4), pp. 202-203]
[Figure 1: Location of Guitar Island, the type area of the Guitar Fm. Major part of the island is occupied by limestone of the Guitar Fm. The southern part, however, exposes calcareous limestone of the Long Fm. (after Sharma and Srinivasan, 2007)]
Lithology and Thickness
Limestone. The formation mainly consists of well-bedded, cream-colored to pale yellowish-grey, hard, moderately sorted foraminiferal limestone. The limestone is at places, affected by recrystallization and ranges from sandy to coarse grained varieties.
[Figure 1: Summary of major facies types characterizing the Piacenzian of Car Nicobar Island, India (after Ghosh and Sarkar, 2013)]
[Figure 2: Cliff section showing limestone of the Guitar Formation at Abville Point, Sawai Bay, Car Nicobar. The raised coral reefs can be seen in the foreground (after Sharma and Srinivasan, 2007)]
[Figure 3: Microscopic views of the observed facies types. (C) Large protuberance of an undetermined non-geniculate coralline alga with fragments of geniculate algae and Heterostegina in grainstone-packstone facies. Sample 7873 H. (D) Non-geniculate Spongites (left) and larger benthic foraminifer Operculina. Sample 7873 I. (E) Foraminiferal packstone with abundant nummulitid (Amphistegina) tests. Sample 7873 K. (F) Dominating corals with encrusting coralline algae. Sample 7873 M. (G) Amphiroa with surrounding corals. Sample 7873 N. (H) Amphistegina grainstone with abundant Amphistegina, abraded fragments of algae and other biogenic components. Sample 7873 (after Ghosh and Sarkar, 2013)]
Relationships and Distribution
Lower contact
Unconformity on silty mudstone of Long Fm in Guitar Island, and unconformable (?) on deep-water claystones of Sawai Bay Fm in Car Nicobar Island.
[Figure 1: Unconformable contact between silty mudstone of the Long Fm (below) and limestone of the Guitar Fm at Guitar Island (after Sharma and Srinivasan, 2007)]
Upper contact
Regional extent
[Figure 1: Detailed geological map of Andaman area (modified after Bandopadhyay 2012) showing locations mentioned in numbers in B. Ghosh et al., 2017, Geo.Soc.London.Memoirs, 47, 95-110)]
GeoJSON
Fossils
Srinivasan and Azmi (1976b) recorded rich assemblage of foraminifera correspond to Neogloboquadrina acostaensis Zone to Globigerinoides fistulosus Zone.
Age
Depositional setting
Shallow neritic
Additional Information