OCCURRENCE OF QUATERNARY MARINE DEPOSITS AT HIGH ELEVATIONS: HIGH SEA LEVEL STAND OR NEOTECTONISM?

The Quaternary miliolitic rocks of Gujarat, occurring at different altitudes are exposed as fringe deposits along the Saurashtra Coast and as isolated patches in the inland areas of Kutch and Saurashtra. These miliolitic rocks contain foraminiferal tests, ooids, pellets and skeletal fragments of lamellibranchs, bryozoans, gastropods, echinoid spines etc. The terrigenous materials include mostly quartz and rock fragments of basalt and sandstones. Petrographically these rocks can be considered as biopelsparite.

The depositional environment of these rocks has intrigued scientists since they were first reported by Carter in 1849. Presence of large-scale cross-bedding in these rocks and their occurrence at an elevation of 80 meters above the present day sea level prompted many workers to attribute an aeolian origin to these deposits and describe the inland exposures as arrested dunes.

The present study focuses on the ichnological aspect of these rocks, which has been overlooked by earlier workers. Boxwork pattern of Thalassinoides burrows has been observed in a chalky substrate exposed near Umrethi area (ca. 85 meters above mean sea-level). This chalky bed is sandwiched between coarse-grained miliolites depicting large-scale cross bedding. Due to the hard nature of the burrow-fill, also composed of carbonate material, it was easy to dig out the anastomosing network of the burrow pattern. Presence of Thalassinoides trace fossils is a clear indication that a moderate to low energy sub-littoral environment prevailed during the time of deposition of these rocks ruling out the earlier proposed aeolian origin. The calculated 230Th /234U age of these rocks has been estimated at 1.2 ± 0.02 Ky.

There is however no data indicating a high sea-level stand in the past few thousand years which could explain the occurrence of the miliolites at such high altitudes. On the other hand, a study of satellite (LANDSAT) imageries show abrupt ending of drainage patterns across a lineament indicating recent tectonic activity in this region. The occurrence of miliolites at high elevations may thus be attributed to neo-tectonism rather than rapid rise in the sea-level in the past. Further studies of these rocks in the light of ichnology might provide a clear understanding of the Quaternary palaeo-environment of the Saurashtra peninsula of India.