Beachrock in St. Martin's Island, Bangladesh: Implications of sea level changes on beachrock cementation

St. Martin's Island has been experiencing local sea level fluctuations and subsequent shoreline changes for the last 2 million years, which produced Pleistocene and Holocene beachrock along the island. The shoreline changes along the island are evidenced by the occurrence and distribution of a few conglomeratic beachrocks containing abundant fossil fragments. This study documents three separate stages of beachrock cementation that mainly depict the late Holocene local sea level fluctuations.

Beachrock exposed along the northwestern, eastern, and southeastern coastlines of the island can be grouped into three categories: conglomeratic, fossiliferous conglomeratic, and calcareous sandy. The first two categories are characterized by aragonite needle cement, micritized grains, and micritic envelopes, which are features of early marine phreatic diagenesis. This cementation phase has been formed by local sea level rise. On the other hand, petrographie features in the calcareous sandy beachrock include low Mg‐calcite cement, evidence of neomorphism of aragonite cement and micritic rims to low Mg‐calcite, random needle‐fiber calcite, calcified organic remains, and micritic networks. The diagenetic features suggest that this beachrock passed through marine‐phreatic to meteoric phreatic and meteoric‐vadose diagenetic environments. This diagenetic sequence commonly forms in response to sea level lowering. Therefore, evidence of initial deposition in shoreline environment and later residence in meteoric phreatic and meteoric‐vadose conditions suggests that the changes of beachrock cement in the study area have a direct relationship to the corresponding changes in the sea level.

Following Umitsu's (1987) relative sea level curve of the Bengal lowland, it is roughly estimated that the sea level rise at St. Martin's Island look place after ca. 10,000 yBP and before ca. 6,000 yBP, and sea level lowering occurred after ca. 5,000 yBP. However, the oldest beachrock in the study area, which forms a 6‐m‐high cliff, belongs to the late Pleistocene (Chowdhury et al. 1994). Their presence at sea level over large parts of the island suggests that cementation is currently active.

Keywords:

• beachrock
• cementation
• petrography
• sea level change
• St. Martin's Island