Eustatic and Tectonic Controls on Shallow Marine Carbonates of South Andaman

Coral reefs play an important role in protecting the coastlines from wave erosion and host a significant proportion of oceanic biodiversity. In this study we focus on south Andaman (India) to understand the eustatic and tectonic controls on fringing reefs along the margins of these tectonically active island.

In October 2022 and May 2023 (two-weeks long each) fieldwork was conducted to map and sample coastal facies in south Andaman. The coastal facies identified include unconsolidated-biodetrital carbonate-sands, pebbly-beaches (mixed carbonate–siliciclastic), exposed in-situ corals (coral-carpets), conglomeratic beach sediments (mixed carbonate–siliciclastic), and rocky coastlines (ultramafic-rocks). The shelf and shallow marine areas surrounding south Andaman boast rich, fringing coral reefs.

Stream power incision and river profile inversion model is used to understand tectonic contribution. The modelling approach indicated uplift rates ranging from 0.06 to 0.1 mm/yr over the past one million year, considering an average erodibility index of 5*10-6 and a specific response time of 0.1 million years.

Even though there is an overall uplift, it is observed that the modern distribution of coastal facies is controlled by subtle variations in regional tectonics, viz. relatively uplifting western vs relatively subsiding eastern margin of south Andaman. Further, the rates of tectonic uplift are a magnitude lower than the eustatic rates. In near future sea level rise scenario, the fringing-reefs are expected to keep up with the rising sea level, however, the ocean acidification might reduce the production rates of these shallow marine carbonate systems.