Submerged beachrock: A tool for reconstructing relative sea level change example from Tekirdag coastline, Sea of Marmara, Turkey

The beachrock formations represent a significant paleo-environmental proxy because they can record both the vertical and the horizontal evolution of the coastline. They have been used to assess Holocene coastline evolution and crustally induced relative sea level (RSL) change, notably in the Sea of Marmara. In this study, we report existence of the less known submerged beachrocks in the nearshore coastal area of Tekirdag city (Altinova), the northern Sea of Marmara. The beachrocks found in the Tekirdag coastline are locally spread, parallel to the coastline with an extend of about 5 km and at depths ranging from -2 to 0m below present sea level. The beachrock is defined by a calcite-cemented shoreline sandstone. The cement mineralogy and morphology of the beachrocks are indicative of the diagenetic environment, and therefore the examination of the cement characteristics and microstratigraphy can allow identify the type of cement and spatial relationship between the past shoreline and beachrock formation zone.

The Tekirdag coastal area is located in the western Marmara Region. The western coasts of the Marmara Region include a number of natural features inherited from their coastal evolution. Besides, relative sea level change during late Quaternary in this region and its vicinity are generally not homogeneous as a result of the tectonic activity controlled by the North Anatolian Fault Zone (NAFZ) which played a crucial role in the coastline evolution at different periods of the region.

The aim of our study to reconstruct the shoreline modification using beachrocks in the study area. For this purpose, we coupled a series of methodologies for the paleo-environmental and geomorphological study of the coastal zone and the shallow submarine area, which included: a) coring from submerged beachrocks, b) petrographic and microstratigraphic analyses of cementation, c) monitoring of underwater beachrocks and coastal zone by drone and e) dating the formation of beachrocks. Through our analyses we aim to better contribute the use of beachrocks as accurate proxies for the RSL variation in the study area.