The Third International Geography Symposium, Antalya, Turkey, 10 - 13 June 2013, pp.242
Since beachrock is mostly indicative for intertidal cementation of loose beach
materials through the precipitated calcium carbonate, thickness and
subsurface extension of the cemented slabs have a privilege importance to
highlight its relation to sea-level changes especially during the Holocene. Until
recently most of publications have been depending upon interpretation of the
emerged beds of beachrock regardless of the nature of buried beds.
In this study, however, we present new data regarding subsurface geometry,
cementation characteristics and optical ages of beachrock lying on the
southern shoreline of Lake İznik, NW Turkey. Similar to cemented beach
deposits on sea coasts, both exposed and buried beds of beachrock in such a
freshwater environment may have significant implications with regard to
lake-level changes and climatic changes during the formation of beachrocks.
We first examined micro-fabrics within beachrock on the basis of scanning
electron microscopy (SEM) analyses of consecutive cements that bind the
grains. A total of eleven samples of beachrock were dated using optical
luminescence dating. Then, ERT survey was performed to image subsurface
extension of buried beds. To determine its extent and thickness as well as
contact relation with the underlying lake sediments, three ERT profiles
oriented perpendicular to coastline were measured using dipole-dipole
electrode configuration with electrodes spaced every 1 m. A total of 11 depth
levels were measured via GF ARES multi-electrode resistivity-meter system.
Our results reveal that beachrock contains abundant calcium carbonate which
occurs as void fills and meniscus bridges. Aragonite is the predominant
component. Beds slightly (5-10º) dipping toward the lake have a maximum
thickness of 1-1.5 m and they are characterized with high resistivity values.
The thickness increases landward (southward) and the tomograms pointed to
the boundary of beachrock being at 22 m-23 m backshore. OSL ages of
samples range between 1470±200 and 7406±952, suggesting that
cementation period spans from early to late Holocene. Much of beds seems to
have cemented during the climatic maximum when lake level was at least 1m
lower than present.