Doktorandin AG Umweltgeologie
SHK AG Umweltgeologie
Betreuer: PD Dr. G. Hoffmann & Prof. Dr. Tom McCann
Titel der Masterarbeit:
Beach rock deposits on marine terraces along the coastline of Oman (Northern Indian Ocean)
Evidentially analyses of beach rock deposits are useful in the reconstruction of regional sea level changes and hold the potential of complementing present coral-based reconstructions (Mauz et al. 2016). Their sedimentological features allow detection of the exact water depth during their formation in contrast to corals, which have a wide vertical living range. Furthermore, they can be used to test coral-based records, or, due to their higher abundance, establish records where no coral markers are available. Any paleo beach rock location above or below present sea-level can be interpreted as evidence for eustatic, isostatic or neotectonic movements. The aim of the ongoing project in Oman is to quantify the uplift rates of the area and the time frame of its occurrence in order to reconstruct coastal evolution.
The study area is located on the north-eastern coastline of Oman. This coastal section shows geomorphological evidence of neotectonic activity and presents us with a unique setting for the investigation of beach rock deposits: six raised marine terraces, showing erosional remnants of paleo beaches along their former shoreline angle. The terraces are cut into Palaeocene to Early Eocene limestone and marl formations and usually consist of a wave-cut platform unconformably overlain by a few meters of Quaternary terrace deposits, including beach rocks. However on the upper, and therefore older, terrace levels only lag deposits in form of beach rocks could be documented. The arid climate of the region in combination with a history of uplift caused several wadis to cut these sediments and left several meters of beach rock profiles in sheltered places.
Nineteen outcrops on three terrace levels were mapped, surveyed, logged and sampled. The investigation focusses on sedimentology, ichnology, and petrology to understand paleoenvironment and conditions at the time of beach rock formation as well as post-depositional changes. With regard to this analysis of the sedimentological and ichnological features allows us to identify lateral facies relationships, energy level, sediment entry, and beach morphology. Thin section-based petrographic microscopy helps to recognize the depositional environment in terms of water chemistry, cementation cycles and mineralogy.
Preliminary results show that all beach rocks in the area classify as calcarenites or calcirudites. Calcarenite beds with a bioturbation index of 4 and higher are commonly occurring. Major sediment components are extraclasts, which show signs of reworking, as well as peloids and fossils, whereas Quartz, Pyroxene, and Olivine grains are secondary. This causes limitations in terms of dating approaches by optical stimulated luminescence (OSL) due to the scarcity of Quartz and Feldspar minerals. Furthermore, in the majority of the samples, over half of the grains have been leached. The oldest beach rocks even show complete porosity inversion. In all samples multiple stages of cementation can be observed, the cement is calcitic or aragonitic and occurs as isopachus or irregular crust cement, passive cement, meniscus cement, or microstalagtites.
MAUZ, B.; VACCHI, M.; GREEN, A.; HOFFMANN, G.; COOPER, A. 2016. Beach rock: A tool for reconstructing relative sea level in the far-field. Marine Geology, 362, 1-16.