Beachrock as sea-level indicator : addressing common problems through sedimentological facies analysis

Falkenroth, Michaela; Reicherter, Klaus (Thesis advisor); Hoffmann, Gösta (Thesis advisor)

Aachen : RWTH Aachen University (2022, 2023)
Dissertation / PhD Thesis

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2022


In times of global climate change and subsequent eustatic sea-level rise, coastal communities are in need of reliable models of future sea-level change. Since instrumental observation of sea level only spans the most recent centuries, other archives are necessary to identify long term trends and extrapolate these into the future. Anything that forms with an undeniable spatial relationship to mean sea level and is preserved over geological times can function as an archive of paleo sea-level. Examples are biological, like corals, geomorphological, like coastal notches or archaeological, like ancient harbours. Another one of these archives, or so-called sea-level indicators, is beachrock.Beachrocks are beach sediments that lithified through a natural cementation with carbonate (or rarely silicate) minerals. The cementation of beachrock can take place rapidly, on a scale of years rather than millennia, and is most commonly observed on tropic and subtropic coastlines. When a beachrock is removed from its coastal context, either elevated significantly above mean sea level or submerged below, its textures, cements and age can yield information on the paleo sea-level of the coastline in question.When reconstructing paleo sea-level from a sea-level indicator, two attributes are of particular interest: the age of the indicator and the elevation change it underwent since formation. Both of these are problematic when the indicator is beachrock. While dating of beachrock is a problem that may or may not be solved by advancing dating technologies, the elevation change is at the centre of this work. To quantify the elevation change that beachrock underwent post formation, its initial relationship with mean sea-level needs to be known.Sedimentological facies analysis is a way to interpret the initial position of beachrock relative to mean sea level by studying its textures, structures, fossils content, cements and other characteristics. Although its benefits are generally recognised, beachrock facies often remain under-described in the literature. In this thesis, beachrocks from two coastlines, northeastern Oman and northwestern South Africa were investigated for their suitability as sea-level indicators.In Oman, one study area shows a set of twelve marine terraces that reach a height of 500 m above mean sea level. The upper terraces are erosional and beachrocks are the only sediment-cover that withstood erosion. The lower terraces, on the other hand, are depositional and beachrocks occur alongside other sediments, which can be fluvial, alluvial, or marine. Sedimentological macro- and microfacies analysis was carried out on five outcrops on three terrace levels and three outcrops south of the terraced coastal section. This work led to the identification of eight lithofacies in six facies associations. It was demonstrated that facies analysis can lower the indicative meaning of beachrock significantly. Former models of beachrock interpretation were completed by ichnological information and observations within gravel dominated beachrocks. The data collected on beachrocks south of the terraced coastline in Sur, led to a survey of the stratigraphical relationship of beachrocks with coastal notches, bedrock fanglomerates, and biological indicators of paleo sea-level. The oldest paleo notches in Sur stem from the last interglacial sea-level highstand of MIS 5e. This is concluded from cosmogenic nuclide dating of the fanglomerate bedrock in Sur Lagoon. All outcrops of paleo notches around Sur Lagoon were investigated in regards to the faunal distribution and notch shape. Furthermore, the notches’ absolute elevation and biological markers relative to msl were measured with a differential GPS. The bioerosional notch occurs at the same height around Sur Lagoon, indicating that the area remained tectonically stable over the last 125 kyr. According to the elevation of the notch-apex, msl was 3.93 ± 0.12 m higher than today during the last interglacial. The distribution of boring and bioconstructing organisms relative to the notch shape displays at least one phase of short-term sea level rise subsequent to the notch formation. The beachrocks that are associated with the bioerosion notches in Sur Lagoon show a larger grainsize than any sediment that is deposited in the lagoon nowadays. This, in combination with the occurrence of exceptionally high and deep abrasion notches, indicates that the coastline was more openly exposed and thus experienced a higher wave energy during the notch formation.At Mission Rocks on the KwaZulu-Natal coastline of South Africa, a narrow beach with isolated sand patches occupies low points of an otherwise continuous 3 m thick, raised shore platform of sandy and gravelly beachrocks. These beachrocks are in the process of disintegrating into megagravel deposits through chemical and mechanical weathering in a wave-dominated, high-energy setting. The breakdown is potentially slowed by a contemporary, fast-forming beachrock facies that blankets the surface and fills fractures and potholes within the older platform. The accumulation and cementation of this recent beachrock was investigated as part of this thesis. The beachrock is dated by historical evidence to post-World War II. Data from field observations, petrographic, and geochemical methods reveal that the cementing agents of the beachrock were precipitated from marine water in a phreatic setting despite its position above the intertidal zone. Not only does this facies have implications for the interpretation of paleo beachrock as a sea-level indicator, it also raises further questions regarding modelling of coastal erosion of beaches associated with outcrops of beachrock.


  • Division of Earth Sciences and Geography [530000]
  • Neotectonics and Natural Hazards Teaching and Research Area [531320]