Abstract
Shell beds represent a useful source of information on various physical processes that cause the depositional condition. We present an automated method to calculate the 3D orientations of a large number of elongate and platy objects (fossilized oyster shells) on a sedimentary bedding plane, developed to support the interpretation of possible depositional patterns, imbrications, or impact of local faults. The study focusses on more than 1900 fossil oyster shells exposed in a densely packed Miocene shell bed. 3D data were acquired by terrestrial laser scanning on an area of 459 m2 with a resolution of 1 mm. Bivalve shells were manually defined as 3D-point clouds of a digital surface model and stored in an ArcGIS database. An individual shell coordinate system (ISCS) was virtually embedded into each shell and its orientation was determined relative to the coordinate system of the entire, tectonically tilted shell bed. Orientation is described by the rotation angles roll, pitch, and yaw in a Cartesian coordinate system. This method allows an efficient measurement and analysis of the orientation of thousands of specimens and is a major advantage compared to the traditional 2D approach, which measures only the azimuth (yaw) angles. The resulting data can variously be utilized for taphonomic analyses and the reconstruction of prevailing hydrodynamic regimes and depositional environments. For the first time, the influence of possible post-sedimentary vertical displacements can be quantified with high accuracy. Here, the effect of nearby fault lines—present in the reef—was tested on strongly tilted oyster shells, but it was found out that the fault lines did not have a statistically significant effect on the large tilt angles. Aside from the high reproducibility, a further advantage of the method is its non-destructive nature, which is especially suitable for geoparks and protected sites such as the studied shell bed.
Highlights
To improve our understanding of hydrodynamics from the past, it is important to study the bivalve shells adapted to specific local environments (Dunca et al 2009; Versteegh et al 2011)
To quantify the effect of post-sedimentary displacement by faults on shell orientation, we analyzed the relation between strongly tilted shells close to the fault lines (Fig. 7): shells shown in blue present roll angle above 31.5° and yellow shells present pitch angle tilt above 18.4°
The aim of this paper is to demonstrate an interdisciplinary method to calculate 3D orientations of elongated objects in Earth Science such as fossilized oyster shells from a densely packed Miocene shell bed based on approaches developed in photogrammetry
Summary
To improve our understanding of hydrodynamics from the past, it is important to study the bivalve shells adapted to specific local environments (Dunca et al 2009; Versteegh et al 2011). The position of elongated objects, such as mollusc shells, was investigated in numerous papers, where the measurements aim at deciphering past depositional environments and hydrodynamic regimes, such as prevalent currents, wave action, storm directions, and turbidite flows (e.g., Grant et al 1992; Hladil et al 1996; Newell et al 2007; Roberts et al 2008; Harzhauser et al 2015). A further drawback of most analyses is the 2D nature of the data, which focus only on the azimuth angle of the orientation This angle is interpreted as an indicator of flow or transport direction, while having two more orientation angles from the shells can be an important source of information to report in more detail about depositional condition of the specimens. The obvious disadvantage of this approach is its destructive nature, resulting in the complete loss of the studied objects
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