Geological fieldwork

UAV-based photogrammetry allows rapid mapping of pipes and tectonic deformation structures over large areas (tens of square kilometers). However, ground-based geologic mapping and sampling is necessary to analyze the interaction of the fluids with the bedrock and reveal the internal architecture. Figure 6.5 shows a representative example of a cliff within Beloslav Quarry looking NNW towards the exposure. Figure 6.5A shows a light grey sand unit that gradually changes towards the top of the interval to sandy brown. Birds nest holes indicate that the sediment is poorly consolidated. Except for some decimeter-scale carbonate cemented horizons there is limited evidence for bedding, suggesting that the sand interval may be heavily bioturbated.

The carbonate pipes are orientated sub-vertically and perpendicular to the carbonate horizons (Figure 6.5C). Most carbonate pipes are light grey in color, which is the same color as the surrounding sandy host rock. We attribute the darker grey color of some pipes to more severe

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weathering (Figure 6.5C). Figure 6.5D shows that the pipes are meter-scale in width and appear to occur in discrete clusters. There is no evidence that the spatial distribution of the individual clusters follows any regular pattern and the distance between individual pipes (~2-3 m) fits the results from the UAV analyses.

Many carbonate pipes bifurcate of towards the tops (Fig. 4E) confirming interpretations based on the UAV imagery. However, convergence of pipes also occurs in some places (two pipes merging into one; see Figure 6.5). Some pipes appear intertwined, leading to more complex geometries. All pipes show a more globular or bulbous outer surface at their top, which often correlates with carbonate cemented horizons (Figure 6.5F). In some areas the overlying carbonate layer dips slightly towards the carbonate pipe indicating a genetic relationship between the pipe and carbonate horizon (Figure 6.5G). Although sub-horizontal fractures and differing color (most likely due to preferential weathering) at the interface between the pipes and upper carbonate unit are present (Figure 6.5G). There appear to be two main carbonate units of meter-scale thickness that the pipes originate from (lower unit, Figure 6.5H) and terminate into (upper unit; Figure 6.5D, G).

The host rock is composed of poorly consolidated, quartz sandstone with minor micritic cement (Figure 6.5I). The overall unit is heavily bioturbated, with an abundance of shell fragments and an abundance of Nummulites, ranging from 0.5 – 25 mm in diameter (Figure 6.5M). The carbonate pipes and the host sandstone have a similar composition and texture but differ with respect to the amount of carbonate cementation and the presence of trace fossils (Figure 6.5L).

The trace fossils in the host rock consist of sub-vertically oriented burrows. They have convex shape, thin towards their base, and show sub-horizontal branching networks (Figure 6.5L). The outer boundary between the carbonate pipe and the surrounding host rock is sharp in weathered examples, but slightly more diffuse in unweathered examples (Figure 6.5I-K). The carbonate pipes and the overlying carbonate unit show the same texture and composition (Figure 6.5J).

171 Figure 6.5. Vantage point looking NNW, providing an understanding of carbonate pipe geometry, and the relationship of the pipes with the surrounding sandy host rock, and the upper & lower carbonate horizons. The letters correspond to the main field observations. Detailed analyses indicated by black box: (I) Two carbonate pipes observed, one less weathered (in the foreground) and one more weathered (the dark grey colored pipe). (J) The interface between the top of the carbonate pipe and the upper carbonate unit. The carbonate pipe emanates into the upper carbonate unit, despite the false appearance of a sharp contact due to a sub-horizontal fracture and white staining of the upper carbonate unit. (K) Another view of the carbonate pipe, appearing to have intruded vertically upwards through the poorly consolidated sand host rock. (L) Horizontal branching burrow network at the base of the upper carbonate unit. (M) An abundance of shells and shell fragments 0.5-25 mm in size within the carbonate pipe, highlighting the similarity in composition between the pipe and the surrounding sandstone host rock.

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To ground truth the UAV-based photogrammetric we collected information on fracture distribution, orientation, and dip in the field. For Pobiti Kamani, we measured a total of 36 fractures. They predominantly trend in NNE/SSW direction and dip at high angles towards the east, matching the NNE/SSW trend of 1016 fracture orientations measured from UAV-based data. From fracture exposure of the fracture surfaces we can measure the dip (Figure 6.6D-F), although erosion may have slightly distorted the true strike and dip directions.

At Beloslav Quarry, the resolution of the UAV imagery is not sufficient to resolve fractures.

From geological field data, the metre-scale carbonate pipes display sub-vertical veining within fractures (Figure 6.6A). The veins are linear in shape, displaying no degree of sinuosity and no evidence of branching of the veins. They are also reactive to HCL, indicating a secondary infill of carbonates in previous fractures. The veins appear to display a predominant N/S orientation.

Within a (lower) carbonate unit, S-shearing is observed (Figure 6.6B-C).

Figure 6.6. (A) Meter-scale carbonate pipes displaying secondary sub-vertical carbonate veining within fractures. The veins are orientated NNE/SSW. (B) Fractures and carbonate veins observed, cross cutting both the carbonate pipes and surrounding host rock, with no clear orientation trend. (C) S-shaped shear fabric within a carbonate horizon, indicating the presence of active N-S shear stress during the formation of this interval. (D) NNE trending fracture network on a carbonate slab surface, at higher elevation than the carbonate pipes observed to the East. (E, F) Zoom-ins of the measured fracture surfaces. Weathering and erosion may have slightly altered the true orientation of the fractures.

173 6.6. Discussion