Morphology of Dome- and Tepee-Like Landforms Generated by Expansive Hydration of Weathering Anhydrite: A Case Study at Dingwall, Nova Scotia, Canada

Abstract

The gypsum-anhydrite rocks in the abandoned quarry at Dingwall (Nova Scotia, Canada) are subjected to physical and chemical weathering, including hydration of the anhydrite, i.e., its transformation into secondary gypsum under the influence of water. This process is known to lead to the localized volume increase of the rock and the formation of spectacular hydration landforms: domes, tepees and ridges. Cavities appearing in the interior of these domes are often unique hydration caves (Quellungshöhlen in German). For the first time, this paper gives detailed geomorphometric characteristics of the 77 dome- and tepee-like hydration landforms growing today at Dingwall based on their digital surface models and orthophotomaps, made with the method of photogrammetry integrated with direct measurements. The length of hydration landforms varies from 1.86 to 23.05 m and the relative height varies from 0.33 to 2.09 m. Their approximate shape in a plan view varies from nearly circular, through oval, to elongated with a length-to-width ratio rarely exceeding 5:2. Length, width and relative height are characterized by moderate mutual correlation with proportional relations expressed by linear equations, testifying that the hydration landforms generally preserve the same or very similar shape independent of their sizes. The averaged thickness of the detached rock layer ranges from 6 to 46 cm. The size of the forms seems to depend on this thickness—the forms larger in extent (longer) generally have a thicker detached rock layer. Master (and other) joints and, to a lesser extent, layering in the bedrock influence the development of hydration landforms, particularly by controlling the place where the entrances are open to internal cavities or caves. Three structural types of the bedrock influencing the growth of hydration forms were recognized: with master joints, with layering and with both of them. The latter type of bedrock has the most complex impact on the morphology of hydration landforms because it depends on the number of master joint sets and the mutual orientation of joints and layering, which are changeable across the quarry. The durability of the hydration forms over time depends, among others, on the density of fractures in the detached rock layer.

Anhydrite Weathering Zone with Hydration Caves at Dingwall (Nova Scotia, SE Canada) as a Potential Geosite and Geodiversity Site

“…The anhydrite was likely deposited as gypsum in Viséan (344-330 Ma) when much of Atlantic Canada was under a vast shallow water marine basin (Schenk 1969;MacNeil et al 2018). The gypsum was later, during burial, transformed (dehydrated) into the anhydrites, which in turn was subjected to hydration and gypsification in the weathering zone during exhumation (Mac-Neil et al 2018;Jarzyna et al 2022b). Just this secondary gypsum from the weathering zone covering the anhydrite bedrock with an approximately several meters thick layer was once exploited at Dingwall.…”

“…Domes are usually round, oval or variably elongated and resemble a loaf of bread in shape, whereas tepee-like forms are triangular in vertical cross-section normal to the crest (Jarzyna et al 2022b). They can be treated as basic hydration landforms which together create a rare type of relief called hydration landscape (Jarzyna et al 2022b). Fractures and rock displacements are very common.…”

“…Subtypes of geoheritage at Dingwall include aspects as, firstly, dissolution of gypsum-anhydrite rocks, which led to the formation of karst relief (Moseley 2017;Vladi et al 2022), specific petrographic features of the rocks recording diagenetic changes and weathering typical of the burial-exhumation cycle of Ca sulphate evaporite deposits (Mossop and Shearman 1973;Adams 1991;Mac-Neil et al 2018;Bąbel et al 2020;Jarzyna et al 2022a), and structural features involving spheroidal weathering and fractures developed during the conversion of anhydrite to gypsum and weathering. Furthermore, in the quarry, several sets of master joints are visible which significantly shape the morphology of hydration forms (Jarzyna et al 2022b).…”

“…The master joints are regularly arranged and form a system composed of five (from A to E) vertical and subvertical sets slightly rotated in some parts of the quarry (Fig. 2G; Jarzyna et al 2022b). According to analysis in ArcGIS, the apparent angle between the two dominant joint sets (A (east orientation) and B (southeast orientation)) is acute and varies from 25 to 39°.…”