Multi-method study of the Roman quarry at Podpeč sedimentary succession and stone products

Abstract

The paper presents a multi-method characterisation of the Roman quarry of the middle Lower Jurassic (Pliensbachian) limestone situated in the village of Podpeč, south of Ljubljana, and examples of the placement of stone products made from micritic, fine-grained, and oolithic facies into the known extent of the quarry. 23 m of the rock succession from the ancient quarry was exposed at the northern tip of the St. Ana Hill by archaeological trenching. Petrological, micropaleontological, mineralogical, geochemical, and isotopic analyses of carbon, oxygen, and strontium were performed in order to characterise the rocks exploited in the quarry. Additionally, a new detailed geological map of the wider Podpeč area was prepared, which defines in detail the lithostratigraphic units in the area.
The recorded succession contains facies that also occur in the modern part of the quarry. Interpretation of the sedimentation environment is consistent with previous interpretations and occurred in an internally differentiated lagoon. The studied succession is characterised by δ¹³C isotope values ranging from -2.44 to +2.5 ‰; δ¹⁸O values ranging from -4.0 to -1.2 ‰; and ⁸⁷Sr/⁸⁶Sr values ranging from 0.707414 ‰ (SD 0.000003) to 0.707329 ‰ (SD 0.000012). The Sr isotope values can prove a decisive factor when studying the provenance of stone products, while δ¹³C and δ¹⁸O values can help narrow the place of extraction within the known extent of the Roman quarry at Podpeč. The high positive correlation of SiO₂ with Al₂O₃, K₂O and TiO₂ recognised both in the logged succession and in the studied stone products indicates a low terrigenous input into the depositional area and further confirms the provenance determination.
By applying a multi-method approach to the characterisation of the known extent of the ancient part of the Podpeč quarry, we have reliably determined the provenance of stone products that have their origin in the quarry and have successfully applied this approach to several stone products made of micritic, fine-grained and oolithic limestones.