Geological Challenges of Archaeological Prospecting: The Northern Peloponnese as a Type Location of Populated Syn-Rift Settings

Abstract

The Northern Peloponnese is not only home of a series of ancient poleis that are being studied by archaeologists, but it is also located on the southern shoulder of the most active extensional crustal structure in the world; the Corinthian rift. This rift has shaped the Northern Peloponnese as we now see it today since the Pliocene. Normal faulting, the tectonic uplift of syn-rift sediments and sea level changes, has shaped a landscape of steps rising from the coast to the ridges in the hinterland that provides challenging conditions to a geophysical survey. Where we can find coarse grained slope and delta deposits of conglomerate on top of banks of marl on ridges and slopes, the lower marine terraces and the coastal plain as well as valleys show the protective caprock eroded and the marl covered by young alluvial deposits. These materials show only a small contrast in their magnetic properties, which reduces the importance of magnetic mapping for the archaeological prospection in this region. The human utilization of the coastal plain and the urban areas pose additional challenges. These challenges have been overcome through various approaches that are shown in exemplary case studies from Aigeira and Sikyon. Whereas a combination of magnetic mapping and ground-penetrating radar (GPR) works very well on the ridges and along the slopes where we find coarser sediments in addition to the magnetic mapping, it is not suitable in the coastal plain due to the attenuating properties of the alluvial sediment. Here, electrical resistivity tomography (ERT) proved to be very successful in mapping entire parts of a settlement in great detail. Seismic soundings were also sucessfully applied in determining the bedrock depth, the detection of walls and in the question of locating the harbor basin. In the presented six exemplary case studies, the following findings were made: (1) A fortification wall and building foundations at a depth of 0.4–1.2 m on a plateau northwest of the acropolis of Aigeira was found by 400 MHz GPR. (2) A honeycomb-shaped pattern of magnetic anomalies that suggested cavities could be identified as a weathering pattern of conglomerate rocks. (3) A rock basement 2.3 m deep and remains of an enclosing wall of the Aigeira theater area were found by shear wave refraction measurements. (4) Extensive ERT surveys detected several building remains in Sikyon like a potential building and grave monuments as well as several small houses. (5) A silted-up depression in the sediments of the coastal plane located through Love wave measurements, could be taken as evidence for either a silted harbor or a navigable riverbed.

Prehistoric chamber tombs or geological pitfall? A multimethod case study from Ancient Aigeira with a focus on seismic full‐waveform inversion

“…As outlined in Rusch et al (2020), additional investigations with GPR (Figure 2c) and ERT (Figure 2d) have uncovered similar patterns. However, the results were not conclusive enough to decide without doubt if this feature is indeed caused by a system of buried or partially filled cavities that could indicate the presence of chamber tombs (Figure 2f; Cavanagh & Mee, 1998) or are related to geological structures consisting of conglomerate blocks embedded in the surrounding marl (Figure 2e).…”

“…But this structure occurs in an area consisting of weathered conglomerate showing similar blocky cross sections. The structure was found in a previous survey by Rusch et al (2020). In this case, the general low magnetic contrasts of archaeological and geological materials had motivated the additional application of GPR and ERT which had been applied successfully in the particular geological setting of Aigeira beforehand.…”

“…As outlined in Rusch et al (2020), additional investigations with GPR (Figure 2c) and ERT (Figure 2d) have uncovered similar patterns.…”

“…Measurements of the physical properties at different investigation sites show that the magnetic susceptibilities of the present materials can be found at the almost non‐magnetic end of the range (Rusch et al, 2020) and do overlap as the values for conglomerates ( κ = 0–8 10 −6 SI) and the fine‐grained marl ( κ = 4–9 10 −6 SI) show. The differences in the specific electrical resistivity and the shear‐wave velocity are higher.…”

“…The Aigeira site has been investigated geophysically since 2012 (Gauß, n.d.;Gauß et al, 2016;Rusch et al, 2016). Major results were presented inter alia by Rusch et al (2020) who also summarized geological challenges of archaeogeophysical prospecting typically connected with the syn-rift setting, to which Aigeira belongs. In the present study, we focus on a honeycomb-shaped anomaly pattern that was revealed by an initial magnetic mapping of the Aigeira site (Figure 2b).…”