Geological Mapping of Islets and Rock Formations of the Argolic Peninsula and the Myrtoan Sea

Header photo: Mesoplate, isoclinally folded marbles of Spathi Islet. Skýlli Islet is visible in the background.

As part of the research projects carried out by the Hellenic Survey of Geology & Mineral Exploration (HSGME) titled: “Geological mapping of islets and rocks of the Northern and Southern Aegean Sea to complete the existing digital Basic Geological Map of Greece at 1:50K scale” and “GEOINFRA – Geological mapping actions in Greece to support innovation and entrepreneurship,” a team of HSGME researchers—Dr. Adonis Fotiadis (Geologist), Mr. Georgios Deliyannakis (Geologist, MSc), Dr. Ioannis Vakalás (Geologist), and Mr. Georgios Efthymiou (Foreman)—completed geological mapping on islets and rock stacks south of the Argolic Peninsula and in the Myrtoan Sea.

More specifically, in September 2020, full geological mapping was carried out on the following islets and rock stacks:

• Islets Tselevinia
• Dokos Island
• Trikeri Islet
• Rock stacks Strongylo, Drapi & Astéri
• Rock stacks Platonisi (a.k.a. Alexandros), Karteli, Veniza, Tagari & Disaki
• Rock stacks Pontikos, Petasi, Kales, Erimonisi & Kivotos
• Stavronisi Islet
• Rock stacks Ag. Ioanni (St. John)
• Velopoula Island
• Falkonera Island

To improve cartographic accuracy, supplementary image acquisition with a drone (UAV) was performed along selected flight transects. These data will enrich the geological sheet now in preparation.

Following processing of satellite analyses, the necessary graphical processing of structural measurements to determine the geometry of the geological maps, and study of the collected samples, a unified geological sheet will be produced with the mapping of the above islets and rock stacks of the Argolid–Myrtoan area.

Finally, the table below lists the rock–islet complexes and the individual islets/rocks to be mapped under the project in areas close to the Asia Minor coast.

Table 1: Islets – Rock stacks near the Asia Minor coast

Below is photographic material and early conclusions on the geological conditions of the surveyed islets and rock stacks south of the Argolic Peninsula and in the Myrtoan Sea.

Figure 1: Alternations of blue-white, thinly bedded, isoclinally folded marbles with chert nodules within silicified greenschists and calcareous schists on Spathi Islet.

Tselevinia Islets: At the SE end of the Argolic Peninsula (termination of the Aderes range) lie, from E to W, Skýlli, Spathi & Galí Islets, forming the upper tectonic unit. This unit consists of semi-metamorphosed and schistose formations of a flysch-like mélange that appears farther north along the SE Argolic Peninsula (see TROIΖINA sheet under preparation within GEOINFRA).

From base to top, these formations comprise purple silicified schistose siltstones (metatuffs), grading upward into alternations of grey-white, thin- to medium-bedded, isoclinally folded marbles with or without chert nodules. The latter occur within silicified greenschists capped by a few meters of metasandstones.

Figure 2: Close view of alternating blue-white thinly bedded, isoclinally folded marbles with chert nodules and calcareous schists on Spathi Islet.

The lower tectonic unit, seen on Skýlli & Spathi, consists of formations identical to the post-flysch of the Aderes mélange.

Post-Eocene compressional tectonics affecting the Pelagonian domain produced a nappe stack whose upper tectonic unit is characterized by NE-trending deformation with reclined fold axial planes at ~N40°E, generating thrusts with transport from SE to NW over the lower unit. Moreover, deformation of the lower unit—and thus of Aderes—is characterized by reclined fold axial planes trending NW–SE, plunging SW.

Figure 3: Tectonically tight-folded post-flysch of the Aderes mélange.

Faulting on Spathi includes left-lateral normal faults trending ~330°.

Figure 4: Alternations of grey-white thin- to medium-bedded, isoclinally folded marbles within silicified greenschists on Spathi Islet.

In summary, the tectonostratigraphic units involved on the Tselevinia Islets are identical to the post-flysch nappe stack of the Aderes mélange of the Argolic Peninsula (Upper Cretaceous), derived from various palaeogeographic settings of the Internal Hellenides, especially the Pelagonian isopic zone.

Dokos Island: Lying SE of Argolis, between Ermioni and the Hydra strait, it comprises:

a) the lower unit of Cenomanian rudist-bearing limestones, which occupy nearly the whole island, passing into red pelagic limestones of Maastrichtian age and then into the strongly tectonized Argolic clastic flysch (Upper Maastrichtian–Lower Paleocene), and

b) the upper tectonic unit, which appears to overthrust the flysch at the NE tip of the island and consists of a schistose tectonic sole of serpentinite bearing brecciated Cenomanian limestone rafts.

Figure 5: Cretaceous limestones thrust over the strongly tectonized flysch (Upper Maastrichtian–Lower Paleocene) at the NE tip of Dokos.

Structurally, the island is a tilted fault block dipping ~30° NW. The upper tectonic unit folds the underlying Argolic flysch into reclined, NE-trending folds verging NW.

Normal faults of E–W trend dissect the steep southern cliffs; the downthrown southern block defines the northern margin of the Hydra Channel.

Figure 6: Cenomanian rudist-bearing limestones, Dokos.

For better mapping, supplementary UAV imaging was carried out along selected transects over the eastern parts of the island by Dr. I. Vakalás and Mr. G. Deliyannakis; their geospatial analysis will enrich the forthcoming geological sheet.

Figure 7: Lower unit of Cenomanian rudist limestones passing into red pelagic Maastrichtian limestones and then into the Argolic clastic flysch (Upper Maastrichtian–Lower Paleocene), overlain by the upper tectonic unit thrusting the flysch at the NE tip of Dokos.

Figure 8: Same succession as Figure 7. Image from a 3D model generated by UAV.

Trikeri Islet: Composed of thin-bedded hemipelagic Triassic limestones with overall NW dips. At their base lie red to purple psammitic and argillaceous schists with carbonate interbeds forming the basement (Upper Paleozoic, Carboniferous–Permian) within a NE–SW zone. These lithologies match the basement formations of HYDRA (red, yellow and brown sandy argillaceous schists with mica and limestone interbeds of Upper Paleozoic age) and the pelagic Anisian limestones (Middle Triassic).

Post-Eocene compression produced internal thrusting of much of the islet towards the NW, detaching along sandy basement horizons. Normal faults of WNW–ESE trend downthrow blocks toward the Hydra Channel.

Rock stacks Strongylo, Drapi & Astéri: These comprise Triassic neritic limestones. Drapi and Astéri are planated geomorphologically due to intense faulting followed by erosion.

These lithologies likely correspond to neritic “Pantokrator” facies limestones of Ladinian–Early Jurassic age on HYDRA.

Rock stacks Platonisi (Alexandros), Karteli, Veniza, Tagari & Disaki: Lying SW of HYDRA, they consist of thin-bedded hemipelagic Triassic limestones with NW dips. Tagari and Disaki also show at their base red to purple psammitic and argillaceous schists with carbonate interbeds (Upper Paleozoic basement). These match HYDRA’s basement clastics/schists and Anisian pelagic limestones (Middle Triassic).

Rock stacks Pontikos, Petasi, Kales, Erimonisi & Kivotos: NW of HYDRA, aligned from SSW to NNE across the strait. Like the previous group, they comprise thin-bedded hemipelagic Triassic limestones, matching HYDRA’s Anisian pelagic limestones (Middle Triassic).

Figure 9: Thin- to medium-bedded hemipelagic Triassic limestones on Stavronisi Islet.

Stavronisi Islet: South of HYDRA, composed of folded thin- to medium-bedded hemipelagic Triassic limestones, likely equivalent to HYDRA’s Anisian pelagic limestones (Middle Triassic) and bedded limestones with chert (Upper Scythian–Middle Lias). Overall NE dips define folds with NW-trending axial planes, indicating an early tangential event of the post-Eocene compressional phase. The islet is cut by WNW-trending normal faults and by NE–SW normal faults downthrowing blocks to the NW.

Figure 10: Recrystallized Triassic carbonate platform with stratiform greenish to purple tuffs of Permo-Triassic age at Velopoula Island.

Velopoula Island: Tectonostratigraphically, a recrystallized Triassic carbonate platform overlies at its base stratiform greenish-olive to purple tuffs—probably Permo-Triassic—across the southern half of the island. Up-section, near the center, thin-bedded marble alternations grade upward into red siltstones–metapelites and red jaspers (pelite–radiolarite horizons). From there to the northern part, an ophiolitic cover appears, consisting of schistose microcrystalline to massive coarse-crystalline gabbros cut by tourmaline-bearing pegmatitic veins.

Figure 11: Alternations of thin-bedded marbles grading upward into red siltstones–metapelites and red jaspers on Velopoula.

Figure 12: Red siltstones–metapelites grading into red jaspers (right) beneath the ophiolitic cover (left), composed of schistose microcrystalline to massive coarse-crystalline gabbros on Velopoula.

Falkonera Island: Composed entirely of oceanic lithosphere rocks in coherent (non-nappe) relation. From base to top—recognized continuously from sea level upward without tectonic discontinuities or plastic deformation—are:

• coarse-crystalline gabbros with microgabbro veins,
• coarse-crystalline gabbros (to ~40 m a.s.l.),
• coarse-crystalline gabbros with diabase veins (to ~60 m),
• gabbros with diabase veins and basal diabases (to ~70 m),
• a sheeted complex of multiple diabase dikes cut by volcanic and aplite veins (to ~100 m), and
• massive, pumiceous to porphyritic lava flows (to ~200 m).

Figure 13: Coarse-crystalline gabbros with microgabbro veins, Falkonera.

Hydrothermal static metamorphism affected these rocks to greenschist facies with chlorite and epidote veinlets.

Figure 14: Coarse-crystalline gabbros in a gully on Falkonera.

Figure 15: Coarse-crystalline gabbros (left) and diabase dikes (lower right) cut by porphyritic veins on Falkonera.