The subdivisions of the Mytilini basin are described in some detail from bottom to top (note that formation names do NOT agree with the location of the namesake villages as Hora is built on Pythagorion Formation, Mytilini on Hora Formation and Kokkarion on Mytilini Formation; note also that we describe some localities in detail and those localities might be visited in the course of an excursion to Samos Island). Our description largely follows Weidmann et al. (1984).
(1) The Basal Conglomerate Formation restricted to the western part of the basin is made up by reddish conglomerates and locally mudstones. The type section is a road cut between Mavradzei and Mytilini in the Kazani district. The contact to the basement is either a fault or an angular unconformity. Some of the boulders above this unconformity reach a size of ~1 m3 indicating that the local relief resulting from mid-Miocene extensional deformation was pronounced. At the top of the formation are brown to yellow silts with thick palaeosoils, the latter of which are commonly incised by channel deposits. The Basal Conglomerate Formation is not dated but has been assigned a Serravallian age (Weidmann et al. 1984).
(2) The Pythagorion Formation is mostly thick-bedded limestone with layers of grey to black mudstone and lignitic horizons. The limestone contains fresh-water gastropods, stromatolites and onkolites. Dessication cracks and wave ripples are common indicating very shallow water levels. Rare clastic horizons contain tuffaceous sands, silts and conglomerate layers containing basement clasts. The Pythagorion Formation is best exposed near Mavradzei and at its type section on the Spilani Hill E of Pythagorion. To the west, the thick-bedded limestone interfingers with bituminous limestone (locally referred to as the Mavradzei beds). They are well exposed in the road cut S of Mavradzei and contain a rich fauna of freshwater molluscs, charophytes, phragmites, grey-green lignitic clays and 1-2 cm thick lignite beds. Because the exposed sediments are extensively oxidized it is impossible to recognize the fine lignitic beds anymore. Burrows and desiccation cracks are common. At the top of the Pythagorion Formation occurs the Basalt and Tuff Member (Weidmann et al. 1984), whose type section is the road cut from Pagondas to Spatherei. The top of the thick-bedded limestone is overlain there by a 3-8 m thick basalt flow. The basalt flow is overlain by beige to light grey lahar tuffs, which may reach 50 m and can be followed to the E as far as Hora, where they only measure 1-2 m. Sedimentary structures indicate a fluvial deposition of the tuff. Within the Pythagorion Formation near Pagondas there is a basaltic sill dated by the 40Ar-39Ar method at 11.2 Ma (see previous paper, Fig. 3), a lahar tuff yielded 10.9-10.8 Ma (Weidmann et al. 1984).
(3) The base of the Hora Formation contains thinly bedded fresh water limestones, which frequently contain cherts and 1-2 m thick beds of white, chalky pure limestone. Fresh-water molluscs and stromatolites occur. Above that sequence follow thin-bedded limestones and marls. In turn above follow yellow-green marls with subordinate lime-rich or siliceous shale. The marls contain frequent slump structures similar to those described at stop #2.7. The slump structures included meter-scale disharmonic folds and breccias of unsorted limestone fragments in a shale matrix. Gastropods, ostracods, diatoms and plant debris are common throughout the sequence. On top of the Hora Formation follow thinly-bedded limestones. 40Ar-39Ar dating of a tuffaceous turbidite ~1.7 km W of Mytilini yielded an age of 9.0 Ma (Weidmann et al. 1984). All three so far described formations are folded and affected by reverse faults of the D4 deformation (Ring et al. 1999a).
(4) Above the Hora Formation is the Mytilini Formation which contains all the known mammalian fossils. In the northwestern parts of the Mytilini basin the Mytilini Formation is a few metres thick and undifferentiated. The Mytilini Formation reaches its maximum thickness of up to ~200 m 2-3 km NNW of the village of Mytilini. There, several horizons can be distinguished: (a) The Old Mill Beds within which two bone beds have been identified: quarries QX and G at Smakia. The Old Mill Beds are a 15-80 m thick sequence of sands, silts, marls and water lain tuffs (grey or tan-reddish). A few layers of massive, gravelly tuffs contain channels with rounded basement clasts capped by palaeosoils. The age of the Old Mill Beds is 8.6-8.0 Ma (Weidmann et al. 1984). (b) The Gravel Beds above the Old Mill Beds contain numerous gravels and sandy marls and are of variable thickness. Channel fills are deep (up to ~5 m) and have little lateral extend. They are interpreted as deposits of immature rivers. The clasts are mainly from the basement but also from the Pythagorion and Hora formations. (c) The White Beds are silty or chalky lacustrine limestones and their exposure is limited to the Rongia and Potamis districts. Beds of poorly sorted limestone breccia occur and occasionally reach 8-15 m in thickness and suggest a marked palaeorelief. These breccias commonly constitute small hill tops or ridges. The White Beds contain fresh-water gastropods and one bone bed (Q4) at its very top. (d) The Main Bone Beds, above the White Beds, contain most of the fossil horizons, mainly near the base and near the top of the beds. The Main Bone Beds itself are tuffaceous marls, silts, sands and mudstones sprinkled with small gravels and reach 60-110 m in thickness. Palaeosoils are common and also may contain bones. Many of the gravels in the Main Bone Bed contain pebbles of the Hora Formation and the basement of the Ampelos nappe. The lowest bone bed is Q4 situated at the uppermost part of the White Beds. The next sequences are Q2, S3, S4 and S6. (4b) Above the Mytilini Formation are the Marker Tuffs containing bone bed L in thick-bedded marl. The quality of bone preservation suggests rapid burial and minimum transport. Radiometric dating indicates an age of 7.6-6.9 Ma for the Main Bone Beds and 6.7-6.5 for the Marker Tuffs above them (Weidmann et al. 1984).
(5) Above the Mytilini Formation, and locally interfingering with it, is the Kokkarion Formation. Thick-bedded and porous limestone containing abundant oncolites, stromatolites and fresh-water gastropods are most common. The Kokkarion Formation was deposited in lakes encroaching from the southeast. Its top is an erosional surface.
The Mytilini and Kokkarion formations show no do not show any sign of contractional deformation indicating that Samos Island is undergoing wholesale extension since 8.6 Ma.