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Located inside the Sagres embayment the pocket beach of Mareta is a key point to the study of the Middle Jurassic marine paleoenvironmental evolution of the Iberian Peninsula. Its cliffs cut through a seaward dipping monocline, exposing the whole Bajocian through Kimmeridgian series and the Callovian-Oxfordian unconformity along a 50-55 m high cliff.
Oolitic limestones and well preserved coral bioherms generated in a Middle Jurassic (Aelenian and Bajocian) carbonate platform can be observed (Rocha, 1976). Lower shallow marine carbonate conglomerates and fossilized coral reefs are off-set by normal faults, karstified and covered by Bajocian fine grained clastic limestones with Zoophycus sp. Rotated syn-sedimentary normal faults of Bajocian age are sealed by Bathonian marls, show that extension occurred during the Upper Bajocian.
The Bathonian and Callovian deep water marls and marly limestones with pelagic fauna and slumps correspond to an important subsidence phase, probably also associated with some flexural bending of the carbonate platform. Slumps in the Callovian can be interpreted as correlative with the Callovian extensional structures observed at Praia da Cilheta, or to the late Callovian compressional episode that probably lasted until the early Oxfordian accompanied by uplift and formation of folds and thrusts, hard-grounds and angular unconformity (Terrinha et al., 2002). This basinal subsidence also led to the mesogean and boreal provinces communication as recorded by the ammonite faunas in the upper part of the cliff profile.
In addition, the regressive phase occurred in the Iberian Peninsula in the Callovian-Oxfordian boundary, is recorded in the Praia da Mareta profile trough an erosional discontinuity. The hard-grounds and conglomerates contain fragments of ammonites of Middle Oxfordian age (Plicatilis zone, Rocha, 1976). The hard-ground at Praia da Cilheta has an oxidised appearance and shows soil-like alteration structures indicating probable uplift sub-aerial exposure of the Callovian substract before deposition of the Oxfordian limestones. The Praia do Tonel outcrop consists of folded Oxfordian and Kimmeridgian limestones and dolomites, respectively.
Much later, during the Last Glacial Maximum (LGM), dunes accumulated against the cliff carved into Jurassic limestones. Mean sea level was ca. -130m above the current one during the LGM and, sand with shells wind blow from the exposed continental shelf, formed the aeolianites that we can now observe in the westernmost part of the Mareta Beach.
The Praia da Cilheta section is a classic outcrop firstly described by Choffat (1887) and later by Rocha (1976) who established that the Callovian unconformity at Sagres corresponds to a hiatus between the ammonite zone of Athleta (Upper Callovian) and the Upper Oxfordian. Similar unconformities occur in other Mesozoic basins of Iberia, involving sediments of the same time interval (Mouterde, 1971, Rocha, 1976, Manuppella, 1988).
The Praia da Cilheta outcrop is interpreted as a small graben that suffered two inversion events, the first of which has equivalents of the same age located far east in the basin, is occurred between the Middle and Upper Jurassic rifting events (Terrinha et al., 2002), the second one corresponds to the major tectonic inversion event of Late Cretaceous – Paleogene age.
The main feature of this section is the existence of a very well marked unconformity between the Middle and Upper Jurassic. At the northern end of the section a well exposed fault plane shows the Upper Jurassic overthrust by the Middle and Upper Jurassic. The age of this thrusting event is very poorly constrained, since the Upper Jurassic is covered by un-deformed Quaternary deposits. The Callovian strata are rather more deformed than the Upper Jurassic strata, folds and thrusts in the Callovian are cross cut by the Callovian unconformity, on top of which lies far less deformed Oxfordian limestones.
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