Aalenian


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Aalenian

[ȯ′lēn·ē‚ən]
(geology)
Lowermost Middle or uppermost Lower Jurassic geologic time.
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It is located on the eastern flank of the NNE-orientated valley of La Bonne river, the basement of which is made of marly limestones of Aalenian age.
These erosion-sedimentation processes affected the island on several occasions, and caused the development of repeated, superposed peneplains that were roughly parallel each other: with the last Middle Jurassic (Aalenian?) uplift (see below), they gave birth to the final surface to be covered by Middle Jurassic deposits, namely the Alpine unconformity.
After the ETMEE, during the Middle-Upper Toarcian and even in the Aalenian, the distribution of the brachiopod faunal successions in the Iberian Range is markedly conditioned by the depositional sequences, particularly by the frequent variations on the bedding thickness and facies (Garcia Joral et al., 1990).
Overlying this hardground there are nodular and well bedded platy, bioturbated limestones of the Gorg Blau Formation with fauna of the early to middle Aalenian and the Bajocian (Discites to Humphriesianum zones), the Concavum Zone of the late Aalenian was not found (Alvaro et al., 1989).
The studied stratigraphic interval, ranging from upper Toarcian to middle Aalenian, corresponds to the Povoa da Lomba Formation (sensu Azeredo et al., 2003; Duarte, 2007; Barbosa et al., 2008) and to the underlying Degracias Formation, interpreted as typical facies corresponding to the middle part of the platform (Azeredo et al., 2003; Azeredo, 2007).
Successively, from the base to the top: 242 [+ or -] 7, 173 [+ or -] 5, 206 [+ or -] 6, 248 [+ or -] 8, 248 [+ or -] 8 Ma, i.e Anisian, Aalenian, Norian, and Olenekian; because of a probable rejuvenation of the K/Ar system during the late tectonosedimentary events.
The Opalinus Clay is an indurated clay, upper Aalenian (180-170 Ma), deposited in a shallow marine environment.
In: Universidad Complutense de Madrid (ed.): International Subcommission on Jurassic Stratigraphy, 1st Toarcian and 4th Aalenian working groups meeting.