Die Deltaphasen der Unterkreide am Basko-kantabrischen Kontinentalrand, Nordspanien
Creators: name: Reitner, Joachim
name: Wiedmann, J.
Publisher: name: FID GEO
Year: 1982

description: Excellent onshore outcrops, DSDP cores and dredge sampies facilitate recognition and definition of successive stages in the evolution of the subsiding southern Biscay margin. Here the Lower Cretaceous episodes of this subsidence are treated. In contrast to the central North;Atlantic, rifting in trus area decreased du ring the Jurassic and later renewed in Late Jurassic time. "Wealden" sediments of extreme thidmesses were deposited in the first stage of pronounced subsidence (Oxfmdian to Barremian?). It is to be considered as the first deltaic episode of the second and final rifting stage of the Biscay Ocean. Late Jurassic ("Late Cimmerian") distension tectonics are the major factor controlling the sedimentary environments. Distribution of continental, limnic and brackish facies exhibit arelief of torted blocks wruch persisted through the entire Lower Cretaceous. The predominance of NW-SE directed faults at that time and the maximum of Wealden thicknesses in the N Celtiberic Ranges point to the possihility that the Bay of Biscay originally intented to open this way. The Valanginian transgression was found to be of minor importance with regard to subsidence, but is used as datum level. The second phase in the Lower Cretaceous history of the southern Biscay margin is the development of Urgonian carbonate platforms (Ap~ian to Lower Albian). These island platforms used the previous relief as far as the subrecifal carbonates were restricted to previous horst structures, while marine pelitic «Parahoplites shales" were accumulated in the channel-like grabens Decreasing subsidence may be the only difference to the previous evolutionary stage; sedimentation is, however, still able to compensate subsidence. At this time, the actual E-W directed tectonic structures became predominant. l1lis platform evolution is followed and simultaneously closed up by a second deltaic phase (Middle Albian to Lower Cenomanian in the N , Albian in the S).A new inerease of subsidenee may be eorrelated with "Austrian" distension teetonies and eonsiderable terrigenous input from the Sand W. This is a time of extreme fa eies diversity: 1) turbiditie sedimentation at the subsiding eontinental margin, 2) deltaie fans in the northern baseo-eantabrie trough, 3) eontinental and fluviatile sands in the S, 4) final persistenee of restrieted island platforms and diapirie atolls on loeal highs, 5) oceanie erust in the eentral basin, and 6) alkaline magmatism in the onshore Biseay syncline. In eorrelation with the distension teetonies this is the first period of pronouneed diapirism. The next abrupt change in sedimentary environment, i.e. co an uniform pelagie sedimentation ' (Middle Cenomanian), is due to the "global" Cenomanian transgression and integrates the whole area into a widespread pelagie faeies realm. At this time, sedimentation is unable co eompensate subsidenee. There is no doubt, that late "Cimmerian" and "Austrian" distension teetonies and the related Val anginian and Cenomanian transgressiom, altogether widespread at the North Atlantie margins, have their origin in the episodie aetivity of the mid-oceanie ridge and eompensating erustal processes. An attempt is made to eorrelate similar stages in the history of subsiding passive margins in various time-per iods.