The Lower-Middle Jurassic of the Anholt borehole: implications for the geological evolution of the eastern margin of the Danish Basin (original) (raw)
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This study of Upper Pliensbachian -Bajocian/Bathonian deposits in a borehole drilled on the island of Anholt, Denmark incorporates sedimentology, biostratigraphy (palynomorphs and foraminifera), palaeomagnetism and coal petrology. The studied succession records a gradual change from marine inner shelf storm-influenced clays to mainly terrestrial sands, clays, and lignite containing a flora of mainly freshwater algae and pollen. The regression was initiated at the Pliensbachian-Toarcian boundary and marine influence ceased during Bajocian-Bathonian times; the regression thus took place earlier at Anholt than in the centre of the Danish Basin. The sediments in the Anholt borehole are referred to the Fjerritslev and Haldager Sand Formations. Although the Lower-Middle Jurassic boundary is commonly placed at the boundary between the two formations, our data indicate that at Anholt the upper Fjerritslev Formation (member F-IV) is of Aalenian age. The Lower-Middle Jurassic boundary occurs close to the boundary between members F-III and F-IV of the Fjerritslev Formation.
Palaeoecology and sedimentaryenvironments of the Jurassic-Cretaceous transition beds in Sweden
Geobios, 1994
A section of the Vitab~ick Clays (Annero Formation) at Eriksdal, SE Scania, Sweden has been investigated petrographically, sedimentologically and biostratigraphically. The section is predominantly argillaceous with occasional sandstone interbeds. The clays are composed mainly of illite, kaolinite, and a mixed layer component of smectiteillite. Fe-oxyhydroxides and siderite constitute important minerals in certain beds. Infill sequences, washover fans and pedogenic horizons together with faunal and floral elements indicate deposition within freshwater-brackishmarine environments such as coastal plains, lakes and lagoons. The fossil content includes molluscs, ostracodes and palynomorphs whose stratigraphical ranges have confirmed and narrowed the age to the Jurassic-Cretaceous transition. The dispersed palynomorphs, mainly pollen grains and spores which were derived from the contemporary parental coastal plants, provide the most complete palaeontological documentation of the transition. From the variations in the vegetation throughout the section it is possible to suggest a climatic change to more humid conditions, An interpretation of the contemporary palaeoecology and sedimentary environment is presented.
Geological Quarterly, 2010
Sedimentological anal y sis of the Early Ju ras sic Höör Sand stone in Cen tral Scania, Swe den shows that two units rep re sent ing dif fer ent lithofacies may be dis tin guished. The lower Stanstorp Mem ber is char ac ter ised by tex tur ally and min er al og i cally het er o ge neous sed iments. Mature arenites and im ma ture quartz and subarkosic wackes form two clearly sep a rated groups, in ter preted as flu vial chan nel and al lu vial plain de pos its respectively. The flu vial char ac ter of this mem ber is supported by a do minance of unidirectionally ori ented trough sets, abun dant plant de tri tus, the pres ence of plant roots and palaeosols and a lack of ma rine body fos sils and trace fos sils. An in ter calated com pos ite polymict con glom er ate layer is in ter preted as the re sult of an ex traor di nary flood. The over ly ing Vittseröd Mem ber is char ac ter ised by ma ture quartz arenites and large-scale tab u lar sets with mul ti di rec tional ori en ta tions. The Vittseröd Mem ber arenites are of storm-and pos si bly tide-dominated shoreface or i gin. Bidirectional wave and tide-induced long shore cur rents and uni di rec tional on shore mi gra tion of sand ridges were the chief depositional pro cesses. Long shore cur rents to wards the NNW pre vailed. Upper flow regime fea tures, and ero sion and re work ing of sed i ments are at trib uted to the storm-induced rip cur rents. Lithofacies char ac ter is tics are used to de fine the fol low ing lithostratigraphical units: the Stanstorp and the Vittseröd mem bers, as well as the Höör Sand stone (Fm) above. Lithofacies and palaeoenvironmental char ac ter is tics of the Vittseröd Mem ber and their com par i son with the fa cies of the Helsingborg and Döshult mem bers in West ern Scania sug gest a Sinemurian age for the Vittseröd Mem ber.