L

Mandala KinderCretaceous Sea Floor
Fig. p. Geography of the North Atlantic region during the Early Cretaceous (Berriasian). Later in the Cretaceous the shelf seas became more extensive. (After Hallam and Sellwood, 1976).

or early Jurassic) there was both a break-up of the North Atlantic continents and a rotation of the Iberian Peninsula, which caused the opening of the Bay of Biscay (see Wilson, 1975, with bibliographic references). Reconstructions show, however, that even during late Cretaceous times, the west of Ireland was situated only a short distance east of Newfoundland (about 900—1200km). Rapid spreading, leading to the development of the present North Atlantic seaway was thus a late Cretaceous and Tertiary event, as is shown both by direct sea floor spreading data (e.g. magnetic stripes on the ocean floor) and by the widespread latest Cretaceous and Tertiary volcan-icity in Ireland, Scotland, Greenland, Iceland and the North Sea.

What is now western Europe was thus at the dawn of the Cretaceous a more or less landlocked epeiric sea (Fig. p), whilst at the close of this period, there was an open, if narrow, ocean to the west. These changes in fundamental geography were accompanied by major sea level fluctuations, perhaps themselves a result of fluctuating sea floor spreading, and minor tectonic activity.

At the close of the Jurassic, the areas of shallow current-swept marine sands in Norfolk and Lincolnshire which are now the lower parts of the Sandringham Sands and the Spilsby Sandstones (Casey, 1973) were separate from the lakes, lagoons and supratidal saline flats of the Purbeck basin to the south. At the beginning of the Cretaceous, a brief marine incursion spread across the English Midlands and linked the two areas, leading in southern England to the development of oyster reefs in what became, albeit briefly, saline bays and estuaries. This incursion was followed by a return to brackish and fresh water conditions in the south, although marine conditions continued in the north, and the facies and broad faunal assemblages of the earliest Cretaceous are comparable to those of the latest Jurassic. This period was terminated by a phase of regional uplift which led to the rejuvenation of rivers and the building up of thick .sequences of mud plain, delta and lacustrine deposits.

In the early Aptian, fully marine faunas in the Lower Green-sand Group show that the sea covered parts of southern England, This transgression marked the earliest phase of one of the greatest known periods of submergence of continental areas, a fact recognized first by Edouard Suess (1885—1901). This transgression started in the Barremian, proceeded most rapidly in the Albian and Cenomanian, and extended into the late Campanian with only minor interruptions and regressions. As a result, the geography of Europe changed from the landlocked basins dominated by terrigenous clastic influences, which characterized the beginning of the period, to a wide-spread epicontinental sea with only scattered islands emerging from it (Fig. o). Lower Cretaceous sediments are largely terrigenous; those of the Upper Cretaceous are predominantly pelagic carbonates: the chalks (Latin — creta) which gave their name to the system.

The disposition of basins and massifs in western Europe during the Cretaceous was broadly similar to that of the Jurassic, and was a consequence of the geological history of the region extending back to the Precambrian (Sutton, 1968). During the mid to late Cretaceous, these massifs dominated sedimentation, facies and hence faunal distributions in several ways. Many were islands, standing above the Cretaceous sea for the whole period while others were encroached upon and submerged as the sea transgressed. Some, such as the Anglo-Brabant Massif, influenced sedimentation long after their submergence and relatively deep burial.

Between these island-massif areas were basinal regions which were permanently submerged from mid Cretaceous times onwards, and subsided evenly so that the Cretaceous sedimentary successions are complete. In some areas, subsidence exceeded sediment supply, and there was actual deepening. There are two major basinal areas in north-west Europe: the North Sea Basin, bounding an area of continuous subsidence since the Jurassic and regarded by some as a failed rift system, and the Anglo-Paris (or, more correctly, Wessex-Paris) Basin to the south. Within basins and on the submerged flanks of massifs are areas of minor variation in thickness which seem to be the result of basement 'highs' movements of salt plugs, and later displacement along pre-Mesozoic faults and fold axes. These areas are generally marked by thinning, condensation and re-working, as they were in the Jurassic (Sellwood and Jenkyns, 1975).

During the early Cretaceous, there was a limited tectonic activity in Europe (see Ziegler, 1975, 1975a); in the Upper Cretaceous it was minimal.

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