Climatic and tectonic controls on Triassic dryland terminal fluvial system architecture, central North Sea - From Depositional Systems to Sedimentary Successions on the Norwegian Continental Margin - page 19

Climatic and tectonic controls on Triassic dryland terminal
fluvial system architecture, central North Sea
TOM M c KIE
Shell UK Exploration and Production, 1 Altens Farm Road, Nigg, Aberdeen, AB12 3FY, United Kingdom
(E-mail: [e-mail])
ABSTRACT
The Triassic succession in the central North Sea was deposited in a dryland alluvial
setting during the early phases of rifting which marked the break-up of Pangaea.
Extension was episodic and triggered widespread halokinesis in the underlying
Zechstein Group halite, particularly during the Early and Late Triassic Hardegsen
and Cimmerian I tectonic events. These movements created a tortuous basin topog-
raphy which temporarily inhibited fluvial sand dispersal and resulted in a domi-
nance of floodbasin fines. Halokinesis also exerted a particularly strong control on
stratigraphic preservation on the basin margins by conveying shallow-buried suc-
cessions, which were vulnerable to erosion, into long-term preservation space
below the truncation level of subsequent Jurassic unconformities. During periods
of relative fault quiescence and reduced aridity Skagerrak Formation terminal fluvial
systems were able to expand across the basin as large transverse Fennoscandian
fluvial fans which interacted with both axial and transverse systems draining off
the Scottish Highlands. These systems, driven by the Tethyan monsoon, ultimately
flowed southward into the northern margin of the Southern Permian Basin to
terminate in widespread playa and coastal sabkha. Expansion of these terminal
fluvial systems probably took place during pluvial climate episodes and the preva-
lence of vegetation and burrowing activity on the Skagerrak Formation floodplain
suggests an availability of year-round soil moisture, indicating that monsoonal
flooding was augmented by intermittent to perennial, dry season, flow. The pluvial
episodes potentially to correspond to episodes of warming in the Tethys region,
which may have resulted in enhanced evaporation, increased thermal convection
and monsoon intensification. The consequent reduction in temperature gradients
may also have weakened north-easterly trade winds, allowing more frequent
impingement of moist Tethyan air masses into the region during the dry season. In
addition to such regional climatic changes, flooding of the Southern Permian Basin
by the Muschelkalk Sea during a period of widespread Anisian aridity briefly altered
the local climate, allowing the maintenance of marshes and floodplain lakes adjacent
to this interior seaway. Contraction of the Skagerrak Formation terminal fluvial sys-
tems apparently occurred during cooling, as the climate reverted to the prevailing
Pangaean aridity and resulted in the deposition of basin-wide playa. By the Middle
Triassic, such playa facies had blanketed much of north-west Europe as the Keuper
Formation and Mercia Mudstone Group and the contemporaneous Skagerrak
Formation fluvial systems stand in marked contrast to these arid, mud-dominated,
basin-fills due to their access to runoff from large Fennoscandian catchments located
outwith the arid rift interior.
Keywords: dryland fluvial, Triassic, Skagerrak Formation, Smith Bank Formation,
climate, tectonics, halokinesis.