Halten Terrace Lower and Middle Jurassic inter-rift megasequence analysis: megasequence structure, sedimentary architecture and controlling parameters - From Depositional Systems to Sedimentary Successions on the Norwegian Continental Margin

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the narrowing of the structurally defined seaways

that existed during Tofte depositional times. Basin-

margin attached deltaic systems were extended

across the central Halten Terrace, whereas the

renewed uplift of the intra-basinal highs (such as

the Sklinna and Nordland Ridges) promoted pro-

gradation of the local, rift-interior sourced deposi-

tional systems. This created a series of local straits

across the western Halten Terrace connecting the

various sub-basins, such as the northern and west-

ern parts of the Halten Terrace. These straits formed

between growing structural highs and the detached,

lowstand deltaic shoreline bodies (Fig. 9B2). in

these narrow straits, strong, commonly unidirec-

tional currents reworked the former deltaic sands

into elongated, strait parallel sand ridges (or com-

pound sand bars), probably in a similar manner as

described by Reynaud et al . (2006) for Miocene

seaways of SE France and by Anastas et al . (2006)

for Oligo-Miocene seaways of New Zealand.

Transgressive stage sand-ridges now appear fully

embedded in shelfal mudstones, whereas sand

ridges formed during high-order regressive stages

appear to be shoreface attached.

basal part of the Not Formation. The upper bound-

ary is represented by the Bathonian Melke Formation

mudstones, which represent the shut-off of Garn

sands to the Halten Terrace and the western

Trøndelag Platform areas.

The progradational (regressive) segment (the Not

Formation and lower Garn member) consists of a

forward stepping stack of wave-structured flu-

viodeltaic lithosomes some 10 m+ to 35 m thick,

representing the outbuilding of wave-influenced

deltas and shorelines, most probably as the infill of

a broad embayment (Fig. 11A, B1). The forward

stepping stack is capped by an erosively based (in

places with a thin coarsening upwards basal part),

sand-rich and laterally extensive fluviodeltaic to

estuarine sandstone sheet (the lower Garn member).

The lower Garn member is comprised by amalga-

mated, mixed wave and tide-influenced fluvi-

odeltaic and marginal marine (estuarine and

shoreline-barrier bar), sandy (sand content normally

more than 90%) facies tracts. The sandy nature

and relatively coarse grain size suggests that the

fluviodeltaic strata represent outbuilding of low-

gradient, shoal-water braidplain deltas (Fig. 11A,B2;

see also Harris, 1989; Ehrenberg et al ., 1992), inter-

preted to represent a forced regressive to late low-

stand normal regressive lithosome stack. in narrower

and semi-protected sub-basins, the delta-front and

outer estuary sandstones were reworked into a

series of stacked sand ridges (Messina et al ., this

volume), whereas the river-structured and wave

reworked mouth-bars were preserved in broader

and less sheltered sub-basins.

The aggradational segment (the middle Garn

member) is separated from the underlying pro-

gradational segment by a bioturbated, muddy

siltstone unit, which represents a higher-order,

minor flooding surface. The remainder of the

succession comprises an aggradational stack of

higher order fluviodeltaic lithosomes, including a

thin forward-stepping basal part. it comprises

mixed wave-influenced, tide-influenced and

river-influenced sandstones (see also Ehrenberg

et al ., 1992; Corfield et al ., 2001; Messina et al .,

this volume), forming a sandstone sheet which

displays thickness variations across faults and

within tilted sub-basins. The unit is interpreted to

represent alternations of sandy, mixed energy

braidplain deltas and inner to outer parts of sandy,

wave-dominated, structurally formed estuaries.

These deltas and estuaries translated laterally into

a wave dominated prodelta and inner shelf areas,

respectively (Fig. 11A,B3).

The Garn Megasequence

The Garn Formation together with the upper part

of the underlying Not Formation and the basal

part of the overlying Melke Formation (Fig. 3)

form a relatively thinner (up to 200 m thick)

megasequence (Figs 4 and 10). The Garn megase-

quence represents a timespan of at least 7 Myr

and has an overall progradational-aggradational-

retrogradational character. The Garn Formation

consists of braidplain and fluvial strata that

are fronted to the south-west by mixed tide-wave-

influenced, shoal-water braidplain deltas and

intervening wave-dominated shorelines, as well

as wave-dominated, sandy estuaries (Fig. 10; see

also Gjelberg et al ., 1987; Harris, 1989; Pedersen

et al ., 1989; Ehrenberg et al ., 1992; Corfield et al .,

2001; Elfenbein et al ., 2005). The lower delta-front,

outer estuary and shoreface areas were dominated

locally by stacked mixed wave- and tide-reworked

sand-ridges (Sæther et al ., 1999; Messina et al .,

this volume), which pass distally into the shelfal

muddy sandstones and mudstones of the Not and

Melke Formations (equivalents to the lower and

upper parts of the Garn Formation, respectively;