Tectonic influence on the Jurassic sedimentary architecture in the northern North Sea with focus on the Brent Group - From Depositional Systems to Sedimentary Successions on the Norwegian Continental Margin - page 403

Local depocentres

are sporadically present indicating episodic
marine incursions. These sandstones are inter-
preted as stacked tidal dunes (see also Mjøs,
2009 with regard to 35/11-7, Fig. 9C), forming
thick aggrading units in a nearshore position
(above the Etive Formation) displaying similari-
ties to estuarine depositional environments
(e.g. Dalrymple & Choi, 2007). The interval from
35/11-7 is dominated by spores and pollen and,
although no in situ dinoflagellate cysts have
been observed, it contains sporadic acritarchs
and a significant proportion of Botryococcus
spp., especially towards the top of the unit.
These data also suggest some influence of marine
waters and the known tolerance of modern day
Botryococcus spp. of 0-4ppt salinity (see Qin,
2005; Rao et al. , 2007) further suggests a brackish
marine influence.

The stacked (tens of metres thick) sandstones at
the basal Ness Formation in 34/10-23 (Fig. 4), are
conspicuously anomalous in the correlation
panel. Similar stacked sandstones at the basal
Ness Formation are seen in several other wells
(Figs 8, 9 and 10). These sand-units are geograph-
ically isolated and are not observed in the
surrounding wells (see Fig. 8). Where these
sand-units are cored (Fig. 9A and C), they display
cross-bedding with mudstone-drapes. Dipmeter
data for such a unit in 34/11-A-15, Fig. 10D)
show landward-directed palaeo-currents (i.e. to
the south). Palynomorphs are continuously
recorded in all samples, particularly spores
and pollen; however, marine palynomorphs
including the dinoflagellate cyst Nannocer-
atopsis gracilis and sphaeromorphs acritarchs

2°0 ′ 4°0 ′

35 36

211 33 34

210

2

3

Unst
Basin

61°0 ′ N

1

4

29 31 32

3 30

East
Shetland
Platform

East
Shetland Basin

Wells with thick
sandstone unit at
the base of the
Ness formation

Wells used in this study
Exploration wells
Permo-Triassic faults
reactivated in Middle Jurassic
Middle to Late Jurassic faults
1 Cross-section, see Figure 4
2 Cross-section, see Figure 5A
3 Cross-section, see Figure 5B
4 Cross-section, see Figure 6

34/7-17
34/10-1
34/10-23
34/11- A-15
35/8-5 S
35/10-2
35/11- 4
35/11- 7

Horda
Platform

0 50 km

60°0 ′ N

Fig. 8. Geographical location of wells at the base Ness sandstone level in the northern North Sea.