1. Late Cenozoic geological evolution of the northern North Sea: development of a Miocene unconformity reshaped by large-scale Pleistocene sand intrusion
by H. Løseth, B. Raulline, and A. Nygård
Journal of the Geological Society
Volume 170(1):
January 4, 2013
© The Geological Society of London

2. Map showing location of the study area (yellow outline) in the northern North Sea.
Map showing location of the study area (yellow outline) in the northern North Sea. Mounds at the top Hordaland Group unconformity were mapped on an open grid of 2D seismic lines by Løseth et al. (2003). The northern mounds are partly located within the study area. The locations of the Snorre, Visund and Gullfaks oil fields are shown in red.
H. Løseth et al. Journal of the Geological Society 2013;170:
© The Geological Society of London

3. Summary of stratigraphy in mid-Norway and the northern North Sea.
Summary of stratigraphy in mid-Norway and the northern North Sea. Age assigned to the top Hordaland Group unconformity (THGU) and Utsira Formation, which is partly time equivalent to the glauconitic sand in the study area, varies from early Miocene to Pliocene in completion logs and the literature owing to large amounts of re-sedimented fossils. Stratigraphic positions of intrusive and extrusive sands are shown. The time scale is from Ogg 2010.
H. Løseth et al. Journal of the Geological Society 2013;170:
© The Geological Society of London

4. Seismic section (a) with interpreted line drawing (b), located above the Snorre Field (location in Fig. 5).
Seismic section (a) with interpreted line drawing (b), located above the Snorre Field (location in Fig. 5). The line crosses through wells 34/7-10, 34/7-1, 34/4-7 and 34/4-6 (black lines) with gamma-ray logs (GR, red curves). The wells were re-dated by Eidvin & Rundberg (2001). Truncations are interpreted below the top Hordaland Group unconformity (white arrows) and glauconitic sand is located on top of the unconformity. A lower Miocene unit is interpreted west of the escarpment. The Pleistocene sands extruded on the sea floor during the glacial period.
H. Løseth et al. Journal of the Geological Society 2013;170:
© The Geological Society of London

5. NW–SE-striking seismic section crossing the position of well 34/8-3A (location shown in Fig. 5) and showing the late Cenozoic stratigraphic subdivision above the Visund Field.
NW–SE-striking seismic section crossing the position of well 34/8-3A (location shown in Fig. 5) and showing the late Cenozoic stratigraphic subdivision above the Visund Field. The stratigraphic position of the base of the lower Miocene unit, top Hordaland Group unconformity, top glauconitic sand and base of the glaciomarine prograding Naust Formation are shown as interpreted by Eidvin & Rundberg (2001). Much of the lower Miocene unit is eroded west of the escarpment.
H. Løseth et al. Journal of the Geological Society 2013;170:
© The Geological Society of London

6. An oblique view of the top Hordaland Group unconformity seen from SE (blue colours indicate deep areas; red colours shallow areas).
An oblique view of the top Hordaland Group unconformity seen from SE (blue colours indicate deep areas; red colours shallow areas). Exact positions, depths (in ms) and horizontal scale are given in the inset map. The yellow line shows the position of the escarpment and the dots are on the lower side. Approximate positions of wells and figures are shown. Escarpments flank a central low area in the northern part. The entire surface is mounded in the southern part. Regionally, the top Hordaland Group unconformity dips 0.3° towards the NNE. The black arrow points to a flat-topped high area and the white arrow an incised valley.
H. Løseth et al. Journal of the Geological Society 2013;170:
© The Geological Society of London

7. (a, b) Detailed perspective map of the top Hordaland Group unconformity above the Snorre Field.
(a, b) Detailed perspective map of the top Hordaland Group unconformity above the Snorre Field. The mounds are up to 100 ms high and the escarpment is 40–80 ms high. Both the escarpment and small circular depressions (red arrows), which are interpreted as palaeo-pockmarks, are clearly expressed on the semblance map (c). The dark areas on the semblance map, which show where seismic reflections are different from neighbouring reflections, highlight escarpment, faults and pockmarks. The escarpment separates the high western area from the lower eastern area. Well 34/7-6 penetrated two sand intervals (54 m and 63 m) where the top is located 146 m below the top Hordaland Group unconformity. Well 34/7-4 penetrated 82 m of sand between the mounds. A 30 ms deep depression is observed in the flat area west of the escarpment.
H. Løseth et al. Journal of the Geological Society 2013;170:
© The Geological Society of London

8. (a) NW–SE-striking seismic section showing the escarpment at the top Hordaland Group unconformity (THGU) just south of well 34/8-3A and an underlying sand injectite (yellow overlay) (location in Fig. 5).
(a) NW–SE-striking seismic section showing the escarpment at the top Hordaland Group unconformity (THGU) just south of well 34/8-3A and an underlying sand injectite (yellow overlay) (location in Fig. 5). Zigzag-shaped peak and trough reflections, which tie to the top and base of sands in nearby wells, can be observed c. 200 ms below the top Hordaland Group unconformity. (b) The same section as in (a) but flattened on the intra Hordaland Group yellow horizon in (a). The flattening removes the smooth mound that was formed above the injected sand. The flattened section is cut at the top sand reflection whereas the lower part of (b) is the lower part of (a) cut at the base of the sand. Together, they show a reconstruction of the Hordaland Group before the sand was injected. The reconstruction is validated by the gap between the two parts being almost equally thick.
H. Løseth et al. Journal of the Geological Society 2013;170:
© The Geological Society of London

9. NW–SE-striking seismic section crossing the high mound at the top Hordaland Group unconformity (THGU) above the Visund Field and underlying sand injectites (yellow overlay) (location shown in Fig. 5).
NW–SE-striking seismic section crossing the high mound at the top Hordaland Group unconformity (THGU) above the Visund Field and underlying sand injectites (yellow overlay) (location shown in Fig. 5). Irregular zigzag-shaped peak and trough reflections are c. 200 ms below the top Hordaland Group unconformity and well ties place them at the top and base of sands. (b) The same section as in (a) but flattened on an intra Hordaland Group horizon in (a). The flattening removes the mounds that were formed when sand was injected. Such flattening restores the pre-injection erosive shape of the top Hordaland Group unconformity. An erosive outlier is observed at the green (westernmost) arrow.
H. Løseth et al. Journal of the Geological Society 2013;170:
© The Geological Society of London

10. East–west-striking seismic section crossing the high flat area at the top Hordaland Group unconformity (THGU) in the northern part of the study area (location shown in Fig. 5).
East–west-striking seismic section crossing the high flat area at the top Hordaland Group unconformity (THGU) in the northern part of the study area (location shown in Fig. 5). Reflections below the top Hordaland Group unconformity are not folded. The escarpments (black arrows) are interpreted as being formed by erosion. The blue arrows point to prograding clinoforms that are interpreted as Gelasian contourites formed by sea-bed currents.
H. Løseth et al. Journal of the Geological Society 2013;170:
© The Geological Society of London

11. (a) Regional north–south line showing gentle (0
(a) Regional north–south line showing gentle (0.3°) northward tilting of the top Hordaland Group unconformity (green line) in the northern North Sea (location shown in Fig. 1).
(a) Regional north–south line showing gentle (0.3°) northward tilting of the top Hordaland Group unconformity (green line) in the northern North Sea (location shown in Fig. 1). (b) was flattened on the yellow line in (a) to remove the regional tilt. The incision and erosion in the top Hordaland Group unconformity in the northern part of the section should be noted (white arrow).
H. Løseth et al. Journal of the Geological Society 2013;170:
© The Geological Society of London

12. Two seismic east–west sections that constrain the timing of the mounding above the Snorre Field and show the position of intrusive (i) and extrusive (e) sands (yellow overlay) (location shown in Fig. 6).
Two seismic east–west sections that constrain the timing of the mounding above the Snorre Field and show the position of intrusive (i) and extrusive (e) sands (yellow overlay) (location shown in Fig. 6). First the Hordaland Group was deposited and eroded, and then the glauconitic sand and Gelasian unit were deposited. The Gelasian unit, which thins and base-laps westward towards the escarpment in (a), is interpreted to comprise contourites. Forced folding above the sand intrusions occurred after the Gelasian unit was deposited, as this unit also was uplifted and deformed in (b). The tops of intrusive sand bodies are interpreted 150– 200 ms below the mounds. The extrusive sand onlaps the mounds and fills in the mounded topography. It was extruded on the sea floor during the Pleistocene through feeder dykes from the sand bodies below the mounds. Irregular ditches (d) overlie feeders on the flanks of the mounds. We interpret them as the latest active sand volcano craters.
H. Løseth et al. Journal of the Geological Society 2013;170:
© The Geological Society of London

13. Seismic section through wells 34/7-4 and 34/7-9 showing intrusive (i) and extrusive (e) sands, expressed as low gamma-ray readings (red curve), above the Snorre Field (location in Fig. 6).
Seismic section through wells 34/7-4 and 34/7-9 showing intrusive (i) and extrusive (e) sands, expressed as low gamma-ray readings (red curve), above the Snorre Field (location in Fig. 6). The deep V-shaped bright anomalies in the lower part of the Hordaland Group are partly carbonate-cemented sand injectites. The parent sands for the injectites and extruded sands are interpreted as turbitides originally deposited in the Palaeocene interval. The feeder (f) from the intrusive sand to the extrusive sands should be noted. The red arrow points to the subtle depression at the top Hordaland Group unconformity (THGU) that is located above an intrusive sand.
H. Løseth et al. Journal of the Geological Society 2013;170:
© The Geological Society of London

14. (a) Seismic section following the tracks of wells 34/8-A-33 H and 34/8-A14 H above the Visund Field (location shown in Fig. 5).
(a) Seismic section following the tracks of wells 34/8-A-33 H and 34/8-A14 H above the Visund Field (location shown in Fig. 5). The wide yellow lines show approximately where the wells penetrated sand. The wide black line just above the top Hordaland Group unconformity (THGU) shows the position of Core 1 in well 34/8-A-33 H ( – m), which penetrated fine sand with a crystalline dropstone. Well 34/8-A14 H penetrated 636 m of homogeneous sand below the mound (yellow arrows), whereas 48 m of homogeneous sand were penetrated at the same stratigraphic level in well 34/8-A-33 H, and 13 m and 755 m of sand-rich sediments were penetrated in 34/8-A-33 H and 34/8-A-14 H in the Gelasian unit, respectively. High-amplitude reflections, which die out 1.5 km from the flank of the mounds, correlate with sand-rich intervals. We interpret the sand as extrusive. (b) shows the internal reflection pattern in segments 1 and 2, within the green dotted lines in (a), juxtaposed in the position they had before sand injection.
H. Løseth et al. Journal of the Geological Society 2013;170:
© The Geological Society of London

15. GR log profiles through wells 34/8-A-33 H and 34/8-A14-H.
GR log profiles through wells 34/8-A-33 H and 34/8-A14-H. The massive sand intervals in the Oligocene succession have low GR-log readings and a blocky log pattern. The most sand-rich interval in the Pleistocene (Gelasian) part of well 34/8-A14-H is shown with a grey zone to the right of the GR-log.
H. Løseth et al. Journal of the Geological Society 2013;170:
© The Geological Society of London

16. Summary of interpreted chronostratigraphy and geological events in the northern North Sea during the late Cenozoic.
Summary of interpreted chronostratigraphy and geological events in the northern North Sea during the late Cenozoic.
H. Løseth et al. Journal of the Geological Society 2013;170:
© The Geological Society of London

17. The main late Cenozoic geological events in the northern North Sea.
The main late Cenozoic geological events in the northern North Sea. (a) The Oligocene ooze-rich claystone was deposited and polygonally faulted. A hiatus developed, overlain by less ooze-rich Lower Miocene sediments (greenish brown). (b) The northern North Sea area was regionally uplifted during the mid- and late Miocene in a long-wavelength dome. Fluvial erosion of the exposed Hordaland Group sediments was initiated. (c) Coastal erosion, similar to that of the present-day east coast of England, created a low wave-cut central area bounded by 40–100 m high coastal cliffs. (d) The northern North Sea subsided quickly when the North Atlantic compression phase terminated at the Miocene–Pliocene boundary, causing rapid transgression. Glauconitic sand was deposited on top of the unconformity. (e) Glaciation began at 2.6 Ma. Prograding contourite clinoforms are observed in the Gelasian unit, which also contains glacial dropstones. Sand was injected into Oligocene sediments above the Visund Field during the Gelasian, and the overlying succession was lifted to form sea-floor mounds of height equivalent to the thickness of the injected sand. Some sand was also transported through dykes to the palaeo sea floor where it was re-deposited and mixed with normally deposited sediments. (f) Prograding glaciomarine shelf clinoforms reached the Snorre area after the Gelasian, when sand was injected into the Oligocene claystones. Consequently, the overburden was lifted and clusters of mounds developed. Sands up to 125 m thick were extruded on the sea floor. The sand mixed with glaciomarine clay and dropstones and filled in the sea-floor topography around the mounds. (g) A depocentre of glaciomarine sediments built up just north of 62°N. The weight of these sediments caused isostatic subsidence and a tilt of the study area 0.3° towards the NNE.
H. Løseth et al. Journal of the Geological Society 2013;170:
© The Geological Society of London