The base of the Upper Pliocene and important Upper Pliocene foraminifera
T. Eidvin, F. Riis, E. S. Rasmussen & Y. Rundberg, 2013. New layout 2021
Seismic profiles of the Upper Pliocene show that these sediments prograded from the east (Scandes mountains) with distinct, shallow-angled clinoforms forming several hundred metre -thick accumulations (up to 3000 m thick in the Bjørnøya fan (Eidvin et al. 1993, Faleide et al. 1996, Map 2). Regional seismic mapping indicates that the Upper Pliocene changes character towards the north. North of approximately 58ºN, the succession is thicker and exhibits a more distinct progradation from land with a depocentre developed closer to the Norwegian mainland (Eidvin et al. 1998a and 2000). It has also been noted that in the western part of the Viking Graben, Late Pliocene progradation occurred from the Shetland Platform, so that parts of the lowermost sandy Upper Pliocene section in this area may be derived from the west.
The base of the Upper Pliocene succession usually coincides with a distinct seismic reflector. In the wells we have investigated in the Tampen area in the northern North Sea, on the Norwegian Sea continental shelf and in the western Barents Sea we have recorded ice-rafted pebbles of crystalline rocks down to the base of the Upper Pliocene. Sidewall core samples in wells 34/4-7 and 34/7-1 (Tampen area, Eidvin & Rundberg 2001, Ottesen et al. 2009), 34/7-12 (Tampen area, Eidvin 2009), 6610/7-2 (Nordland Ridge, Eidvin et al. 1998a, Ottesen et al. 2009) and 7316/5-1 (western Barents Sea, Eidvin et al. 1998b) verify that the ice-rafted pebbles are in situ. Conventional cores taken from the base of the Upper Pliocene unit in wells 34/8-9 S and 34/8-A-1 H (Tampen area) contain gravity flow deposits with a glacio-marine imprint (Eidvin et al. 1998a, Ottesen et al. in 2009). Since the large input of ice-rafted material in the Norwegian Sea did not started before about 2.75 Ma (recorded from ODP boreholes by Frondval & Jansen 1996), it seems that most of the lower part of the Upper Pliocene is missing in these areas (e.g. deposits with an age of about 3.5-2.75 Ma). However, planktonic foraminiferal correlations show that the base of the succession is older than 2.4 Ma (Eidvin et al. 2000, Ottesen et al. 2009). We suggest that at about 2.75 Ma, expanding glaciers started loading large volumes of material off the coastal areas which probably triggered the extensive submarine mass flowing and erosion.
In the wells we have investigated from the southern part of the Viking Graben and northern part of Central Graben (Fig. 7), the Upper Pliocene is more fine grained and contains fewer ice-rafted pebbles. In the Viking Graben the lower part of the Upper Pliocene is very condensed. In well 15/9-A-11 in the Sleipner Field (southern Viking Graben), a short conventional core was obtained from the base of the Upper Pliocene at about 20-25 m above the Lower Pliocene part of the Utsira Formation (Head et al. 2004). The core was dated to 2.4-1.9 Ma, but contained no ice-rafted pebbles. The seismic data show that the uppermost sandy units of the Utsira Formation interfinger with the lowermost units of the mudstone drape. This indicates that the Utsira sand and the mudstone drape formed a depositional continuum in this area (Chadwick et al. 2004, Head et al. 2004). However, a small hiatus may be present locally.
Of the wells that we have studied, the foraminiferal correlations show that it is only in well 2/4-C-11 from the northern Central Graben (Fig. 7) that the lower part of the Upper Pliocene is well developed (see also King 1989 and Eidvin et al. 1999). According to the new time scale of the International Commission on Stratigraphy (ICS 2013), where the base Pleistocene is moved back in time to 2.588 Ma, it is just in this area that Upper Pliocene deposits are present. According to that time scale, Pleistocene sediments lie unconformably on Lower Pliocene or older deposits in the other investigated areas of the Norwegian continental shelf.
In wells and seismic data it is commonly observed that the base of the Upper Pliocene represents an erosional surface. Below the western Barents Sea wedge, such erosion is evident in several wells including 7117/9-1, 7117/9-2 and 7316/5-1. In the northernmost North Sea, an irregular erosional surface can be mapped in the Viking Graben. The surface cuts into the pre-existing mud diapirs and has probably locally removed the Miocene section.
With the exception of C. grossus and Elphidiella hannai, all the in situ benthic foraminifera recorded in the Upper Pliocene units are extant. According to King (1989) C. grossus is recorded in the northern North Sea in Upper Pliocene to Lower Pleistocene (based on the time scale of Berggren et al. 1995). In the southern parts of the North Sea it becomes extinct somewhat earlier, close to the Pliocene/Pleistocene boundary. The FAD of C. grossus, in the southern parts of the North Sea, is observed somewhat above the Lower/Upper Pliocene boundary. King (1989) established the time for the LAD of C. grossus in the northern North Sea by registering its LAD above the FAD of Neogloboquadrina pachyderma (sinistral, encrusted). A similar range is described for E. hannai. However, according to Skarbø & Verdenius (1986) and King (1989), E. hannai inhabited shallow water, whereas C. grossus was a deep- to shallow-water form. The FAD of N. pachyderma (sinistral, encrusted) at about 1.8 Ma (Spiegler & Jansen 1989) is a good marker for top Upper Pliocene in cored ODP/DSDP boreholes on the Vøring Plateau, but this biostratigraphic event is difficult to detect in drill cuttings in wells on the continental shelf due to caving.
Spiegler & Jansen (1989) also described a N. atlantica (sinistral) Zone from the Vøring Plateau (Norwegian Sea) from Upper Miocene to Upper Pliocene deposits. The LAD of N. atlantica (sinistral) in that area is approximately 2.4 Ma. A G. bulloides Zone is described from the North Atlantic (DSDP Leg 94) in Pliocene sediments as young as 2.2 Ma (Weaver & Clement 1986). On the Vøring Plateau G. bulloides is common in Pliocene deposits older than 2.4 Ma (Spiegler & Jansen 1989). G. bulloides is also common in the warmest interglacials of the last 0.5 Ma in the North Atlantic (Kellogg 1977). An upper N. atlantica (dextral) Zone is described from the Vøring Plateau in Upper Pliocene deposits, and is dated to 2.4-1.9 Ma (Spiegler & Jansen 1989). A latest Pliocene Neogloboquadrina pachyderma (dextral) Zone is described by King (1989) for the North Sea, by Weaver (1987) and Weaver & Clement (1986) from the North Atlantic and by Spiegler & Jansen (1989) from the Vøring Plateau. On the Vøring Plateau the zone is dated to 1.9-1.8 Ma. Several of these biostratigraphical events are also recorded in the Upper Pliocene units in the wells that we have studied on the continental shelf.