INGENIERÍA BÁSICA DE UNA PLANTA PROCESADORA DE SANGRE

Therefore, the correlation o f the Palaeocene bio-events in wells 205/10-2B and 208/22-1 is given in a separate figure to make graphic correlation possible (Fig. 3.29, p. 127). Both correlation diagrams show that there are only few correlation problems with some biomarker events, indicated by the crossing of correlation lines. These will be discussed below.

In the top o f the studied sections, the samples in all wells were almost barren o f any microfossils. These intervals represent periods with locally high input o f terrigenous material, characterised by sand deposits (see Chapter 2.3, p. 44). This has significantly reduced the microfossil record in the sediments. Therefore, the Impoverished Assemblage as described from well 205/10-2B, is not representative o f a specific time interval. It is an indication of unfavourable conditions for a representative microfossil assemblage preservation in relation to sedimentary processes.

Just below the Impoverished Assemblage in wells 205/10-2B and 208/22-1, the FDOs of many agglutinated foraminiferal taxa are observed, including that o f Reticulophragmium pauperum.

In well 205/10-2B, this event occurs close to the FDO o f Ammoanita ruthvenmurrayi and the interval o f high foraminiferal abundance (Fig. 3.29, p. 127). However, in well 208/22-1, the FDO o f A. ruthvenmurrayi and the high foraminiferal abundance interval are observed well below the FDO of R. pauperum (Fig. 3.29, p. 127). This longer range interval of/?, pauperum is also observed in the northern North Sea Basin, where it is reported to range towards the early Eocene (Gr a d s t e in & BÀCKSTRôM, 1996). In the probabilistic RASC zonation, a R. pauperum

Zone (NSR2b) is distinguished from the concurrent range interval o f A. ruthvenmurrayi and R. pauperum (Zone NSR2a; GRADSTEIN & BÀCKSTRÔM, 1996). Furthermore, the high

foraminiferal abundance event is a widespread reliable correlation event, occurring throughout the Faeroe-Shetland Basin (KNOX et a/., 1997). These observations indicate that an assemblage characterised by the occurrence o f R. pauperum is present on top o f the R. pauperum - A. ruthvenmurrayi Assemblage in well 208/22-1. The lower boundary o f this R. pauperum

assemblage is taken at the FDO o f A. ruthvenmurrayi. The FDO event o f A. ruthvenmurrayi

near the bottom of the Impoverished Assemblage in well 205/10-2B suggests that this might not represent the true stratigraphie FDO event o f this taxon.

Both taxa Remesella varians and Caudammina ovula are reported from Palaeocene samples of wells 205/10-2B and 208/22-1 (Fig. 3.29, p. 127). This is in agreement with the reported range o f R. varians into late Palaeocene in other study areas (KING, 1989; KUHNT & KAMINSKI, 1997;

Ol s z e w s k a, 1997), although C. ovula is known to range into the late Eocene (KAMINSKI eta/.,

1988a; CHARNOCK & JONES, 1990). Therefore, the observed FDOs o f both taxa near the Base Tertiary unconformity in wells 206/3-1 and 206/5-1 are not the true stratigraphie events (Fig. 3.28, p. 126).

In well 205/10-2B, the FDO of abundant Cenosphaera lenticularis is observed below the FDAO of Spiroplectammina spectabilis (Fig. 3.29, p. 127). However, in well 208/22-1, these events both occur in the same sample (Fig. 3.29, p. 127). It is important to note that in the latter well abundant occurrence of S. spectabilis is only observed in a single sample. Furthermore, high abundances of C. lenticularis are reported from the top o f Unit VI o f the Vaila Formation (K nox et o/., 1997). By correlation of the C. lenticularis FDO event to the lithostratigraphy (see Figs. 2.2 and 2.5, p. 40 and 43), it seems that this event occurs at the correct level in both wells (while taking into account that the sample depths are correct within a 33 ft. interval o f wireline log depth). This suggests that the order observed in well 205/10-2B is the correct one, although it can not be tested with the current data.

Although the planktonic foraminiferal taxon Pseudotextularia elegans is not observed in well 205/10-2B, specimens of this species are present in samples near the Base Tertiary unconformity o f wells 206/3-1 and 208/22-1 (Fig. 3.28, p. 126). This planktonic foraminiferal taxon is reported to be restricted to the latest Maastrichtian (FCN21b Zone of KING et a/., 1989;

Rid i n g, 1996). The occurrence o f this warm water species in the higher latitudes is associated

with poleward migration during the latest Maastrichtian global warming event (Ol s s o n et al.,

2001; see discussion in Chapter 5.2, Planktonic Foraminiferal Distribution, p. 260). This indicates that in the two wells 206/3-1 and 208/22-1 part o f the late Maastrichtian is still present, whereas this P. elegans Assemblage, which is also characterised by abundant occurrences o f Planoglobulina acervulinoides, is not present in the other wells. The lower boundary o f the P. elegans Assemblage is difficult to define, as downhole caving poses problems by downhole extensions o f the range o f the nominate taxon. However, provisionally it can be taken at the FDCO o f a planktonic foraminiferal assemblage, which is not characterised by high percentages o f the taxa P. elegans and P. acervulinoides.

In three o f the studied wells, the FDO of Reussella szajnochae (upper event) is recorded close to the Base Tertiary unconformity (Fig. 3.28, p. 126). As is already mentioned, due to its rarity in places and the lack o f independent biostratigraphic controls, the precise stratigraphie position of the FDO of R. szajnochae within the Maastrichtian is not clear, although it is thought to mark the mid/late Maastrichtian boundary (WILKINSON et a l, 1993; RIDING, 1996). However, from the southern Norwegian margin this species is also reported to range into latest Maastrichtian, where specimens are recorded from the P. elegans Zone (GRADSTEIN et al., 1999). For the area west o f the Shetland Islands this is not supported by observations from wells 206/3-1 and 208/22-1, although R. szajnochae co-occurs in samples with low percentages o f P. elegans (see Appendix C, Tables C.6, C .l, C.9 and C.IO, p. 394, 397, 399 and 402). Only in well 206/5-1, the FDO is reported to occur well below the Base Tertiary unconformity, near the bottom o f the uppermost calcareous foraminiferal rich interval. Furthermore, the downhole reappearance o f R.

3.4 .1 Fence Diagram

szajnochae (lower event) is not found to occur within the same (lower) calcareous foraminiferal rich interval (Fig. 3.28, p. 126). The common occurrence o f this taxon in the early part of the late Maastrichtian (second) calcareous foraminiferal rich interval o f wells 206/3-1 and 206/5-1 is not in agreement with the observations from the northern North Sea Basin, where it is reported to be absent during the early part of late Maastrichtian (Kin get a l, 1989; WILKINSON et a l, 1993). Therefore, the conclusion is that both the FDO (upper event) and the downhole reappearance (lower event) o f R. szajnochae in itself are not reliable biomarker events. From various studies it seems that this species is facies dependent, and therefore its distribution is very likely diachronous. However, as its occurrence is associated with calcareous foraminiferal rich intervals, these events may be used for correlation.

Apart from well 208/22-1 (lower drilled limit is reached at 7500 ft.), three calcareous foraminiferal rich intervals are recognised in all the wells (Fig. 3.28, p. 126). Although the distribution o f calcareous foraminifera is affected by post-mortem preservation, these three occurrence intervals imply that the calcareous foraminiferal rich intervals are basin-wide related events, and therefore may be regarded as reliable correlation markers.

Preservation also plays an important part in the biosiliceous microfossil distribution

(MiTLEHNER, 1995). Only in well 205/10-2B, pyritised moulds o f the diatom Fenestrella bellii

occur abundantly, with just few occurrences in wells 206/3-1 and 206/5-1. The FDO o f F. bellii

marks the top o f the biosiliceous rich F. bellii Assemblage, which is characterised by very abundant occurrences o f pyritised infillings of radiolarians and other diatom species. Already at somewhat higher stratigraphie levels above the FDO o f F. bellii, the abundance o f biosiliceous microfossils is increasing downhole towards a maximum in the lowermost samples o f the studied sections (see Chapter 3.3.1.2, Figs 3.7-3.10, p. 82-85). Therefore, the FDAO of biosiliceous microfossils might serve as an alternative marker event when F. bellii is rare.

An alternative marker for the FDCO o f Inoceramus spp. shell debris is the FDO o f big siliceous spherical radiolarians, which is recorded very close it. However, these siliceous spherical radiolarians are only abundant in well 205/10-2B (see Appendix C, Table C.4, p. 390). The distribution is controlled by silica preservation, and therefore these siliceous spherical radiolarians might be rare in certain areas.

Reticulophragmium pauperum 7100 - - 5520

Ammoanita ruthvenmurrayi 7130 - - 5760

high foraminiferal abundance 7100-7310 - - 5880-6120

Spiroplectammina spectabilis FDAO 7700 - 6840

Remesella varians 7760 5840/5860 4950 7020

Caudammina ovula 7820 5840/5860 4950 6120

Cenosphaera lenticularis FDCO 7940 - - 6840

Pseudotextuiaria elegans - 5840/5860 - 7140

Reussella szajnochae (upper) 8510 5840/5860 5160 7170

calcareous foraminiferal rich interval (upper) 8480-9140 5880/5900-6200/6220 4950-5190 7170-7320

calcareous foraminiferal rich interval (middle) 9290-9680 6320/6340-6520/6540 5280-5550 7320-7500

Reussella szajnochae (lower) 10280 6400/6420 5670 -,

calcareous foraminiferal rich interval (lower) 10280-10520 6880/6900-6920/6940 5670-5910 -

agglutinated foraminifera dominant 10520 6920/6940 5910 -

biosiliceous microfossils FDAO 11090 7680-7700 6660 -

Fenestrella bellii 11330 7840/7860 6660 -

big siliceous radiolarians 11570 8080/8100 6990 -

Inoceramus spp. FDCO 11720 8200/8220 6990 -

K) LA

Table 3.2 List o f first downhole occurrence (FDO), first downhole common and first downhole abundant occurrence events (FDCO/FDAO) o f nominate taxa in the four studied wells. Indicated are sample depths in the studied wells (given in feet). List also includes the sample depth intervals of the high foraminiferal abundance intervals and o f the calcareous foraminiferal rich intervals (given in feet).

depth well 205/10-28 well 206/3-1 well 206/5-1 well 208/22-1 (In N e t) 4 5 0 0- 5 0 0 0- R pauperum A. ruthvenmurrayi C. ovula 5 5 0 0- P. elegans 6 0 0 0- 6 5 0 0- biosiliceous FDAO

S. spectabilis FDAO/C. lenticularis FDCO

R varians R szajnochae (upper) 7 0 0 0- P. ele\ 7 5 0 0- S. spectabilis FDAO

/

/

calcareous foraminiferal rich interval

10000-

almost barren interval no samples available unconformity

possibly not true FDO event

10500-

11000-

11500-

t^

Os

Figure 3.28 Biostratigraphic correlation of first downhole occurrences (FDOs) and first dowiihole common and abundant occurrences (FDCO/FDAO) of nominate taxa in the four studied wells from the Foula Sub-basin. Depths of Base Tertiary unconformity (at 8223,5890,4895 and 7140 ft. respectively) and mid- Campanian unconformity (at 7240 and 6250 ft, in respectively wells 206/3-1 and 206/5-1) are taken from the lithostratigraphy.

Oj

r

depth w ell 205/10-2B (in feet) 4500 w ell 208/22-1 A. ruthvenmurrayi ,K szajnochae (upper) S. spectabilis FDAO C. lenticularis FDCO K pauperum A. ruthvenmurrayi C. ovula

S. spectabilis FDAO/ C. lenticularis FDCO

R. varians

R szajnochae (upper)

P. elegans

B

0 ^ 0 1 high foraminiferal abundance interval I I calcareous foraminiferal rich intenral

almost barren interval no samples availabie unconformity

possibly not true FDO event

Figure 3 ^ 9 Biostratigraphic correlation o f first downhole occurrences (FDO) and first downhole common and abundant occurrences (FDCO/FDAO) o f nominate taxa in wells 205/10-2B and 208/22-1. Depths from Base Tertiary unconformity (at 8223 and 7140 ft. respectively) are taken from the lithostratigr^hy.