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(LCOs) and a First Common Occurrence (FCO) also could be identified. The
scaling result is shown in Fig. 9.32 . This dendrogram served as a template to
build a Cretaceous zonal model with 16 stratigraphically successive interval
zones that are middle Albian through late Maastrichtian in age. Five large breaks
(at events 52, 84, 205, 255 and 137 in Fig. 9.32 ) indicate transitions between natural
microfossil sequences; such breaks relate to hiatuses or facies changes, some of
which are known from European sequence stratigraphy (see Gradstein et al. 1999
for more details): (1) The una-schoelbachi break reflects a latest Albian lithofacies
change and hiatus, connected to the Octeville hiatus in NW Europe. (2) The
delrioensis (LCO)- brittonensis break reflects the mid-Cenomanian lithofacies
change and hiatus, connected to the mid-Cenomanian non-sequence and Rouen
hardground of NW European sequence stratigraphy. (3) The Marginotruncata-
polonica break, above the level of Heterosphaeridium difficile LCO, which repre-
sents a maximum flooding surface, may be the turn-around in the middle Coniacian
tectono-eustatic phase, near the end of the Lysing sand phase. (4) The belli-dubia
break, is again (near or) at a maximum flooding event, this time correlated to the
LCO of T. suspectum in the early middle Campanian, above the change from marly
sediments to siliciclasts at the base of the Campanian. (5) The dubia-szajnochae
break reflects the abrupt change from siliciclasts to marly sediment at the
Campanian-Maastrichtian boundary, only noted in the southern part of the study
region.
Figure 9.33 is machine output for CASC correlation of nine events in eight wells.
The PPLs for these wells were based on the ranked optimum sequence with
MNS
7. In Gradstein et al. ( 1999 ) this sequence, which is almost the same as
the sequence of events in Fig. 9.32 , was used for RASC variance analysis. Individ-
ual events deviate from their PPL in each section. These deviations, which are
either positive or negative, have standard deviations that differ from event to event.
Good markers have small standard deviations. Nine such events are connected by
lines in Fig. 9.33 , which is CASC output with 95 % confidence limits. The eight
wells in Fig. 9.33 are arranged from north to south. CASC has a flattening option
according to which the line of correlation for a specific event between sections is
made horizontal. Event 16, the last occurrence (LO) of Hedbergella delrioensis was
used for flattening in Fig. 9.33 . These events and other events including several with
relatively large standard deviations also are correlated in Fig. 9.34 . The poor
markers with larger standard deviations show cross-over inconsistencies in this
diagram. Large standard deviations can be due to a variety of reasons. For example,
Foraminifera that are benthic tend to show more inconsistencies than planktonic
forms. Separate RASC plots of deviations for an event in all wells may reveal
patterns of diachronism. For example, L. siphoniphorum , observed in 19 wells,
appears to be time transgressive, ranging into younger strata southward. The same
may be true for E. spinosa , observed in 13 wells (Gradstein et al. 1999 , p. 69).
In Fig. 9.34 variance data on fossil events are used to create a different, but
effective type of correlation plot. Cretaceous turbiditic sands (with yellow or gray
patterns in Fig. 9.34 ) occurring offshore mid and southwestern Norway are corre-
lated in five wells. The Lower Cenomanian Hedbergella delrioensis FCO and LCO
ΒΌ
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