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TECTONO-STRATIGRAPHIC FRAMEWORK OF THE NORTH FALKLAND BASIN:
POST-DRILLING ANALYSIS
P. C. Richards*and B. V. Hillier**
Search and Discovery Article #10016 (2001)
Adaptation of article, entitled Post-Drilling Analysis
of
the North Falkland Basin—Part 1: Tectono-Stratigraphic Framework, by the same
authors, published in Journal of
Petroleum Geology, vol. 23 (3), July 2000, p. 253-272.
Appreciation is expressed to the authors and to Christopher G. H. Tiratsoo,
Scientific Press Ltd. and Journal of Petroleum Geology (www.jpg.co.uk).
*British Geological Survey, West Mains Road, Edinburgh, EH9 3LA.
**Shell Production and Development Ltd (Falklands Branch), Shell Centre, London.
Correspondence to [email protected].
Six wells were
drilled in the extensional North Falkland Basin in 1998. The wells encountered a
Devonian to Cenozoic stratigraphy dominated by thick Mesozoic syn-and post-rift
successions. Although most previously published models predicted that the
succession would most likely be of marine origin, it is in fact predominantly
terrestrial; marine conditions did not become established in the basin until the
Late Cretaceous. The oldest rocks recorded are Devonian and these were
penetrated in only one well
. The overlying succession comprises: a
fluvio-lacustrine, early syn-rift interval of ?mid-Jurassic to Tithonian age; a
late syn-rift fluvio-lacustrine interval of Tithonian to Berriasian age; a
rift-sag transitional unit of Berriasian to Valanginian age; an early post-rift
lacustrine unit of Valanginian to early Aptian age; a middle post-rift,
transgressive unit of Aptian to Albian age; a late post-rift, terrestrial to
marine unit of Albian to early Palaeocene age; and a post-uplift thermal
subsidence unit of Palaeocene to Recent age.
Much of the
sediment appears to have been derived from volcanic and/or metamorphic terranes,
probably located to the north or NW of the basin. As well
as the volcanic
material which occurs in the ground mass and as lithoclasts in many of the units,
some volcaniclastic rocks and minor amounts of ashfall tuffs are observed,
particularly within the late syn-rift succession.
Fig. 1. The North Falkland Basin: (a) regional location map; (b) structural outline showing locations of the six wells drilled in
1998,and profiles of sections in Figs 3 and 4.
Fig. 2.
Log
correlation of the eight
tectono-stratigraphic sequences and the
associated sub-units in the six wells in the North Falkland Basin.Gamma-ray
log
scale on each
well
is from 0 to 150 API, sonic
log
scale is from 140 to 40
microsecs/ft. The Late Jurassic to Early Cretaceous lacustrine claystone that
forms the primary source rock interval in the basin is shaded grey. See Fig.1
for location of wells.
Fig. 3. Composite SW-NE cross section based on interpretations of seven
intersecting seismic lines (four oriented E-W and three oriented N-S) traversing
the North Falkland Basin. The section illustrates the relationships between the
tectono-stratigraphic units in three of the wells. Vertical scale in
milliseconds of two-way travel time (TWT). See Fig.1 for location.
Click here for view of cross-section (Fig. 3) with seismic section (Fig. 4).
Fig. 4. Part of an E-W seismic line across the Eastern
Depocentre,
illustrating the relationship between the early syn-rift, late syn-rift,
transitional unit, early post-rift and middle post-rift units. Data reproduced
courtesy of Spectrum Energy and Information Technology Ltd. PR=pre-rift;
ES-R=early
syn-rift; LS-R=late
syn-rift; TU=transitional
unit; EP-R=early
post-rift; MP-R=middle
post-rift; LP-R=late
post-rift; P-US=post-uplift
sag. See Fig. 1 for location.
Click here for view of cross-section (Fig. 3) with seismic section (Fig. 4).
Fig. 5. Cartoon illustrating the depositional setting of the early syn-rift
sequence
(mid-Jurassic to Tithonian) in the northern part of the North Falkland
Basin. Note that the Minke High was probably exposed, whereas the Intra-Graben
High was probably the site of fluvial deposition.
Click here for sequence
(animation) of Early
Rift (Fig. 5) and Early Post-Rift (Fig. 7).
Fig. 6.
Sequence
-stratigraphic
representation of
well
correlation in the North Falkland Basin.
Fig. 7. Cartoon of depositional setting of the LC4 sub-unit of the early
post-rift
sequence
(Barremian to Aptian)in the northern part of the North
Falkland Basin.Note that the Intra-Graben High was probably at least partially
emergent at this time.
Click here for sequence
(animation) of Early
Rift (Fig. 5) and Early Post-Rift (Fig. 7).
Table 1. The six wells drilled in the North Falkland Basin in 1998.
STRATIGRAPHY OF THE NORTH FALKLAND BASIN
The pre-rift interval (Devonian)
Lithology, Palynology, Depositional Environment, Age
The early syn-rift unit (? Mid-Jurassic to ?Tithonian)
Lithology, Palynology, Depositional Environment, Age
The late syn-rift unit (Tithonian to Berriasian)
Lithology, Palynology, Depositional Environment, Age
The rift to sag transition (?Berriasian to ? Valanginian)
Lithology, Palynology, Depositional Environment, Age
The early post-rift unit (Valanginian to ?Aptian)
Lithology, Palynology, Depositional Environment, Age
The middle post-rift unit (Aptian to Albian)
Lithology, Palynology, Depositional Environment, Age
The late post-rift succession (Albian to early Palaeocene)
Lithology, Palynology, Depositional Environment, Age
Post-uplift sag subsidence (Palaeocene-Recent)
Lithology, Palynology, Depositional Environment, Age
This paper summarises the stratigraphy of the six exploration wells which were drilled in the North Falkland Basin in the period April to November 1998 (Fig. 1 and Table 1). On the basis of these wells and an extensive database of seismic reflection profiles, a new tectono-stratigraphic framework is erected to describe the evolution of the basin and to provide a predictive model to aid further exploration. Changes in our understanding of the petroleum geology and remaining exploration potential of the basin resulting from this drilling campaign are discussed in a companion paper by Richards and Hillier (2000).
The regional geology
and palaeogeographic evolution of the offshore Falkland Islands has been well
documented (Richards and Fannin, 1994; Richards, 1995 and 1997; Lawrence and
Johnson, 1995; Platt and Philip, 1995; Richards et al., 1996 a and b;
Richards and Fannin 1997). Four Mesozoic-Cenozoic sedimentary basins surround
the Falkland Islands: the Falkland Plateau Basin, Malvinas Basin and South
Falkland Basin lie to the east, west and south of the islands respectively,
while the North Falkland Basin lies directly to the north (Fig.
1a).
Richards et al. (1996 a and b), Richards and Fannin (1997), Thomson and Underhill (1999), Bransden et al. (1999)and Lawrence et al. (1999)gave differing overviews of the structural and stratigraphic setting of the North Falkland Basin, and the associated main rift zone which was termed the North Falkland Graben by Richards and Fannin (1997)(Fig. 1b). Although these papers were compiled before drilling started, many (but by no means all) of the conclusions and interpretations presented in them are still valid following the 1998 drilling campaign. The biggest changes in understanding resulting from drilling concern the basin’s stratigraphy, sedimentary geology and petroleum system mechanics.
Richards et al. (1996a and b) and Richards and Fannin (1997)suggested that the infill of the North Falkland Basin may be analogous to the successions observed in the San Jorge Basin to the west, in Argentina (Fitzgerald et al., 1990). The analogy proved to be a reasonable one since both basins appear to have been dominated by fluvio-lacustrine deposition until quite late in their development. The stratigraphy of other nearby Argentine basins, particularly the offshore Malvinas Basin and the San Julian Basin (Fig. 1a), proved, after drilling, to have very little similarity to that observed in the North Falkland Basin. Figueredo et al. (1996) noted that before drilling in the San Julian Basin took place, it was anticipated that this basin would have an infill comparable to that of the San Jorge Basin. However, drilling proved that the succession was different in many respects, with a rift-to-sag transition occurring there much later than in the San Jorge Basin. The San Julian Basin exhibits a more comparable stratigraphy to that of the Deseado Masssif which separates the Malvinas and Magellanes Basins (Figuerido et al., 1996), and is unlike the drilled succession in the North Falkland Basin.
The North Falkland Basin lies beneath 150 m to 2, 000 m of water. It consists of a complex system of offset depocentres following two dominant structural trends (NW-SE and north- south), and displays a structural style quite different to the other offshore Falkland Islands basins. Its margins are mostly faulted (Fig. 1b), and it is surrounded by a structural platform composed of sedimentary rocks of probable Devonian age. The geometry and tectonic evolution of the basin was documented by Richards and Fannin (1997), who noted that two major depocentres, the Western Depocentre and the Eastern Depocentre, are separated by a faulted ridge termed the Intra-Graben High. Another fault-bounded high within the Western Depocentre is referred to here as the Minke High. The Intra-Graben High appears to have been a positive feature influencing sedimentation during much of the basin’s evolution. The Minke High appears to have been emergent, and the site of non-deposition, throughout the deposition of the syn-rift succession and through into the early part of the post-rift phase.
Richards et al. (1996 a and b), Richards and Fannin (1997), Thomson and Underhill (1999) and Bransden et al. (1999)noted that the infill of the North Falkland Graben could be subdivided into tectono-stratigraphic units on the basis of seismic geometries. These authors recognised various rift and post-rift phases, and assigned tentative ages to their units on the basis of correlations with offshore Argentine and South African basins. Elements of each of the predicted basin-infill models appear to be confirmed by drilling.
Six wells were drilled in the North Falkland Graben in 1998 (Fig. 1b); three were drilled in the Eastern Depocentre (14/5-1A, 14/10-1 and 14/24-1 ), two on the Intra-Graben High (14/9- 1 and 14/9-2 ), and one on the Minke High in the Western Depocentre (14/13-1 ). The wells allow significant refinements to be made to the original models of the tectono-stratigraphic history of the basin.
The collection of a
full suite of downhole logs was attempted in each well
, although technical
problems with hole-integrity resulted in the abandonment of some logging runs,
particularly in the deeper part of
Well
14/13-1. A full set of wet and
dried cuttings, rotary-and percussion side-wall core samples were collected from
each
well
, and three conventional cores were cut: two from
Well
14/9-1 and
one from
Well
14/24-1. Biostratigraphical analyses were carried out at
the
well
site during drilling and/or at various specialist contractors’
laboratories; post-drilling biostratigraphical analyses were made at two
different laboratories. The results were variable, with no consistent age
determinations being made by analysing palaeontological and palynological data
from different wells. Thin-section and XRD petrological studies were conducted
by all the drilling companies at a number of contractors’ laboratories, while
sedimentological studies were undertaken by contractors and also by the authors.
Detailed matching of mineralogy and lithology to wireline
log
response was
undertaken by the drilling companies for calibration purposes. Heavy-mineral
analysis
of the entire succession at
Well
14/9-1 was undertaken by the
British Geological Survey in order to determine the provenance area for the
sediments and to determine the likely contribution from the Falkland Islands
themselves.
The stratigraphic
correlation and subdivisions used in this paper are defined using a framework
based on seismic stratigraphy and seismic geometry, established as a result of
interpreting and mapping all of the seismic data (including proprietary data) in
the basin (over 20, 000 km of 2D data plus over 300 km2 of 3D data).
The major bounding surfaces of the tectono-stratigraphic units defined below are
based on the identification of regionally-mappable unconformities and
correlative unconformities. The mapped seismic horizons were tied to well
logs
and lithology, as
well
as to the biostratigraphy framework (where sufficient
data were available), in order to define meaningful correlation units.
STRATIGRAPHY OF THE NORTH FALKLAND BASIN
Eight regionally-correlatable
tectono-stratigraphic units are now recognised in the basin (Figs. 2 and
3).
Each of the seismically-identified sequence
boundaries defining the units has
been tied to downhole logs in the six wells. The eight tectono-stratigraphic
units are: a pre-rift succession; an early syn-rift interval; a late syn-rift
interval; a transitional unit; an early post- rift interval; a middle post-rift
interval; a late post-rift interval; and a post-uplift sag unit. These major
units are described in turn below.
In addition to these major units, eight sub-units of basin-wide significance have been identified in the early, middle and late post-rift succession (Fig. 2), and these do not coincide with changes in the tectonic regime. These sub-units are termed LC2-7, L/UC1 and UC1 (where LC is Lower Cretaceous and UC is Upper Cretaceous). The sub-units are interpreted to form distinct sequences and/or parasequence sets, and are bounded by seismic reflectors that separate intervals with differing depositional environments, reflecting the basinward or landward shift of facies belts. Sub-unit boundaries also represent changes in stacking patterns, and in the nature of the seismic offlaps, downlaps or terminations observed at their boundaries. Detailed description of the sub-units is beyond the scope of this paper.
The pre-rift interval (Devonian)
The pre-rift interval
was encountered only in Well
14/9-1 on the crest of the Intra-Graben High
(Figs. 2 and 3), which penetrated 49 m of the unit. Samples from this location
are not necessarily typical of the Devonian succession elsewhere in the basin,
nor of pre-rift rocks in the region. For example, Galeazzi (1998)noted that the
pre-rift succession in the Malvinas Basin to the SW consists of low-grade
meta-sedimentary rocks and granites.
The unit lies beneath
a prominent, high-amplitude reflector some 0. 38s TWT below the crest of the
Intra-Graben High, and appears to form the buried core of the high. The
reflector can be mapped into deeper parts of the basin, where it is interpreted
to represent the base of the Mesozoic sedimentary succession. The top of the
unit is picked at a downwards increase in bulk density and a decrease in
interval transit time (Fig. 2), as well
as at a downwards change in dip
direction from 180 ° to about 50 °. It is defined by the downwards transition
from Jurassic or indeterminate Mesozoic rocks to Devonian sedimentary or
metasedimentary rocks.
Lithology
The only thin section
available shows a moderately well
-sorted silty mudstone with subrounded to
subangular grains, a weak to moderate compaction fabric and some evidence of
low-angle laminations defined by silt-sized grains. The detrital mineralogy is
dominated by illite clays, with abundant carbonaceous material and minor amounts
of monocrystalline quartz, muscovite, biotite and chlorite.
Palynology
Some unidentifiable and poorly preserved spores of terrestrial origin were recorded, together with a single Palaeozoic marine algal specimen. All the palynological material is in a post- mature state of thermal alteration.
Depositional environment
The palynoflora is dominated by miospores of terrestrial origin suggesting a lacustrine or fluvio-lacustrine depositional environment, although the occurrence of a single marine alga may indicate at least intermittent marine conditions.
Age
Although only sparse, poorly preserved microfossils have been recovered from this unit, the occurrence of Emphanisporites spp and Perotrilites microbaculatius suggest a Devonian age.
The early syn-rift unit (?Mid-Jurassic to ?Tithonian)
This unit was
encountered in both Wells 14/9-1 and 14/9-2, drilled respectively
on the crest of the Intra-Graben High and in a near-crestal location on its
eastern flank, and also in Well
14/5-1A near the centre of the Eastern
Depocentre. It is absent at the 14/13-1 locality (Fig.
2), having pinched
out westwards onto the Minke High within the Western Depocentre (Fig.
3). The
unit lies below TD in Wells 14/10-1 and 14/24-1.
This unit is the lowermost wedging seismic package above Palaeozoic basement. Its top is defined by a high-amplitude seismic marker in the Eastern Depocentre which separates the early and late syn-rift seismic units (Fig. 4). The reflector is not easily identifiable over the crest of the Intra-Graben High, but is recognisable on its eastern flank as an onlap surface.
In both Wells 14/9-1 and
14/5-1A, the top of the unit is picked at the base of a
relatively low gamma interval which coincides with the top of an interval with a
gradual downwards decrease in interval transit time (Fig.
2). The top of the
unit corresponds to a downwards decrease in miospore abundance and degree of
preservation in the Well
14/9-1. Sidewall core samples from both 14/9-1 and 14/5-1A
indicate that kaolinite is virtually absent in this
sequence
, while illite forms a more pervasive clay phase than in younger units.
Lithology
The sequence
varies
laterally between Wells 14/9-1, 14/9-2 and 14/5-1A. In Wells 14/9-1 and 14/9 – 2
drilled on the Intra-Graben High, the unit is
dominated by conglomerates, conglomeratic sandstones, sandstones and tuffs with
minor quartzites and interbedded claystone. In
Well
14/5-1A near the
middle of the Eastern Depocentre, the unit comprises mostly silty claystone,
with sandstones up to 5-m thick. The claystones are silty to very silty, medium
brown to white in colour, soft, sticky and plastic. In the lowest part of the
sequence
at
Well
14/ 5-1A, the claystones contain more tuffaceous
material. The sandstones are feldspathic, fine-to medium-grained, sub-angular to
sub-rounded, moderately
well
sorted, with white calcareous coatings to grains
giving a frosted appearance.
Palynology
Palynomorph recovery
is significantly poorer in this unit than in the overlying units. Sparse, poorly
preserved miospores are characteristic of the unit in the finer-grained
sediments of the 14/5-1A well
. The miospore Cyclusphaera psilata is
occasionally recorded here, and may be caved. Other palynomorphs recovered
include frequent inaperturate pollens, rarer, bisaccate pollen, and some
pteridophyte spores.
Depositional Environment
The unit was probably
deposited in a fluvial or fluvio-lacustrine setting, possibly with alluvial fans
shedding sediment into the basin from the margins (Fig.
5). The Intra-Graben
High was probably a slight topographic ridge at the time, and was present to the
west of the tilt-fulcrum during the early syn-rift phase in the Eastern
Depocentre, which was structurally deepest adjacent to the eastern
basin-bounding fault. This region may have been the locus of fluvial
sedimentation, prior to the avulsion of fluvial channels towards the developing
low area adjacent to the eastern basin-bounding fault. Alternatively, the
developing Intra-Graben High may have formed the shoreline area of a
fluvial/lacustrine depositional environment, and thus received coarser sediment
than the central parts of the Eastern Depocentre. The finer- grained, claystone-dominated
succession found in the more centrally located 14/5-1A well
possibly
represents lacustrine and intermittent fluvial deposition closer to the deepest
part of the evolving rift.
Age
Although all taxa
recovered are generally long-ranging, they suggest an age no older than Middle
Jurassic. A number of biostratigraphers have examined sample preparations from
different wells; consequently, the sequence
has been interpreted in some wells
as ranging from Middle to Late Jurassic, but in others as ranging up to possibly
Valanginian. Given the probable age of the overlying
sequence
, it is most likely
that the
sequence
ranges in age from ?Middle Jurassic to Tithonian (Fig.
6),
although its top may be slightly younger. A seismic
sequence
with a similar
geometry and position near the base of the rift section was identified by
Figuerido et al. (1996)in the San Julian Basin but was interpreted as
being of Permian age.
The late syn-rift unit (Tithonian to Berriasian)
This unit was
encountered in all wells except 14/13-1, from where it is absent due to
pinchout towards the Minke High in the Western Depocentre (Figs. 2 and
3). The
unit forms the late syn-rift phase of basin infill, and comprises a wedge-shaped
seismic package displaying some onlap of reflectors onto the tilted top surface
of the early syn-rift sequence
(Fig. 4). Fig. 2 shows that the top of the unit
is generally marked by a downwards decrease in interval transit time and a
downwards increase in gamma-ray values; there is also a downwards decrease in
Compensated Neutron
Log
porosity values.
Lithology
Lithofacies types vary
according to the palaeogeographic position of the wells. In Well
14/ 9-1 located
on the crest of the Intra-Graben High, the
sequence
comprises a lower
conglomerate interval and an upper claystone interval. The conglomerate is pale
green to brownish grey, with a clay matrix in places. Side-wall core samples
from near the base of the conglomerate consist almost entirely of
cryptocrystalline silica glass and microcrystalline quartz. The overlying
claystone interval is light to medium grey, pale brown and brownish grey.
Well
14/5-1A, located
nearer the middle of the Eastern Depocentre, encountered a claystone- dominated
succession with thin sandstone beds, which had a total thickness of about 120 m.
The
sequence
in
Well
14/24-1 in the southern part of the basin is
dominated by tuffaceous claystones, which are pale to dark grey-green,
moderately hard, with some relict granular texture and occasionally visible
crystals. Most of these tuffs represent reworked volcanic material deposited by
sedimentary processes. Minor amounts of matrix-and clast-supported conglomerates
and current-rippled sandstones are also present, as is the local development of
rootlet structures. Similar reworked tuffaceous sedimentary rocks have been
described by Figuerido et al. (1996)from the rift succession in the
nearby San Julian Basin, although Galeazzi (1998)described a
sequence
dominated
more by volcaniclastic and rhyolitic rocks (the so- called Tobifera
volcanics)from the Malvinas Basin.
Palynology
In Well
14/9-1, the
unit has a rich to moderately rich, non-marine palynomorph assemblage. In
Well
14/5-1A,
the palynofloral assemblage contains the fresh-to brackish-water alga Celyphus
rallus, and Botryococcus. Kerogen is dominated by abundant, pyritised
amorphous organic matter, with subordinate structured inertinite and vitrinite
phytoclasts. In
Well
14/24-1, the unit is dominated by miospores
particularly gymnosperm pollen, while non-marine to brackish- water algae are
also common.
Depositional Environment
In common with the
underlying sequence
, the late syn-rift interval was probably deposited in a
fluvio-lacustrine environment. The coarser-grained sedimentary rocks found at
the base of the
sequence
at the crest of the Intra-Graben High probably
represent, as in the underlying unit, lacustrine shoreline deposits fringing a
more permanent lake, before the establishment of more pervasive lacustrine
conditions at this location (as interpreted from the overlying units). The
succession in
Well
14/5-1A near the middle of the Eastern Depocentre has
a mixed sandstone/claystone basal unit of possible lacustrine ?turbidite origin,
but becomes less sand- rich upwards, possibly reflecting the establishment of
permanent lacustrine conditions towards the end of the late syn-rift period. In
Well
14/24-1 in the southern part of the Eastern Depocentre, the
claystones are largely tuffaceous. These were probably deposited in a low energy
lake, with some infrequent high-energy depositional events indicated by the
presence of matrix- and clast-supported conglomerates and current-rippled
sandstones. The localised presence of ?early diagenetic mottling and rootlets
may indicate the development of incipient soil profiles at the lake margin or in
a waterlogged, marshy setting. Less tuffaceous claystones overlie the
tuff-dominated succession in
Well
14/24-1, and may represent the
re-establishment of permanent lacustrine deposition together with a decrease in
the availability of volcanic material, as a result of drowning of a local source
area as lacustrine conditions expanded. Lambiase (1990) demonstrated that syn-rift
successions in continental basins tend to be dominated by semi- permanent lakes
in their central portions.
Age
This sequence
has been
variously assigned (by different biostratigraphers working on different wells)as
Tithonian to Valanginian, Valanginian to Barremian, or Tithonian to Berriasian.
Most of the taxa recovered are long-ranging, but they are comparable to other
Late Jurassic to Berriasian assemblages known from the Springhill Formation of
Argentina and Chile. This unit is probably of Tithonian to Berriasian age (Fig.
6), although it is possible that its top lies within the Valanginian.
The rift to sag transition (?Berriasian to ?Valanginian)
This unit can be
identified in all wells except 14/13-1 ; it pinches out onto the flanks
of the Minke High within the Western Depocentre (Figs. 2 and
3). It has a
lensoid profile (Fig. 3), indicating that it may represent the first phase of
sag-infill of the basin. The top of the unit is defined as the base of a series
of high amplitude reflections (Fig. 4), and in most wells by a downwards
decrease in interval transit time, a downwards decrease in gamma values (Fig.
2),
a downwards decrease in Compensated Neutron Log
porosity values and a slight
downwards decrease in bulk densities.
Lithology
This unit is generally claystone dominated. Thus, in Well
14/5-1A near
the centre of the Eastern Depocentre, the unit comprises an organic-rich
claystone which is medium to dark greyish brown, firm to moderately hard, blocky
to sub-fissile, micromicaceous, silty in part, generally clean and homogenous,
and increasingly indurated with depth. Traces of
well
sorted, kaolinitic
sandstones are recorded at some levels.
Well
14/9-1 shows the only
variation to the dominant claystone lithology; here, as
well
as tuffaceous
claystones, the unit comprises weathered conglomerates which are poorly sorted
and contain angular to subrounded lithic clasts.
Palynology
The unit is not distinguishable as a distinct palynological unit in any of the wells. It appears to contain assemblages similar to those in the overlying early post-rift succession, being dominated by fresh-to brackish-water alga and non-marine miospores.
Depositional Environment
As in the underlying
units, the succession at the crestal Well
14/9-1 possibly represents a
shoreline depositional environment fringing a lake. The claystone-dominated
successions in the other Eastern Depocentre wells (including
Well
14/9-2 on
the flank of the ridge)may represent sites of more permanent lacustrine
deposition.
Age
This unit is
relatively thin and it is difficult to differentiate it in terms of
biostratigraphy from the overlying claystone succession of the early post-rift
sequence
. The most likely age range for this unit is probably Berriasian to
Valanginian (Fig. 6).
The early post-rift unit (Valanginian to ?Aptian)
The early post-rift
unit is probably present in all six wells, although the lowermost part is
present only in the Eastern Depocentre and on the eastern flanks of the Intra-Graben
High (Figs. 2 and 3). The early post-rift succession is represented by a
saucer-shaped seismic package developed above the syn-rift wedges and the
transitional unit. The top of the unit is picked at a seismic reflector which
defines the top of a lowstand seismic package occurring in front of a Lower
Cretaceous delta (Fig. 2)originally identified by Richards and Fannin (1997),
which prograded into the basin from the north. The top of the unit is marked by
a downwards increase in gamma-ray log
response and a significant downwards
increase in interval transit time and bulk density (Fig.
2).
The early post-rift
unit is divisible into three seismic or sequence
stratigraphic sub-units (Fig.
2). Sub-unit LC2 at the base of the succession is characterised by an interval
of high- amplitude, sub-parallel reflectors, and is observed on seismic sections
to pass northwards into the oldest deltaic foreset deposits recognised in the
Eastern Depocentre. The overlying sub- unit LC3 is more seismically transparent
over much of the basin, and in the northern part of the Eastern Depocentre
contains the main, progradational part of the Lower Cretaceous delta. The
topmost sub-unit, LC4, is present only locally within the Eastern Depocentre,
directly to the south of the delta foresets, and is interpreted to represent a
period of lowstand deposition following maximum delta progradation into the
basin.
Lithology
The early post-rift succession is dominated by massive claystones with minor sandstones, siltstones and interbedded dolomite. A sandier succession comprising much of sub-units LC2 and LC3 is present in the northern parts of the basin, representing deltaic progradation mainly from the north, although minor deltaic lobes also prograded from both the western and eastern margins of the basin.
The claystones which dominate the succession are grey to brown coloured, soft to firm, amorphous or blocky to sub-fissile, slightly calcareous, rarely pyritic, carbonaceous, and occasionally micromicaceous, with rare plant remains. Interbedded with them are buff-coloured, soft to firm, amorphous to sub-blocky, silty and carbonaceous dolomite beds, up to 2-m thick (which cause the high seismic reflectivity of the unit), and also rarer, silty sandstones.
Well
14/5-1A, the
northernmost
well
in the Eastern Depocentre, penetrated the deltaic system
draining from the north. Near the top of this deltaic interval, presumed topset
deposits comprise interbedded argillaceous sandstones and claystones. The
argillaceous sandstones are very fine grained and moderately
well
sorted, with
detrital components comprising abundant mono-and polycrystalline quartz grains,
K-feldspar and volcanic and metamorphic rock fragments. The topset claystones
are dark brown, with abundant flakey (woody?)particles, locally abundant silt
grains, and scattered, small, golden-brown phosphate grains. Beneath these
seismically-imaged topset deposits, the dipping foreset interval comprises
claystones interbedded with sandstone, silty or sandy claystones and rare
dolomite. The sandstones are translucent to orange green and include mixed
lithic fragments including volcanics, cherts and quartzites. They are fine-to
very coarse-grained, poorly-sorted and angular to sub-rounded. The sand grains
have occasional white kaolinitic coatings and are rarely cemented. Lithic clasts
within the sandstones include trachytic and rhyolitic volcanics, and the
dominant quartz grains generally have a thin veneer of microcrystalline quartz.
The seismically imaged, flat- lying bottomset interval comprises claystones and
silty claystones with thin sands and dolomite stringers that produce
high-amplitude seismic reflections.
Palynology
The succession is dominated by the fresh-to brackish-water alga Botryococcus, Celyphus rallus and rarer Pediastrum.
Depositional Environment
The dominant claystone lithologies of this succession were deposited in a lacustrine setting, as indicated by the general absence of marine indicators in the palynological assemblages. A similar lacustrine setting was reported for the D129 succession in the nearby San Jorge Basin (Fitzgerald et al., 1990), although Galeazzi (1998)reported that generally similar claystones in an early post-rift setting in the Malvinas Basin to the SW are of marine origin.
The presence of
interbedded dolomites within the claystones may suggest possible drying phases
with evaporitic conditions in relatively shallow-water areas. Alternatively,
carbonate- rich layers may represent deposition in a relatively deep lake during
dry climatic periods, and organic-rich rocks may represent deposition during
wetter periods (cf. Collinson, 1978). The occurrence of thin coals at Well
14/24-1
point to shallow-water, lake margin conditions at this location, although
water depths may have been considerably greater elsewhere. Dipping seismic
reflectors interpreted as progradational foresets in the northern part of the
Eastern Depocentre (see below) have a height of around 500m, suggesting that
lake depths were of this order of magnitude.
The occurrence of fish
remains and ostracods in places indicates that the lake was oxygenated, at least
in the upper parts of the water column. However, total organic carbon values
averaging 5%, and oil yield values (determined by Rock-Eval pyrolysis: Richards
and Hillier, 2000) up to 74. 5 kg HC/tonne of rock in well
14/10-1 suggest the
development of anoxic bottom conditions. In present-day East African lakes, the
highest TOC values are recorded in lakes that are either permanently stratified
or are only episodically overturned (Talbot, 1988). However, TOC values of 1. 4%
to 4% have been recorded in lakes that are occasionally stratified (Talbot,
1988). Katz (1990) showed that the highest organic concentrations, and the
highest hydrocarbon generating potentials, are found in East African lakes of
intermediate water depths (50 to 400m), possibly because of the increased
phytoplankton production in these lakes compared to deeper,
permanently-stratified water bodies.
It is quite unusual for large lake systems to be recorded within post-rift rather than syn-rift settings, although the broadly age-equivalent D-129 Formation lacustrine claystones of the San Jorge Basin (Eastern Argentina)also occur in an early post-rift setting (cf. Fitzgerald et al., 1990). By contrast, modern lakes in the East African Rift Valley are syn-rift systems. Lambiase (1990), in a review of ancient rift-basin lakes worldwide, suggested that the topography required for the development of a large lake occurs only in the early phase of rift evolution, although lacustrine sedimentation can occur at any stage in the evolution of a rift system. It is probable that the early post-rift lacustrine deposits are a remnant of a more localised lake which first became established during the late syn-rift phase.
Delta progradation
from the north (which began during deposition of sub-unit LC2 and continued
throughout deposition of sub-unit LC3)resulted in the development of a
significant axial delta system within the Eastern Depocentre. The source area
for the delta is uncertain, although petrographic and heavy mineral analyses
(see below) on side-wall core and cuttings samples suggest that it was derived
by unroofing of a metamorphic and granitic terrane probably located to the north.
However, no detailed provenance-typing studies have yet been conducted. In Well
14/5-1A,
a range of deltaic environments and associated sedimentary rocks are
preserved in seismically-defined intervals interpreted as topsets (1, 750m to 1,
780m below rig floor) and foresets (1, 780m to 2, 008m)in a 258-m thick section
which rests above generally fine-grained deposits interpreted as bottomset beds
(2, 008m to 2, 274m). The foresets dip at approximately 5°, which contributed
to pre-drilling prognoses that they were composed predominantly of sand-grade
material.
Although the delta
succession in Well
14/5-1A is relatively sand-rich, it is possible that
much sand-grade material was carried further offshore, into the deeper-water
lake areas by sediment-laden underflows, producing density-flow deposits within
the lake. Such density deposits have not yet been encountered by wells in the
basin, but mounding visible along some seismic reflections within the early
post-rift unit may indicate the sites of such sand deposition. Progradation of
the axial deltas ceased, probably during the Valanginian to early Barremian,
when an apparent relative lake level fall led to the development of lowstand
deposits (sub-unit LC4)located in the deeper parts of the basin (Figs. 2 and
7).
Age
In common with the
other stratigraphic units in the North Falkland Basin, the early post- rift
succession has been dated differently in different wells. In some wells it has
been assigned to the Valanginian to Barremian, whereas in others a Valanginian
to Aptian or even Albian age has been assigned. These differences possibly
result from the generally long age-ranges of the dominant non-marine taxa. In
Well
14/5-1A, miospores typical of the Aptian to early Albian in
Antarctica and Australia, including Inaperturopollenites elegans, Trilites
tuberculiformis and Cyatheacidites annulatus are observed near the
top of the
sequence
, while the freshwater alga Celyphus rallus, which is
characteristic of the Barremian, is recorded from near the middle of the
sequence
.
The base of the
sequence
is hereby taken to lie within the Valanginian, while its top is
assigned to the Aptian (Fig. 6), although it may range up into the Albian.
The middle post-rift unit (Aptian to Albian)
This unit is
recognised in all six wells (Figs. 2 and 3). No distinct log
breaks are observed
at the top of the unit, and lithologies are generally similar above and below.
However, its top corresponds with a downwards transition from sedimentary rocks
bearing marine or marginal- marine palynomorphs, to rocks with predominantly
non-marine forms.
Three sub-units are identified within the middle post-rift succession (Fig.
2). The lowest sub-unit, LC5, overlies the lowstand deposits of sub-unit LC4 at
the top of the early post-rift sequence
, and is interpreted as a transgressive
systems tract before the renewal of fluvial deposition over the delta surface.
The overlying sub-unit (LC6)is significantly coarser-grained than sub-unit LC5,
and probably therefore represents a low-stand systems tract (a basinwards shift
of facies). A thin, uncored transgressive systems tract must cap sub-unit LC6,
as the succeeding unit (LC7)represents highstand systems tract fluvial
sedimentation that continued throughout the remainder of the middle post-rift
interval.
Lithology
The succession
comprises sandstones, conglomerates, claystones and minor coals. Nearly half of
sequence
stratigraphic sub-unit LC5 at the base of the succession was cored in
Well
14/ 9-1. This cored interval predominantly comprises fining-upwards
cycles about 0. 5-to 1. 5-m thick, with sharp, erosively-based, coarse-grained
or granule-rich sandstones fining up to fine- or very fine-grained sandstone,
and in one case to siltstone. Occasional carbonaceous wisps and laminae in the
sandstones define both wave and current ripples which can be disrupted by
bioturbation; although some indistinct, possibly vertical burrow forms are
present, no definitive types can be identified. Muddier and siltier facies are
sharply interbedded with the sandstones, and range in thickness from a few
centimetres to about 70cm. The siltstones are lenticular bedded, with intense
bioturbation of the sandy lenses in places, and small, vertical burrows are
present near the tops and bases of some beds. Some small (cm scale) gutters are
present at the bases of current-rippled sand lenses in the lenticular-bedded
units.
The overlying sequence
stratigraphic sub-unit LC6 was also partially cored in
Well
14/9-1. The
cored interval comprises very fine to medium-grained, fining-upwards sandstones
in beds up to about one-metre thick. Rootlets and small, vertical burrow
structures are observed in places, with widespread bioturbation. The detrital
mineralogy of the cored interval is dominated by fragments of devitrified acidic
lavas, with minor components of claystones, sandstones, granites and sparse
schists and quartz. Sub-unit LC6 is not cored in Wells 14/5-1A, 14/9-2 and 14/10-1,
where it appears to be significantly coarser-grained, with a
number of conglomeratic horizons identified in side-wall core samples. A
side-wall core sample from a conglomeratic bed in
Well
14/5-1A was found
to contain clasts of K-feldspar-rich volcanic rocks, devitrified volcanic glass,
granite and metamorphic rock in a matrix of very coarse-grained sandstone.
Sub-unit LC7, which forms the remainder of the middle post-rift succession,
comprises interbedded sandstones, conglomerates, claystones, tuffaceous
claystones, siltstones and minor limestone, dolomite and coal beds.
Palynology
Sub-units LC5 and LC6
contain humic vitrinite debris and miospores, with locally high concentrations
of vitrinite and low concentrations of inertinite and amorphous kerogens. Sub-
unit LC7 contains marginal to non-marine palynoflora, with some vitrinitic
debris and Botryococcus algae. A diverse pteridophyte spore assemblage
was recorded from Well
14/13- 1, suggesting a humid, temperate
palaeoclimate and non-marine conditions.
Well
14/5-1A encountered rare
marine dinocysts within this interval, although they are significantly less
common than in the overlying L/UC1 unit, and the organic material is dominated
by carbonised woody matter.
Depositional Environment
The absence of
widespread marine palynological indicators probably precludes a fully- marine
setting, suggesting that the depositional environment may have been a marginal
lake or a lacustrine lagoon. Common bioturbation, coupled with some evidence of
vertical burrows, suggests that the near-surface sediment was well
oxygenated.
The occurrence of carbonaceous laminae and chips indicate that there was
abundant vegetation in the hinterland.
The fining-upwards
sandstones that dominate the cored interval in sub-unit LC5 indicate high energy
traction deposition of sand in either small, confined streams or more widespread,
unconfined flows, that scoured into a muddy substrate, probably in a
shallow-water environment. Large-scale cross-bedding is generally absent,
suggesting that the channels and/or sheetfloods were of relatively minor size.
The sandstones may represent deposition in a range of fluvial, crevasse splay,
storm sheet and other nearshore to lagoonal settings. Although not cored, the
conglomerates in sub-unit LC6 are interpreted to represent coarse-grained
fluvial deposits; they are recorded in the same sub-unit as small-scale
fining-upwards sandstones with rootlets and burrows, interpreted to represent
deposition in a vegetated fluvial floodplain. The thin coals recorded in Well
14/24-1
indicate deposition in waterlogged lake margin conditions, which may have
been widespread across the Eastern Depocentre at this time.
Age
A wide range of ages
has been proposed for this sequence
in the six wells. In some wells it has been
assigned a Valanginian to Albian age, in others a Barremian to Albian age, and
in others an Aptian to Cenomanian age. As with the older units, the general
absence of marine dinocysts and the long-ranging nature of the dominant
miospores hinder accurate age determination. In
Well
14/9-1, a marked
influx of Classopollis spp near the very top of the unit suggests an age
no younger than early Albian, whilst the presence of Callialasporites
trilobatus a few tens of metres below the top suggests a probable Aptian age.
In
Well
14/9-1, the presence of the miospore Classopollis turbatus, in
association with Cyclusphaera psilata near the top of the
sequence
suggests an Aptian to early Albian age. This unit is therefore assigned a
probable Aptian to Albian age (Fig. 6).
The late post-rift succession (Albian to early Palaeocene)
This interval is
present in all six wells, and represents continued overstepping of the basin
margins which had begun during deposition of the underlying succession.
Continued inundation of the basin margins was partly facilitated by relative
lake-level rise and coincides with the first significant marine incursion into
the basin. The sequence
therefore represents a combined transgressive and marine
highstand systems tract. It is divisible into two sub-units, L/UC1 and UC1.
These are separated by a regionally-mappable seismic reflector and can be
differentiated on the basis of their palynological content. Sub-unit L/UC1 is of
apparent marginal-marine origin, whereas sub-unit UC1 is fully-marine.
The top of the succession is picked at a seismic event that marks a significant unconformity recognisable throughout the basin. There are insufficient logs and samples to characterise the top of this unit adequately, since in most of the six wells the first sample retrieval occurred only just above or below this major unconformity.
Lithology
The succession is either sand-dominated (with interbedded claystones)or claystone dominated with interbedded sands, depending on the precise location. The sands are occasionally sphaerosideritic.
Palynology
Sub-unit L/UC1 yielded very few microfossils, but caved dinocysts are abundant at some levels in a palynofloral assemblage dominated by miospores with occasional, probably in situ dinocysts. The dinocyst Palaeoperidinium cretaceum, and the agglutinating benthonic foraminiferid Spiroplectammina praelonga are present, together with rarer examples of other dinocysts, suggesting a marginal-marine setting, probably partially restricted.
Sub-unit UC1 contains
a diverse marine microfauna, dominated by a range of dinocysts, calcareous
benthonic and agglutinating foraminiferids with more minor radiolaria and sponge
spicules. Low numbers of planktonic foraminifera are also present. The amount of
terrigenous, humic debris is variable throughout the unit, but it is more common
in the lower parts of the sequence
. The fresh-to brackish-water alga Botryococcus
is also present in places within the lower parts of the unit.
Depositional Environment
The presence of sphaerosiderite within sub-unit L/UC1 indicates the possible development of waterlogged soils, and this, together with the palynological data, suggests a terrestrial to marginal-marine environment. The restricted-marine character of the dinocysts and foraminifera present suggest possible lagoonal conditions, which may have developed only periodically. By contrast, the overlying sub-unit UC1 was deposited in a more pervasive marine environment which developed as relative sea levels rose. Although there are indicators in the palynofacies for an open-marine setting, the presence of some agglutinating foraminiferids suggests the possibility of at least locally restricted conditions. The intermittent presence of humic debris and Botryococcus suggests that terrestrial run-off was important at times, possibly in a near- coastal setting.
Age
The range of ages
proposed for this unit in the various wells is less than that observed for the
underlying units: the sequence
is assigned to the Albian to Maastrichtian or
Danian in most wells. No palynomorphs characteristic of the Turonian to
Santonian periods are present in any of the wells except
Well
14/5-1A, where
the first downhole occurrences of the dinocysts Surculosphaeridium
longifurcatum, Odontochitina porfera and Cleistosphaeridium huguoniotti are
(respectively) taken to indicate a Coniacian age, a Coniacian-to-Albian age and
a Coniacian age. The presence of abundant Manumiella spp in
Well
14/9-1
near the top of the
sequence
supports an early Palaeocene age for the top of
the interval. This tectono-stratigraphic interval probably spans the Albian to
early Palaeocene (Fig. 6).
The middle and late post-rift successions together possibly equate more or less with Megasequence 3 described from the Malvinas Basin by Galeazzi (1990). He proposed that the base of this Megasequence in the Malvinas Basin is defined by an unconformity at 112 Ma (Aptian), and that the top of the succession may be marked by an unconformity at 68 Ma (Maastrichtian).
Two potential phases of uplift have been invoked for the North Falkland Graben. Lawrence and Johnson (1995)and Thomson and Underhill (1999)proposed mid-Cretaceous uplift leading to deltaic progradation, followed by Tertiary inversion of the basin. Richards et al. (1996 a and b), noting comparisons with the Malvinas and Magallanes Basins and with various South African offshore basins, suggested that there was evidence only for Palaeogene uplift of the entire region. However, they acknowledged that there could be evidence for a Cenomanian unconformity (significantly younger than deltaic progradation) similar to that observed in the Falkland Plateau and Malvinas Basins.
The age data now
available from the six wells in the basin indicate that there are a number of
regional or sub-regional unconformities (Fig. 6); the two most significant of
these span the late Cenomanian to Santonian and much of the Maastrichtian. Plots
of Vitrinite Reflectance (VR) versus depth from Well
14/9-1 show a
surface intercept value of 0. 27% Ro, indicating that some 800m of
Cretaceous-Cenozoic section is missing over the crest of the Intra-Graben High.
However, surface intercept values for VR observed in the other wells range from
near 0 in
Well
14/5-1A in the centre of the basin to almost 4% Ro in
Well
14/24-1.
This range of values probably indicates that there has been
differential uplift and/or erosion and/or non- deposition in different parts of
the basin, probably as a result of local tectonic readjustments over individual
fault blocks.
Post-uplift sag subsidence (Palaeocene-Recent)
This unit was penetrated in all six wells but was neither sampled nor fully logged in any of them. The top of the unit is seabed.
Lithology
The unit consists of massive claystones which are medium to dark brown, locally chocolate brown, soft to moderately hard, non-calcareous and generally very silty. Interbedded with these are minor, very fine to fine-grained sandstones.
Palynology
Microfossils from this
unit are more or less restricted to samples from near sea bed, as there were
very few returns to any of the rigs during penetration of the sequence
. The
samples from the seabed and 36-in hole in
well
14/13-1 contained a rich
and diverse assemblage of planktonic and calcareous benthonic foraminiferids,
together with sponge remains, radiolaria and dinocysts. The samples from the
24-in hole in
Well
14/13-1 also yielded a rich microfauna and flora, with
a different composition to the overlying samples. This fauna was dominated by
fairly diverse, calcareous benthonic foraminifera with rare planktonic
foraminifera, while the flora is dominated by marine microplankton, principally
dinocysts.
Depositional Environment
Deposition probably followed early Palaeocene uplift of the basin, and commenced with the northwards progradation of a major deltaic wedge into a fully marine basin.
Age
This unit ranges in age from Palaeocene to Recent.
Before drilling
commenced, it was assumed that much of the sediment infilling the basin was
derived from the area of the present-day Falkland Islands, or from the Devonian
platform surrounding the basin, while the Lower Cretaceous deltas were thought
to have been sourced from areas to the west and NW (Richards and Fannin, 1997).
Analysis
of lithoclasts, detrital mineralogy and particularly, the heavy
minerals suggests that all the sediments encountered to date may in fact have
been derived from a mixed volcanic, regionally metamorphosed and perhaps
granitic terrane, probably located to the north or NW of the basin. Much of the
kaolinite that fills pore throats in the middle post-rift sandstones may have
been derived from the breakdown of feldspars weathered from a granitic terrane.
The precise location of the source region has not been established, and no heavy
mineral typing studies have yet been conducted to compare the North Falkland
Basin successions with outcrops in Argentina or South Africa.
The lowermost part of
the early syn-rift succession in Well
14/9-1 contains clasts of Devonian
sedimentary rocks indicating its provenance. However, the remainder of the
Mesozoic section in this
well
contains distinctive, dark brown rutiles which
indicate that the source area was mineralogically different from the Devonian
rocks that form the bulk of the Falkland Islands, as these contain mostly yellow
to red rutiles (analyses conducted by R. W. O’B. Knox, pers. comm., 1998).
Apatite, anatase, topaz, calcic amphiboles, garnet, monazite and zircon are
commonly found throughout the succession in
Well
14/9-1, and indicate
derivation from a range of igneous, metamorphic or sedimentary rocks. The
predominant monocrystalline quartz present as lithoclasts indicates, in addition,
a possible volcanic source. The syn-rift succession at
Well
14/24-1 (the
most southerly of the wells drilled) comprised a variety of tuffs, tuffaceous
claystones and other volcaniclastic rocks. The variable degree of rounding in
some intervals indicates variable distances of transportation and/or variable
times spent in different depositional environments.
We propose, therefore, that the provenance area was not therefore located in the region of the present-day Falkland Islands, but was probably constituted by a progressively-unroofed granitic intrusion with associated metasomatized and volcanic rocks located possibly to the north or NW (R. W. O’B. Knox, pers. comm., 1998).
The infill of the northern part of the North Falkland Basin has been divided
into eight tectono-stratigraphic units, tying the six recently-drilled wells to
a regional seismic database. Biostratigraphic information is at present not
sufficiently refined to allow the erection of a regional sequence
-stratigraphic
framework based on palynological and palaeontological data. Lithostratigraphic
subdivisions are adequate to describe the stratigraphy in individual wells, but
could not be used to correlate between wells because of lateral lithological
variations within units.
The eight tectono-stratigraphic units (and the eight sub-units) recognised are:
The basin was the site of fluvio-lacustrine deposition throughout the early and late-syn rift periods, and a more permanent lake became established during the transitional phase before post-rift subsidence. The early post-rift phase was tectonically quiescent, and was characterised by the progradation of several deltaic systems (dominated by a southwards-prograding delta in the Eastern Depocentre) into a large lake system. A relative lake-level fall at the end of the early post-rift phase was followed by deposition in a fluvial and lacustrine lagoonal setting, before the establishment of highstand fluvial sedimentation throughout the remainder of the middle post-rift phase.
The late post-rift phase, from the Late Albian or Cenomanian to the Early Palaeogene, was characterised by the establishment firstly of marginal-marine conditions and then of fully- marine conditions (in the Campanian), as a marine connection was established into the basin. By contrast, marine conditions became established earlier (Late Jurassic) in the Malvinas Basin (Galeazzi, 1998)and later (Cenozoic) in the San Jorge Basin to the NW (Fitzgerald et al., 1990), suggesting that the regional development of marine conditions spread from the south or SE. Regional uplift in the Palaeogene was followed by thermal sag, and marine/ deltaic deposition occurred through the remainder of the Cenozoic.
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