a rift-to-drift record of vertical crustal motions in the ... · the seismic-sequence correlation...

A rift-to-drift record of vertical crustal

motions in the Faroe-Shetland Basin,

NW European margin: establishing

constraints on NE Atlantic evolution*

M S Stoker1, S P Holford1 & R R Hillis2

1Australian School of Petroleum, University of Adelaide, Adelaide, SA 5005, Australia 2Deep Exploration Technologies Cooperative Research Centre, 26 Butler Boulevard, Burbridge

Business Park, Adelaide, SA 5950, Australia

1 *For full details see: Journal of the Geological Society, https://doi.org/10.1144/jgs2017-076

Road cutting in the Rhiconich

Terrane of the Lewisian Gneiss

Complex, NW Scotland, showing

grey tonalitic gneisses cut by

black Scourie dykes and pink

Laxfordian granite

Photo: Martyn Stoker

NE Atlantic Workshop, Durham, November 2017

https://doi.org/10.1144/jgs2017-076



2

NE Atlantic Workshop, Durham, November 2017 Some key issues.....

Paradigm Problem

• The entire thickness (+6 km) of the Faroe

Islands Basalt Group (FIBG) was erupted in

latest Thanetian (Late Paleocene)–earliest

Ypresian (Early Eocene) time (BP T-

sequences T40–45), about 56–54 Ma,

associated with continental breakup.

• This commonly-held view is based solely upon biostratigraphic data

(palynology of terrestrial flora) which conflicts with radiometric age dates,

magnetostratigraphic data and the stratigraphic relationship between the

sedimentary and volcanic formations in the Faroe–Shetland region, all of

which support a more prolonged period of volcanism, i.e. Selandian (Mid-

Paleocene)–earliest Ypresian (Early Eocene) (BP T-sequences T22–45),

about 62–54 Ma, which includes intraplate and breakup volcanism*.

• The FIBG (as defined above) and its

association with a Late Paleocene

unconformity (herein referred to as the Flett

unconformity) are a consequence of transient

uplift linked to (Iceland) plume processes.

• The conflicting interpretations of the chronology of the FIBG (described

above) strongly question its association with a single late Paleocene

unconformity. Whilst the existence of this unconformity is unequivocal, it is

not a unique surface; it is just one of numerous Cretaceous–Cenozoic

unconformities that preserve evidence of pre-, syn- and post-breakup

differential uplift within the Faroe–Shetland region*.

*Although the disagreement between biostratigraphers and radiometric age daters (in particular) has the

appearance of a minor, parochial academic dispute, its lack of resolution has held back stratigraphic investigations

and understanding of the North Atlantic Igneous Province for almost 20 years.

The following slides present a flavour of this ongoing controversy, highlighting the importance of the stratigraphic

record; an awareness of temporal and spatial context in developing an understanding of the tectonostratigraphic

framework; and the importance of palaeogeographic reconstruction as a test for both model and observation.

Structural framework

• Many of the structural highs comprise,

or are underlain by, Archaean basement

of Lewisian affinity

• The structural trend of the basement

blocks is dominated by a NE–SW-

trending Caledonian structural grain,

with this pattern also cut by NW–SE-

trending faults and transfer zones,

though the existence and significance of

some of these features is debated .

• The Mesozoic and early Cenozoic

structural development of the region has

been influenced by the interaction of

these two sets of lineaments, especially

in the south where the Rona and Judd

faults create a NW-trending offset – the

Judd High – that marks a long-lived

offset in the axial trend of the developing

Faroe-Shetland and NE Rockall basins

3

JUDD FAULT

RONA FAULT GEOSEISMIC PROFILE IN SLIDE 4


4

Tectonostratigraphic framework

• The structural framework comprises a series of basins and highs that record

a prolonged history of extension and rifting that took place episodically

during the Late Palaeozoic, Mesozoic and Early Cenozoic

• It is well-established that tectonics, including tilting, contraction and

differential uplift and subsidence, influenced post-breakup (Eocene and

younger) development of the shelf-margin/basin system linked, at least in

part, to the ongoing dynamics of the adjacent spreading system

• Inspection of the above profile reveals that the structural disposition of the

Paleocene–Eocene succession resembles that of a mildly inverted basin-fill;

that is, the configuration of the original basin at depth is retained whilst its

shallower parts are gently deformed and arched upwards (axial thickening).

This arching is ultimately expressed as the Judd anticline in this part of the

basin, which influenced the development of the overlying Rift-to-Drift and

post-Rift sequences. On this basis, a KEY QUESTION is: was this axial

thickening (early onset of basin inversion?) instigated by contractional

processes (early growth of anticlines, response to wrench movements, etc.)

during the Paleocene pre- and syn-breakup phases, and thus unrelated to

mantle processes as has been previously speculated?

Key to unconformities shown on geoseismic profile

T2a Late Priabonian-early Rupelian (essentially ‘Top’ Eocene)

T2b Late Bartonian/Priabonian

T2c Mid- to late Bartonian

T2d Mid-Lutetian

BSU Early Eocene (Base Stronsay unconformity)

FU Late Thanetian (Flett unconformity)

MPU Late Selandian (Mid-Paleocene unconformity)

NTDU Late Danian-early Selandian (Near-Top Danian unconformity)

BTU Latest Cretaceous-earliest Paleocene (Base Tertiary unconformity)


Palaeogeographic

Timeslices in maps a–f

(slides 8–13)

a

b

c d

e

f

Rift-to-drift

transition

Stratigraphic framework

5

The stratigraphic

evolution of the

Stronsay Group, with

its history of

fluctuating shelf

development,

unconformities and

channel incisions,

has many features in

common with the

development of the

Moray Group.

KEY POINT:

• On the southern margin of the Faroe-Shetland

Basin, the transition from the Moray Gp into the

Stronsay Gp is not marked by a rapid deepening of

the basin (as implied by Smallwood & Gill 2002,

and Shaw Champion et al. 2009); instead, the

stratigraphy (backed-up by the palaeogeographic

maps, below) reveals that a stacked assemblage

of unconformity-bound terrestrial and coastal plain

deposits characterise these Ypresian sequences.

Key to unconformities

T2a Late Priabonian-early Rupelian (‘Top’ Eocene)

T2b Late Bartonian/Priabonian

T2c Mid- to late Bartonian

I2c Late Lutetian

T2d Mid-Lutetian

BSU Early Eocene (Base Stronsay unconformity)

FU Late Thanetian (Flett unconformity)

MPU Late Selandian (Mid-Paleocene unconformity)

NTDU Late Danian-early Selandian (Near-Top Danian unconformity)

IDU Mid- to late Danian (Intra-Danian)

BTU Latest Cretaceous-earliest Paleocene (Base Tertiary unconformity)

Sedimentary facies

BFF Basin-floor fans

CPSH Coastal plain/shallow-marine shelf (includes deltaic)

CSH Clastic shallow-marine shelf

PSM Prograding shelf-margin

SAB Slopa apron–basinal

TSH Transgressive shelf

NE Atlantic Workshop, Durham,

November 2017

(FIBG Faroe Islands Basalt Group)

6

Model A

• Palaeomagnetic data suggests that the Beinisvørð Formation was erupted during

magnetochrons C26r through C24r, with the Malinstindur through to Enni formations

emplaced during C24r (e.g. Waagstein et al. 2002 and references therein).

• Radiometric age dating of the Beinisvørð Formation has yielded K-Ar and Ar-Ar

ages between 54.6 and 64.9 Ma (Waagstein et al. 2002). A separate set of pre-

Eocene ages for the Beinisvørð Formation range between 57and 61 Ma (Unpubl.

Sindri reports).

• Independent review of the Palynoflora (see ‘comment on biostratigraphy’ at bottom

of slide) has concluded that it is not specifically age-diagnostic.

• Sequence stratigraphy has identified three major sequence boundaries (Near-Top

Danian, Mid-Paleocene and Flett unconformities) that have been tentatively traced

from the sedimentary succession into the volcanic pile (Ebdon et al. 1995; Mudge

2015; Unpubl. BGS data, e.g. Smith et al. 2013; Stoker et al. 2015). This challenges

the seismic-sequence correlation of Ellis et al. (2002) and Jolley (2009).

Model B

• Biostratigraphic correlation of the Lopra and Beinisvørð formations to unit 1 of the

Flett Formation (T40) implies that the FIBG cannot be older than about 57 Ma, Late

Paleocene (e.g. Ellis et al. 2002; Passey & Jolley 2009); more recently Schofield &

Jolley (2013) stated that the FIBG is of earliest Ypresian (Early Eocene) age and

coeval with the PETM (e.g. Jolley 1997; Jolley et al. 2002, 2012). The radiometric

age dates and magnetochron interpretation of Model A are disregarded in Model B,

as is the consideration of other Paleocene hyperthermal events.

Comment on biostratigraphy

The palynological assemblage of the FIBG is based upon a correlation with the Ardtun Leaf Beds (Island of Mull) in the British Tertiary Igneous Province, whose flora

(commonly referred to as the ‘Staffa flora’) was linked to the PETM by Jolley (1997), and perpetuated in subsequent papers (Model B references). However, it should be

noted that: (1) independent analyses of the ‘Staffa flora’ describe it as a generic Paleocene pollen/spore flora that is deciduous and temperate, and not characteristic of the

PETM or the Paleocene-Eocene transition (Aubrey et al. 2003; Unpubl. BGS data 2016); and (2) it is overlain by lavas that have yielded U-Pb and Ar-Ar dates of 57.5–

60.54 Ma (Chambers & Pringle 2001).

The significance of this issue....

The Cambridge models of maximum transient mantle plume uplift in

the Faroe–Shetland region (e.g. Smallwood & Gill 2002; Shaw

Champion et al. 2008; Hartley et al. 2011) are largely predicated on the

assumption that there is a direct link between the development of the

Late Paleocene (Flett) subaerial unconformity and the emplacement of

the Faroese Volcanic Province (i.e. FIBG Model B, opposite (after

Schofield & Jolley 2013). Other tectonic models and/or volcanic

histories are not generally considered.

The Faroe Islands Basalt Group:

a stratigraphic conundrum

Model A Model B

Other data sources:

Onset of tuffaceous activity (Watson et al. 2017)

FSSC (Schofield et al. 2017)


LATE PALEOCENE–EOCENE PALAEOGEOGRAPHIC MAPS

7


The six palaeogeographic maps (a–f) illustrated below document the Late Paleocene–Eocene syn- to early post-breakup development of the Faroe–Shetland region. The maps are based upon the vast wealth of data acquired and published by the British Geological Survey (BGS) over the last 50 years as part of the regional offshore mapping programme (cf. Ritchie et al. 2011), integrated with other published information. BGS borehole 99/3 is a key Eocene stratigraphic site (Stoker et al. 2013) and provides an important control point in the Faroe-Shetland Basin. The six timeslice intervals reflect the punctuated sedimentary record preserved within the Moray and Stronsay groups; as such, they can be utilised as a sensitive recorder of the processes involved in basin development across the rift-to-drift transition.

8

Late Thanetian–early Ypresian (T40–T50)

Syn-rift/initiation of breakup (C24)

• At the end of the Thanetian (T40), uplift

along flanks of Faroe-Shetland Basin

resulted in erosion of Lamba Fm

sediments (dendritic incision pattern on

southern flank) as well as the top of the

Beinisvørð Fm basalts

• In the early Ypresian (T45 & 50), the

dendritic channel network was infilled

and buried beneath fluvial and paralic

sediments of the Flett and Balder fms

• Deeper-water basinal sedimentation

restricted to northern part of the Faroe-

Shetland Basin

• Prograding hyaloclastites associated

with the Enni and Malinstindur fms built

up the Faroe-Shetland Escarpment

• Tuffaceous deposits associated with the

Balder Fm probably mark the instigation

of plate breakup north of this region


Late Ypresian–earliest Lutetian (T60–T91)

Rift-to-drift transition (C24–C21)

• Initial basin morphology influenced by

syn-breakup volcanic terrain

• Major deltaic build-out – Munkagrunnur

Ridge delta – on SW margin of Faroe-

Shetland Basin

• Sediment also being shed from West

Shetland region

• Deltaic/shallow-marine setting implies

oscillating relative sea level –

contemporary uplift (flexural rebound?),

e.g. Munkagrunnur Ridge (Ólavsdóttir

et al. 2010)

• Marine shelf with tuffaceous limestone

west of Faroe-Shetland Escarpment?

• Emergent Iceland-Faroe Ridge

9


Early/mid-Lutetian (T91/T93)

Rift-to-drift/early post-rift (C21)

• Subaerial exposure of much of southern

end of Faroe–Shetland region?

• Emergent Iceland-Faroe Ridge

• Tectonic forcing, e.g. early growth of

inversion domes, such as the Judd

Anticline?

• No eustatic sea-level minimum at this

time

10


Mid-Lutetian (T93–T94)

Early post-rift (1) (C21–C20)

• Southern margin of Faroe-Shetland

Basin was transgressed and a lower

shoreface to shallow-marine setting

prevailed

• Regional evidence (e.g. Robinson 2004)

suggests increase in the geographic

extent of the basin (>200 m WD); Faroe-

Shetland Escarpment submerged and

overlapped

• West Shetland region remained a major

source of sediment

• Westward extent of shelf controlled by

the still-emergent Iceland-Faroe Ridge

11


Late Lutetian (T96–T97)


• In Q205, mid-Lutetian shelf deposits

incised by submarine channels up to

200 m deep (Robinson 2004; Robinson

et al. 2004)

• This channelisation might be part of a

wider zone of subaerial to submarine

erosion that extended around southern

margin of Faroe-Shetland Basin

• Renewed tectonic activity (domes, uplift

of intrabasinal highs) and/or eustatic

fall?

• Erosion of southern shelf margin led to

redeposition as Middle Eocene basin-

floor fans in Q’s 213 & 214

12


Bartonian–Priabonian (T98–T99)


• Basinal deepening across region; major

shelf-margin progradation from West

Shetland region

• Faroe-Shetland Basin between 350 and

500 m deep, but probably a restricted,

anoxic or sub-oxic, semi-enclosed

basin; also fed by deltas sourced from

the Wyville-Thomson Ridge – a further

response to contemporary tectonics?

• Faroe region still emergent at this time

(Waagstein & Heilmann-Clausen 1995)

though Iceland-Faroe Ridge beginning

to submerge (DSDP site 336)

13


SOMETHING TO PONDER....

The six palaeogeographic maps (a–f) are repeated below, but with an arbitrarily-placed oval covering the southern margin of the Faroe–Shetland region. This is intended to demonstrate that the same area that is assumed to have been uplifted by a mantle plume remained tectonically active long after plate breakup, by which time the axial plume would have been hundred’s of kilometres away.

14


Indicator of shallowing/uplift:

Subaerial drainage network incised into

Lamba Fm shelf:

• Valleys with maximum vertical relief of 200 m

(Smallwood & Gill, 2002)

Postulated mechanism:

Common paradigm is that uplift was linked

to mantle plume, located under Greenland

(at this time)

See Stoker et al. (2017 online) Journal of the

Geological Society, https://doi.org/10.1144/jgs2017-

076 for discussion and alternative tectonic mechanism

The bigger picture:

Continental margin rifting, volcanism and

breakup

• Evidence for wrench tectonics observed along the

East Greenland continental margin (Guarnieri 2015)

Late Thanetian–early Ypresian (T40–T50)

Syn-rift/initiation of breakup (C24)

15

Arbitrary oval






Munkagrunnur Ridge delta


Contractional deformation

• Compressional uplift associated with Munkagrunnur

Ridge (Ólavsdóttir et al. 2010) and southern margin

of Faroe-Shetland Basin

The bigger picture:

• Incipient seafloor spreading; segmented

margin with episodic ridge jumps

•Several readjustments (C22 & C21) in

spreading direction between Eurasia and

Greenland

•Tectonic instability and continuing

volcanism along UK/Faroes margin, e.g.

Rockall Plateau (Stoker et al. 2012)

Late Ypresian–earliest Lutetian (T60–T91)

Rift-to-drift transition (C24–C21)

16

Arbitrary oval



Uplift and subaerial erosion of

Munkagrunnur Ridge delta region


Contractional deformation; basin-margin

uplift, as well as growth of Judd (and

Westray) anticline(s)

The bigger picture:

•Readjustment (C21) in spreading

direction, Eurasia and Greenland, prior

to/coincident with (?) full breakup in

Iceland Basin (Stoker et al. 2012)

•Extension and counter-clockwise rotation

of southern JMMC; clockwise fan-shaped

spreading in Norway Basin (Gaina et al. 2009;

Gernigon et al. 2012; Blischke et al. 2017)

•No eustatic sea-level low at this time

Early/mid-Lutetian (T91/T93)

Latest rift-to-drift transition? (C21)

17

Arbitrary oval


Mid-Lutetian (T93–T94)


18

Arbitrary oval

The bigger picture:

Independent propagating rifts of proto-

Reykjanes & Aegir–Mohns ridge systems

•Continuous spreading (unbroken ridge

axis) in Iceland Basin

•Fan-shaped spreading in Norway Basin



Erosion of shelf; Incised channel network,

up to 200 m deep


Contractional deformation; continued

growth of Judd (& Westray) anticline(s);

uplift of Flett High (Robinson et al., 2004)

The bigger picture:

• Independent propagating rifts of proto-

Reykjanes & Aegir–Mohns ridge systems

persisted, with continuous spreading in

Iceland Basin and fan-shaped spreading

in Norway Basin

• Iceland hotspot minimum

•No eustatic sea-level low at this time

Late Lutetian (T96–T97)


19

Arbitrary oval



•Rejuvenated, prograding shelf-margin

with episodic incision (tens of m’s deep)

of shelf (Robinson et al. 2004)

•Wyville Thomson Ridge delta


Linked to contractional deformation in

western part of area (?), and regional

uplift/tilting of Orkney–Shetland high (?)

The bigger picture:

•Fan-shaped spreading in Norway Basin

•Extension and counter-clockwise rotation

of southern JMMC

•Readjustment (C18 & C13) in spreading

direction between Eurasia and Greenland

•No eustatic sea-level low at time of shelf-

margin progradation

Bartonian–Priabonian (T98–T99)


20

Arbitrary oval


• It is significant that interpretations of seismic

reflection data that aim to reconstruct the

tectonic history of the Moray Group in the

southern Faroe-Shetland Basin, must first

remove the effects of later Cenozoic

deformation in the same area to restore

Moray Group coal horizons to their original

horizontal disposition. That this restoration is

necessary highlights the fact that the Late

Paleocene, Eocene and later Cenozoic

deformations are co-extensive and spatially

linked to the southern margin of the Faroe-

Shetland Basin

• This observation provides some support for a

common tectonic origin of these features; by

the end of the Eocene, however, the North

Atlantic Ocean was probably up to 600

kilometres wide, and its axial mantle plume is

unlikely to provide a consistent explanation

for these repeated episodes of localised

deformation.

21

Structural framework: something to ponder... (1)

JUDD FAULT



Arbitrary oval

22

• This map shows all of the Cenozoic fold axes

described in various papers and reports

•Not all of these are necessarily contractional

folds; their antiformal or synformal geometry

might be a result of differential movements

•Whereas most of these folds are described in

terms of their post-breakup activity, it remains

unclear which, if any, might have been active

during pre- and/or syn-breakup time

•Work by Dean et al. (1999) and the BGS has

indicated that the uplift of the Corona High

and the developmeny of synclines (or

synformal sags) in the Flett and Foula sub-

basins, respectively, were active during pre-

breakup Early and Mid-Paleocene time

• It might be a reasonable conclusion –

bearing in mind earlier comments on the

geoseismic profile – that some of the

features of uplift at the southern margin of

the Faroe-Shetland Basin, including the

Munkagrunnur and Wyville Thomson ridges,

as well as anticlines such as Judd and

Westray, might have been developing prior to

breakup

Structural framework: something to ponder... (2)

JUDD FAULT



Arbitrary oval

23

Paleocene–Eocene tectonostratigraphic framework (1) NE Atlantic Workshop, Durham, November 2017

A few thoughts and conclusions:

• The preserved rock record indicates that the development of the Faroe-Shetland Basin during plate breakup characterised by long-term tectonic

instability, vertical motions and unconformity development that persisted long after the cessation of volcanism.

• The Late Paleocene Flett unconformity is one of a series of prominent unconformities that punctuate the Paleocene–Eocene succession, several of

which were formed by subaerial processes. Evidence linking its formation to uplift driven by a mantle plume remains equivocal.

• The majority of the syn- to early post-breakup unconformities post-date volcanism, and are most probably linked to vertical movements associated with

compressional deformation. The possibility that the Flett unconformity had a similar tectonic origin should not be discounted.

• The process of breakup and passive margin development in the Faroe–Shetland region is underpinned by a long history of vertical crustal motions that

arguably extended from the Paleocene to the Early Miocene. This long-ranging tectonic signature most probably reflects the protracted process of

breakup in the wider NE Atlantic, which remained partial until the JMMC finally separated from Greenland in the early Neogene.

24

Paleocene–Eocene tectonostratigraphic framework (2) NE Atlantic Workshop, Durham, November 2017

A few thoughts and conclusions:

• It is sensible to assume that the tectonic development of the Faroe–

Shetland region is linked to the process of breakup between Greenland

and NW Europe.

• Although the detail remains to be worked out, the broad correlation

between local and regional tectonic events suggests that basin

development was modulated by rift- and plate-boundary processes linked

strongly to the complex kinematics and tectonic evolution of the adjacent

ocean basin.

Late Paleocene–Early Eocene reconstruction

a rift-to-drift record of vertical crustal motions in the ... · the seismic-sequence correlation...

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