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Genesis of honeycomb buildups in the Permian Zechstein Group, Southern
North Sea
Thomas D. Houghton
*
, Joyce E. Neilson , John R. Underhill , Rachel E. Brackenridge
Centre for Energy Transition, School of Geosciences, Meston Building, University of Aberdeen, AB24 3UE, United Kingdom
ARTICLE INFO
Keywords:
Permian Zechstein
Mid North Sea High
Carbonate buildups
Palaeogeography
Palaeobathymetry
ABSTRACT
Seismic interpretation has revealed a hitherto unreported honeycomb pattern of carbonate buildups within the
Orchard Platform (Southern North Sea). The Z2 Stassfurt Halite Fm. onlaps the southern margin of the Orchard
Platform and is also found inlling Z2 intra-platform lagoons to form salt lakes. Post Z2 evaporation, the deeper
Z3 water column drowned the Orchard Platform inhibiting the platform recovery attempted by the Z3 Plat-
tendolomit Fm. The palaeobathymetric variability of the drowned Orchard Platform was sufcient to bring parts
of the seaoor into the photic zone allowing for the sporadic growth of the Z3 Plattendolomit Fm. However, the
palaeobathymetric lows remained beneath the photic zone ensuring an incomplete regeneration of the Orchard
Platform with the creation of a high-frequency network of intra-platform lagoons which mimic the polygonal
texture of a honeycomb. Whilst previously accepted as collapse structures or karst systems, this study correlates
the development of the honeycomb buildups to variations in seaoor palaeobathymetry which in turn mimic the
structural lineaments of the Zechstein subcrop. Syn-depositional instability in the Zechstein subcrop caused the
topsets of the Z2 salt lakes to become warped. The warped halite provided seed points for Z3 Plattendolomit Fm.
growth which allowed for linear ridges of carbonate to traverse the Z2 salt lakes and eventually connect with the
honeycomb buildups. Deposition in the Mesozoic lead to loading of the Zechstein. Halite-lled Z3 lagoons
accommodated this loading, which caused a pinching effect on the Z3 honeycomb buildups. The sedimento-
logical understanding provided by this study not only de-risks frontier exploration but also provides insight into
carbonate growth in restricted platform recovery scenarios.
1. Introduction
The Mid North Sea High (MNSH) is the intermediate west-east
striking palaeohigh that separates the Anglo-Polish Basin from its
Northern Permian Basin counterpart (Fig. 1). Despite hosting the rst set
of exploration wells, the MNSH was neglected due to a perceived lack of
hydrocarbon prospectivity and has remained largely underexplored.
Overall, the North Sea has gained the status of a mature petroleum
province and is now being re-evaluated for exploration (Brackenridge
et al., 2020; Underhill and Richardson, 2022). Recent studies and sub-
sequent drilling campaigns have revealed a play fairway within the
Upper Permian Zechstein Group across Quadrants 41–43 on the MNSH
(Patruno et al., 2018; Browning-Stamp et al., 2023). Recent success
involving the Ossian-Darach, Crosgan and Pensacola discoveries have
demonstrated hydrocarbon prospectivity in the Zechstein Z2 Hauptdo-
lomit Fm. 3D seismic and sedimentological facies mapping (Garland
et al., 2023; Browning-Stamp et al., 2023) unveiled the Orchard Plat-
form: A Z2 Hauptdolomit Fm. carbonate platform spanning Quadrants
36–38 and 42–44 (Fig. 2). Whilst our understanding of the character-
istics of the Orchard Platform has improved signicantly, the greatest
uncertainty now resides with the overlying Zechstein Group formations
which must be analysed to de-risk future exploration of the Zechstein
system on the MNSH. Using the TGS MNSH ION Survey (a pre-stack
depth migrated 3D seismic reection dataset), this study improves
stratigraphic understanding by analysing the distribution, thickness,
and characteristics of the Z2 Stassfurt Halite Fm. and the Z3 Platten-
dolomit Fm., both of which overlie the prospective Z2 Hauptdolomit Fm.
2. Geological context
Differential subsidence associated with early Permian rifting gener-
ated two west-east striking intracratonic basins bounded by massifs
* Corresponding author.
E-mail addresses: t.houghton.22@abdn.ac.uk (T.D. Houghton), j.neilson@abdn.ac.uk (J.E. Neilson), john.underhill@abdn.ac.uk (J.R. Underhill), rachel.
brackenridge@abdn.ac.uk (R.E. Brackenridge).
Contents lists available at ScienceDirect
Marine and Petroleum Geology
journal homepage: www.elsevier.com/locate/marpetgeo
https://doi.org/10.1016/j.marpetgeo.2024.107116
Received 31 July 2024; Received in revised form 14 September 2024; Accepted 15 September 2024
Marine and Petroleum Geology 170 (2024) 107116
Available online 16 September 2024
0264-8172/© 2024 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ ).
(Fig. 1). The Mid North Sea – Ringkøbing Fyn High is one such massif
which functioned as a partition between the Northern Permian Basin
and Anglo-Polish Basin (McCann et al., 2006; Glennie and Underhill,
1998). This palaeotopographical ridge of Palaeozoic strata remained
buoyant throughout the subsidence due to a Caledonian granite core and
as such inhibited communication between the basins (Donato et al.,
1983; Brackenridge et al., 2020). The only major known connection
between the basins is “Jenyon’s Channel” which was a north-south
striking, 40 km wide fault-controlled depression through the MNSH
(Jenyon et al., 1984; Houghton et al., 2024). This structural gap allowed
for cool Panthalassic marine waters to spill from the Northern Permian
Basin into its southern counterpart the Anglo-Polish Basin, thus initi-
ating sedimentary transgression from desert sands (Rotliegend) to ma-
rine carbonates (Zechstein) (Ziegler, 1988; Coward et al., 1995;
Sørensen et al., 2007; Doornenbal et al., 2010; McKie, 2017). Intermit-
tent marine connection to the Panthalassic Ocean resulted in the depo-
sition of ve major cycles (Z1-Z5) of carbonates and evaporites within
the Zechstein Sea across less than ve million years (Glennie, 1995;
Tucker, 1991; Jackson and Stewart, 2017). The carbonates were
deposited during sea-level highstand, followed by the precipitation of
evaporites during sea-level lowstand. The evaporitic facies progressively
onlapped against the basin margins whilst the basins were inlled dur-
ing lowstand. This study focusses on the transition between the end of
the second (Z2) cycle and the beginning of the third (Z3) cycle.
The rst ooding event deposited the Z1 Kupferschiefer Fm. (known
as the Marl Slate onshore) comprised of a thin, organic-rich, sapropelic
marine shale (Kotarba et al., 2006; Fyfe et al., 2023) after which came
the Z1 Zechsteinkalk Fm. carbonates followed by the Z1 Werraanhydrit
Fm. (Fig. 3). The Z2 cycle began with the deposition of the Z2 Haupt-
dolomit Fm. which thins distally into an organic-rich layer colloquially
known as the Stinkdolomit.
Shallow marine Z2 Hauptdolomit Fm. progradation on top of Z1
Werraanhydrit Fm. clinoforms helped to create a tabular carbonate
system known as the Orchard Platform (Fig. 3; Patruno et al., 2018;
Garland et al., 2023) at the southern entrance to Jenyon’s Channel
(Fig. 2a). As straits and seaways control the exchange of nutrients be-
tween basins (Bahr et al., 2022), it is envisaged that nutrient-rich waters
arriving via Jenyon’s Channel could have helped stimulate the growth of
the Z2 Orchard Platform at its intersection with the Anglo-Polish Basin.
The development of the Z2 Orchard Platform began to isolate the
Anglo-Polish Basin and led to a period of limited marine replenishment.
Accordingly, sea-level lowstand ensued and the subsequent hypersa-
linity precipitated prolic volumes of the Z2 Stassfurt Halite Fm. which
onlapped the southern margin of the Orchard Platform whilst the basins
were lled. The subsequent (Z3) ooding event generated a higher
sea-level, and the Orchard Platform was drowned resulting in restored
communication between the basins (Houghton et al., 2024). The Z3
cycle began with the deposition of the Z3 Grauer Salzton Fm. which
consists of calcareous shales, above which the Z3 Plattendolomit Fm.
carbonates began to develop.
Patruno et al. (2018) helped revive interest in the MNSH by identi-
fying intra-Zechstein clinoforms that bound a large tabular buildup and
similarities were drawn between this feature (now named the Orchard
Platform) and the Crosgan eld. In particular, the erosional unconfor-
mity at the base of the Zechstein stratigraphically connected the Orchard
Platform to potential reservoirs and source rocks in the folded and
fractured underlying Carboniferous strata therefore establishing uid
migration pathways. When combined with the three potential intra-
Zechstein source rocks (the Z1 Kupferschiefer Fm., the Z2 Stinkdolo-
mit Fm., and the Z2 lagoonal facies; Kotarba et al., 2006; Słowakiewicz
et al., 2013), the Orchard Platform has the potential for a multi-phase
hydrocarbon charge.
Fig. 1. Regional map of general Zechstein facies distribution in the North Sea after Ziegler (1988) with improvements to the MNSH after Brackenridge et al. (2020).
T.D. Houghton et al.
Marine and Petroleum Geology 170 (2024) 107116
2
Fig. 2. a) Map of the MNSH study area showing the locations of Z2 carbonate platforms and basins along with the seismic coverage of the TGS MNSH ION Survey b)
Map of the Orchard Platform study area including the subset of the 3D seismic volume (rectangle) and the locations of the local well penetrations that were tied to
guide seismic interpretation. The shape of the carbonate platform is after Browning-Stamp et al. (2023) and Garland et al. (2023).
T.D. Houghton et al.
Marine and Petroleum Geology 170 (2024) 107116
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The Z2 Stassfurt Halite Fm. basin inll allowed the Z3 Plattendolomit
Fm. platforms to extend further into the basin than their Z2 counter-
parts. As few wells penetrate the Orchard Platform, only limited cuttings
and cores have been recovered for the Z3 Plattendolomit Fm. Based on
these few sporadic wells, the Z3 Plattendolomit Fm. has been charac-
terised as a low energy, muddy, peritidal to subtidal unit (such as in
wells 36/13- 1, 36/15- 1 and 38/29- 1); however, higher energy envi-
ronments (such as at 38/25-1) yielded ne-grained backshoal facies
(Browning-Stamp et al., 2023). Nearby wells (42/10b- 2Z and 38/16- 1)
indicate potential prospectivity as the Z3 Plattendolomit Fm. interval
has yielded traces of benzene and toluene which indicates hydrocarbon
migration from the Carboniferous into the Zechstein (Harbour Energy,
2022). Whilst facies models for the Z3 Plattendolomit Fm. will remain
poor without further exploration, the stratigraphic model from this
study helps to characterise the Z3 continuation of the Orchard Platform.
3. Data and methodology
This study used SLB’s Petrel software to analyse seismic reection
data from the TGS MNSH ION Survey across Quadrants 36–38 and
42–44. This study utilised the 10,930 km
2
of 3D seismic data from Phase
1 and Phase 2 of this survey and was analysed at 5m resolution. Key
wells residing within the boundary of the MNSH ION Survey were used
to guide the seismic interpretation (marked in Fig. 2b). The seismic
reection dataset was specically acquired to analyse the Orchard
Platform and therefore there is high condence associated with the
Fig. 3. Tectonostratigraphic summary of all Zechstein formations and members, including the Z3 Roof Anhydrite Mbr. (Houghton et al., 2024) and the Z2 Stassfurt
Potash Mbr. (described by Garland et al., 2023). Figure uses the Southern North Sea classication of the Zechstein (after Duguid and Underhill, 2010, Patruno et al.,
2018; Grant et al., 2019, Fyfe et al., 2023). Where the carbonates and anhydrites are thin, they are interpreted together as they are represented by one
seismic reector.
T.D. Houghton et al.
Marine and Petroleum Geology 170 (2024) 107116
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Fig. 4. Seismic lines through the Orchard Platform showing important stratigraphic relationships at ve times vertical exaggeration. The lack of pullup lends
condence to the velocity analysis used to produce the depth volume. a) Well control on seismic interpretation and introduction to Z3 Plattendolomit Fm. char-
acteristics b) Relationship between subcrop faulting, the Z2 Orchard Platform, and the Z3 Plattendolomit Fm. c) Greater buildups of the Z2 Orchard Platform and
their control on salt lake distribution, along with Z3 Plattendolomit Fm. ridge preserved on a former salt lake d) Ridges between former salt lakes and some insight on
the evolution of the honeycomb buildups e) Examples of syn-Zechstein faulting and minor karstication.
T.D. Houghton et al.
Marine and Petroleum Geology 170 (2024) 107116
5
Zechstein-specic pre-stack depth migration. The polarity convention is
“North Sea Normal” and as such red troughs and blue peaks represent
increases and decreases in acoustic impedance, respectively (indicated
in Fig. 3, in line with the interpretation of this dataset by
Browning-Stamp et al., 2023). The formations that comprise the Zech-
stein Group have distinct petrophysical and seismic responses (recently
discussed by Patruno et al., 2018; Grant et al., 2019; and Fyfe et al.,
2023). Well tops were taken from Houghton et al. (2024), the
Fig. 4. (continued).
T.D. Houghton et al.
Marine and Petroleum Geology 170 (2024) 107116
6
methodological details of which can be found therein. Well-to-seismic
ties were undertaken to determine horizons for seismic interpretation.
In addition to top and base Zechstein surfaces, the high resolution of the
MNSH ION Survey facilitated mapping of three intra-Zechstein seismic
horizons: The top Z3 Plattendolomit Fm., the top Z2 Stassfurt Halite Fm.,
and the top Z2 Hauptdolomit Fm. As the Z2 Hauptdolomit Fm. and Z3
Plattendolomit Fm. are often quite thin, the horizons interpreted
sometimes includes the overlying Z2 Basalanhydrit Fm. and Z3 Haup-
tanhydrit Fm., respectively (see Fig. 3). Originally, the full dataset was
interpreted; however, the results section focusses on a smaller area of
interest that resides in the east of the Orchard Platform (outlined in
Fig. 2b). Surface attributes such as dip angle and dip azimuth were used
to analyse the characteristics of the Zechstein subcrop and the Z3 Plat-
tendolomit Fm. Complete mapping and facies analysis of the Z2
Hauptdolomit Fm. was undertaken by Browning Stamp et al. (2023) and
Garland et al. (2023), an extensive assessment of the Z2 Orchard Plat-
form can be found therein.
Fig. 4. (continued).
Fig. 5. Detailed petrophysical well-tie to seismic interpretation. Integration of seismic and petrophysical data shows that repeated stratigraphy correlates to bulging
associated with halite migration from Triassic loading.
T.D. Houghton et al.
Marine and Petroleum Geology 170 (2024) 107116
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4. Results
4.1. The Zechstein subcrop
The MNSH was above the depositional limit of the early-middle
Permian Rotliegend Group (Underhill et al., 2023) and therefore the
Zechstein subcrop (indicated in Fig. 4) consists of tilted and folded
Carboniferous strata which generates a strong impedance contrast with
the base of the Z1 Werraanhydrit Fm. (the Z1 Kupferschiefer Fm. and Z1
Zechsteinkalk Fm. are often below seismic resolution). Fig. 5 demon-
strates the well-tie to seismic interpretation. The Zechstein subcrop
features an extensive network of faulting and folding corresponding to
the highly deformed Carboniferous strata (Fig. 6). Whilst general
faulting trends are difcult to identify across the entire Orchard Plat-
form, the study area exhibits two distinct laterally orthogonal fault
systems: One that strikes WNW-ESE and another that strikes NNE-SSW
(Fig. 6). The seismic cross sections shown in Fig. 4 show an exten-
sional network of small horsts and grabens, the faults of which extend
less than 500m into the subcrop with offsets of less than 100m. This
system is separate to Mesozoic extensional faulting that reaches into the
lower Palaeozoic strata (an example of which is indicated in Fig. 4d).
4.2. The Z2 Stassfurt Halite Fm
The Z2 Stassfurt Halite Fm. is absent across much of the study area as
this evaporitic package onlaps and terminates against the margins of the
Orchard Platform. When present, the unit has limited thickness (up to
200m) and is conned to rounded polygonal pockets. The longest axis of
these major pockets can reach up to almost 20 km and have a very
approximate WNW-ESE strike (Fig. 7), with three examples of NNE-SSW
trending axis (matching the orthogonal pattern previously described in
the Zechstein subcrop). The seismic reectors within the Z2 Stassfurt
Halite Fm. tend to be horizontal with relatively little deformation
(Fig. 4). When present, the top of the Z2 Stassfurt Halite Fm. features a
distinct bright reector associated with a strong impedance contrast
with the overlying carbonate; however, when the halite pinches out the
brightness of the reector is lost, giving way to dull anhydrite reectors
which then, in turn, often fall beneath seismic resolution.
4.3. The Z3 Plattendolomit Fm
The Z3 Plattendolomit Fm. generates a strong impedance contrast
with the overlying Z3 Leine Halite Fm. and is therefore a very reliable
horizon to map. The reector consistently alternates between two levels
in the seismic reection data which produced highs and lows (as seen in
Fig. 4a–e). The vertical difference between the reectors is always
approximately 100m. Rather than a at reector, the lows tend to be
concave/depressed with the lowest point being equidistant between two
highs. Conversely, the roofs of the highs tend to be at with occasional
exceptions (Fig. 4e). The Z3 Plattendolomit Fm. dip angle map (Fig. 9)
reveals that the edges of the highs are extremely consistent with slopes
that approach 90◦. Fig. 5 reveals an example where the edges of the
highs are slightly concave.
When observed in map view (Fig. 8), it becomes clear that the highs
and lows observed in seismic data are all interlinked into one network.
When overlain by the major faults in the Zechstein subcrop, it is
apparent that there is a correlation between the strike of subcrop faults
and the geometry of the Z3 Plattendolomit Fm. surface (as indicated
across Fig. 4). A 3D schematic of the Z3 Plattendolomit Fm. (derived
from seismic data) is provided in Fig. 10.
5. Interpretation and discussion
5.1. Faulting and Z2 palaeogeography: setting the scene for the Z3
Plattendolomit Fm
The Z2 Hauptdolomit Fm. was deposited on top of the Z1 Werraan-
hydrit Fm. sulphate platform which began to form the Z2 Orchard
Platform (Patruno et al., 2018; Garland et al., 2023). Deposition of the
Z1 Werraanhydrit Fm. sulphate platform brought the seaoor well
within the photic zone which was conducive to carbonate growth
(Fig. 11b). Consequentially, the Z2 Hauptdolomit Fm. followed typical
platform models: intra-platform highs developed over the Carboniferous
subcrop (Fig. 11a) creating a network of subtidal lagoons linked by tidal
channels in the interior platform (Fig. 11b), all of which was protected
by a rimmed margin (Browning-Stamp et al., 2023). An evaporitic
period succeeded carbonate deposition which facilitated the precipita-
tion of the Z2 Stassfurt Halite Fm. forming salt lakes within the
intra-platform lagoons (Fig. 11c).
The Z2 Stassfurt Halite Fm. is thickest in the Anglo-Polish Basin
where it deformed into a network of diapirs and salt walls before thin-
ning against the southern margin of the Orchard Platform. To the north
in Jenyon’s Channel, the Z2 Stassfurt Halite Fm. is thin. The Northern
Permian Basin (and Jenyon’s Channel) maintained near-normal salinity
throughout the Z2 evaporation due to continuous replenishment from
the Northern Panthalassic Ocean which resulted in reduced halite pre-
cipitation (Houghton et al., 2024). Replenishment of the Anglo-Polish
Basin via Jenyon’s Channel was partially inhibited by the
Fig. 6. 2D surface of the dip angle of the Zechstein subcrop. An azimuth attribute was used to highlight the lineations in the surface that correlate to subcrop
fractures. These are particularly clear in the west of the study area where an orthogonal network of bright lines can be observed.
T.D. Houghton et al.
Marine and Petroleum Geology 170 (2024) 107116
8
Fig. 7. Thickness map of the Z2 Stassfurt Halite Fm. including the major salt lakes. Large areas of the Orchard Platform are non-depositional for this formation (blank
parts of the map).
Fig. 8. 2D surface of the Z3 Plattendolomit Fm.
Fig. 9. Dip angle of the Z3 Plattendolomit Fm. which demonstrates the even, predictable nature of the buildup slopes and ridges across the entire platform. This is
overlain by the locations of the Z2 salt lakes.
T.D. Houghton et al.
Marine and Petroleum Geology 170 (2024) 107116
9
development of the Orchard Platform which facilitated the creation of
the extensive halite deposits found in the basin today. The aforemen-
tioned intra-platform lagoons were inlled with halite in the east of the
Orchard Platform which demonstrates how marine water from Jenyon’s
Channel ooded onto the platform. The salt lakes are variable in size but
are ovoid with lengths of 10–20 km and widths of 5–10 km and contain
halite thickness of up to 200m (Fig. 7). The onlapping of intra-halite
reectors against the intra-platform highs suggests that the halite
within the salt lakes is bedded and non-chaotic (such as in the centre of
Fig. 4c). This opposes the internal characteristics of the diapirs found in
the Anglo-Polish Basin which evolved from bedded halite into chaotic,
highly deformed structures through halokinesis. As such, the halite on
the platform slowly and uniformly precipitated to inll the palae-
otopographical lows. The bedded nature of the Z2 salt lakes resulted in a
attened topset which levelled parts of the Z2 Orchard Platform and in
doing so generated a at surface for Z3 Plattendolomit Fm. deposition
(Fig. 11c).
In general, the axis of the salt lakes matches the strike direction of
subcrop faulting (Figs. 6 and 7). Moreover, distinct groups of subcrop
faults spatially bound the salt lakes (which is particularly clear in Fig. 4c
and d). Where two salt lakes run parallel, they are often separated by
horst blocks in the subcrop. The subcrop variations inuenced the dis-
tribution of the Z2 Hauptdolomit Fm. such that less carbonate deposition
occurred within the subcrop lows which left the space for halite inll. In
turn, this laterally restricted the salt lakes leaving them with the same
underlying trend as the subcrop (the evolution of which is characterised
in Fig. 11). The orthogonal NNE-SSW subcrop structural trend is also
mimicked by the Z2 salt lakes. The Z2 Hauptdolomit Fm. highs are less
pronounced in this direction such that halite often precipitates over the
structures; however, the most prominent and well-established highs
inuenced the connection between salt lakes (Fig. 4d). Occasionally the
uniform roof of a salt lake became raised by minor mobilisation (right of
Fig. 4d). This mobilisation fails to disrupt intra-halite reectors and
merely creates a slight distortion in the topset of the salt lake. These
distortions often correlate to faults in the subcrop which shows that
some fault movement (perhaps caused by loading) was occurring during
Zechstein deposition (as indicated in Fig. 4c and e). This feature is not
only observable in the Z2 salt lakes, but also on the basin margins
(Fig. 4a). Here the faulting extends into the Z2 Hauptdolomit Fm. which
supports the model for basin subsidence under the accumulating weight
of halite (Van den Belt and de Boer, 2007). The minor distortion of the
salt lakes and major salt deformation on the southern margin of the
Orchard platform have important consequences as they affected the
seaoor palaeobathymetry for the deposition of the Z3 Plattendolomit
Fm.
5.2. Evolution of the Z3 Plattendolomit Fm
Seismic mapping revealed a honeycomb textured network of
buildups and ridges in the Z3 Plattendolomit Fm. in the east of the Or-
chard Platform. Ridges of carbonate can extend unimpeded for many
kilometres and are anked by slopes of approximately 90◦. The Z3
Plattendolomit Fm. ridges follow the same general trend as the local
laterally orthogonal fault system observed in the Zechstein subcrop. The
ridges always rise to a height of approximately 100m where they
develop a at or slightly bulbous roof (Fig. 4). Whilst the ridges are
associated with individual faults, broader areas of elevated Carbonif-
erous subcrop correspond to signicant intra-platform isolated carbon-
ate buildups (Fig. 4c). Importantly, even in these isolated buildups the
Z3 Plattendolomit Fm. still exhibits heights of 100m.
The sea-level was higher in Z3 times (Houghton et al., 2024) and as
such the Z2 Orchard Platform was submerged in a deeper water column
than before. In an ideal system, sea-level rise is gradual to allow time for
carbonate growth to catch-and-keep-up with the upwards migration of
the photic zone. In the case of the Z3 Orchard Platform, the opposite was
true: The sea-level rise was sudden as it was associated with reooding
of the Zechstein basin network. Sea-level rise resulted in the base of the
photic zone scarcely reaching the now-drowned Z2 Orchard Platform
(Fig. 11d). Structural highs in the Zechstein subcrop were preserved in
the Z1 and Z2 stratigraphy; however, in Z3 times, these highs were the
only parts of the seaoor to reach into the photic zone. Accordingly,
growth of the Z3 Plattendolomit Fm. nucleated from the palae-
obathymetric highs which resulted in the network of buildups that as-
sume a honeycomb pattern (Fig. 8). Internal stratigraphic geometries
within the ridges display up to three major stages of growth within
seismic resolution (Fig. 11e and f). This left the palaeobathymetric lows
to become rounded polygonal lagoons bounded by ridges within which
occurred only minimal carbonate deposition and as such the network of
honeycomb buildups protected the lagoons.
The lows between Z3 Plattendolomit Fm. honeycomb buildups were
inlled with marl, debris, and perhaps pelagic deposits (depending on
environmental conditions). Consequentially, the signicant strati-
graphic thinning that occurs between the ridges and palaeobathymetric
lows of the Z2 salt lakes is preserved in the Z3 cycle (Fig. 4c and d;
Fig. 11d, e and f). As such, the major Z2 salt lakes became Z3 Platten-
dolomit Fm. lagoons bounded by ridges. As discussed, intra-Zechstein
faults provide evidence for Zechstein aged movement or loading
which prompted localised warping in the Z2 salt lakes. The warping was
Fig. 10. 3D diagram of the study area showing the honeycomb structure. Pink marking show where Z3 lagoons overly Z2 salt lakes and therefore indicate inherited
palaeobathymetric lows.
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(caption on next page)
T.D. Houghton et al.
Marine and Petroleum Geology 170 (2024) 107116
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sometimes sufcient to reach into the photic zone which allowed for the
development of Z3 Plattendolomit Fm. ridges across the areas which
were once palaeobathymetric lows in Z2 times (as seen in cross section
in Fig. 4c and laterally in Fig. 8, the evolution of which is shown in
Fig. 11). The Z3 Plattendolomit Fm. ridges that run across the Z2 salt
lakes adhere to the height, width, and slope geometries of the honey-
comb buildups.
Deposition of carbonates and evaporites ceased in the Triassic due to
climatic changes along with the introduction of increased volumes of
clastic material into the Anglo-Polish Basin, sourced from the erosion of
the London-Brabant Massif and Pennine High (Rushton et al., 2023). The
accumulating weight of clastic sediments in the overburden began to
cause a loading effect on the underlying Zechstein Group. The compo-
sition and mechanical strength of the Z3 honeycomb buildups allowed
them to withstand the initial loading; however, this was not the case for
the intra-buildup halite lled Z3 lagoons. The laterally uniform pressure
in the overburden began to cause a spreading effect in the weaker Z3
Leine Halite Fm. which caused the Z3 honeycomb buildups to become
pinched and slightly concave. Fig. 5 provides evidence for this phe-
nomenon by demonstrating a vertical repetition in the Z3 Plattendolo-
mit Fm. due to the Z3 Leine Halite Fm. bulging into the honeycomb
buildup.
5.3. Stratigraphic analogues
A useful analogue is provided by Alves (2015) and comes from the
late Permian of the Barents Sea where sustained organic productivity
generated buildups until the onset of the Permian-Triassic extinction
event. Whilst polygonal buildups are evident in the Carboniferous, the
polygonal geometries return in the late Permian which provides an
age-equivalent analogue to the honeycomb buildups of the MNSH. In
addition to a correlation to the underlying Carboniferous buildup sys-
tem, there is a suggestion that the unusual polygonal geometries in the
Barents Sea could be inuenced by biotic self-organisation (Alves, 2015;
Schlager and Purkis, 2015). In this theory, they (ibid) suggest that ma-
rine builders seek the easiest access to nutrient rich marine currents and
therefore organise themselves onto palaeobathymetric highs. In the case
of the honeycomb buildups of the MNSH, biotic self-organisation could
have concentrated growth on the palaeobathymetric highs which were
shallower in the photic zone, and upward moving nutrient currents were
focused through the deeper water column towards them. Furthermore,
biotic self-organisation resulting in polygonal buildups can be observed
in modern analogues such as at the Mataiva Atoll, French Polynesia (see
Schlager and Purkis, 2015).
The Loppa High on the Barents Shelf yields other examples of
polygonal buildups (Elvebakk et al., 2002; Colpaert et al., 2007, 2010);
however, these buildup systems date to the late Carboniferous to early
Permian and therefore do not provide accurate age-equivalent ana-
logues for the honeycomb buildups of the MNSH.
5.4. Discussion on the karstication hypothesis
Patruno et al. (2018) helped to reinvigorate interest in the MNSH by
describing a sulphate-carbonate platform complex where Carboniferous
horst blocks elevated the subcrop, facilitating the precipitation of a great
Z1 shallow marine sulphate platform. The pinnacles of the Z1 Wer-
raanhydrit Fm. clinoforms encouraged the deposition of Z2 shallow
marine Hauptdolomit Fm. facies which have better reservoir
characteristics than the low energy facies deposited in the adjacent
grabens. Patruno et al. (2018) recognised a “rugose” (rough) texture (a
wrinkled texture usually used to describe corals) across the Z3 Platten-
dolomit Fm. surface on the Orchard Platform. They (ibid) hypothesized
that this texture originated from karstication during the sea-level
lowstand that precipitated the Z3 Leine Halite Fm.
If the Z3 Plattendolomit Fm. had endured extensive karstication,
the interpretation should have revealed stratal geometries indicative of
exposure and downstepping (Schlager, 1981). This is where sea-level
falls and growth then continues from further down the slope; howev-
er, this is not observed in the seismic data over the easter part of the
Orchard Platform. Petrophysical evidence shows that the Z3 Leine Halite
Fm. precipitated above the Z3 Plattendolomit Fm. rather than onlapping
against the side of the platform (Houghton et al., 2024), suggesting that
the honeycomb buildups were submerged in hypersaline water at
sea-level lowstand (which is not conducive for extensive karstication
which requires fresh water). By extending the interpretation of the Z3
Plattendolomit Fm. further eastward, this study correlated the honey-
comb texture to underlying palaeobathymetric variations originating in
the subcrop. Whilst on a higher-frequency scale, the model presented in
this study shares similarities with the sulphate-carbonate platform
model: In both models, variations in the Carboniferous subcrop incite
shallow-marine deposition. However, Patruno et al. (2018) was working
with large-scale platforms associated with great horst blocks whereas
this model examines a localised fault network in the Zechstein subcrop
which highlights the need for high resolution seismic mapping to un-
derstand the complete tectonostratigraphic narrative of a geological
system. Patruno et al. (2018) noted that the Z3 Plattendolomit Fm.
features a similar texture in 3D seismic mapping of the Crosgan eld
(approximately 30 km south of the study area) which suggests that this
model is applicable to other Zechstein platform recovery scenarios.
The seismic geometries observed within the honeycomb buildups are
akin to those of patch reefs; however, rather than growth outwards from
a single nucleation point on the seaoor, we nd linear growth along
subcrop faulting. The at pinnacles of the buildups are a consistent
height across the study area, which suggests that the buildups completed
aggradation by reaching sea-level. Ultimately, the seismic geometries of
the Z3 Plattendolomit Fm. are more complicated than can be explained
by one model. Whilst sea level rise should result in a backstepping of the
platform (Schlager, 1981), the precipitation of the Z2 Stassfurt Halite
Fm. attens parts of the inner platform and distally extends the shallow
palaeobathymetry. This results in the Z3 Plattendolomit Fm. honeycomb
platform extending slightly further than the Z2 Orchard Platform
without the need for backstepping. Following this, the honeycomb
buildups were emphasised due to pinching associated with Triassic
sedimentary loading.
The seismic line in Fig. 4e shows evidence for minor karstication as
the top 30m of the Z3 Plattendolomit Fm. is lightly perforated. This
texture is not widespread and as such is restricted to the western part of
the study area. The geometry of the karstication is chaotic and does not
correlate to the underlying stratigraphy. It seems that the very top of the
buildups in the west of the study area were sometimes exposed. As the
topset of the buildups and ridges are generally at and uniform, the
minor karstication is neither widespread nor responsible for the hon-
eycomb buildups.
The Grosmont is perhaps one of the most famous examples of large-
scale karstication (Machel et al., 2012). At the Grosmont, karstication
lead to the loss of stratigraphic packages and the development of
Fig. 11. Evolutionary schematic showing the development of Z3 honeycomb structures. The model is based on seismic geometries and therefore has approximately
ve times vertical exaggeration. The blue and yellow lines indicate the sea-level and photic zone, respectively. a) A fault network in the Zechstein subcrop featuring
horsts, grabens and half-grabens b) The deposition of the Z1 and Z2 stratigraphy creates a gentle topography reecting the subcrop highs c) The inll of Z2
palaeotopographical lows as salt lakes d) the early Z3 Plattendolomit Fm. e) The start of aggregation on the palaeobathymetric highs f) Continued aggregation of
buildups (revealed through internal seismic geometry), which produces honeycomb structures g) The Z3 palaeobathymetric lows are inlled with Z3 salt; deposition
of the Z4 and Z5 packages (mostly halite) h) Sedimentary loading in the Triassic deforms the Z3 halite which gives the honeycomb buildups a bulbus geometry.
T.D. Houghton et al.
Marine and Petroleum Geology 170 (2024) 107116
12
large-scale circular to oval sinkholes of up to 150m diameter, the dis-
tribution of which could be compared to those identied in the Z3
Plattendolomit Fm. surface. Geometric consistency is the main differ-
ence between the sinkholes of the Grosmont and the buildup-bounded
lagoons of the Orchard Platform. Even across the Z2 salt lakes, the
honeycomb buildups always rise and fall to exactly the same height and
have a strong sedimentological correlation with the underlying strata. At
the Grosmont, the different carbonate platforms were dissolved to
random depths. Furthermore, polyphase post-depositional karstication
across millions of years was required to generate such extensive sink-
holes at the Grosmont: Indeed, Machel et al. (2012) note that the kar-
stication is ongoing today. In the case of the Orchard Platform, the
underlying Z2 Hauptdolomit Fm. is devoid of the same sinkholes and so
the timescale for widespread karstication is limited to the few hundred
thousand years between Z3 Plattendolomit Fm. deposition and the onset
of the Z4 cycle when the Z3 system was sealed in halite. As such, post Z4
karstication would not be possible without extensive halite dissolution.
Moreover, the end of the Permian was extremely arid (McKie, 2017)
which raises concerns for the source of such large volumes of fresh
water. As discussed, the topographical lows of the Z2 salt lakes were
preserved in the Z3 cycle. These lows can be up to 20 km along their
longest axis. As such, the diameter of the depressions in the Z3 Orchard
Platform can be tenfold greater than those of the Grosmont, and yet
developed in a fraction of the timeframe with no clear evidence for
extensive exposure. The simplest solution with the most evidence is that
the Z3 Orchard Platform lagoons were primary features with a strati-
graphic origin.
5.5. Subsurface dissolution
Another alternative hypothesis is that the lows in the Z3 Plattendo-
lomit Fm. were generated in response to extensive dissolution of the
underlying Z2 Stassfurt Halite Fm. (Peryt et al., 2010; Patruno et al.,
2018); however, the Z3 Plattendolomit Fm. is stable on many of the Z2
salt lakes. It therefore seems unlikely that the texture of the Z3 Plat-
tendolomit Fm. was caused by collapse via a dissolution effect
(Browning-Stamp et al., 2023) as there are examples of perfectly pre-
served salt lakes with unbroken overlying carbonate ridges. Moreover,
the Z2 salt lakes on the Orchard Platform demonstrate no evidence of
signicant mobilisation. Fig. 9 shows that the Z3 Plattendolomit Fm.
ridges and buildups have uniformly vertical dip. Widespread collapse of
the Z3 Plattendolomit Fm. would have led to a less predictable pattern
along with a consistent package thickness; however, this is not the case
and stratigraphic thinning is associated with palaeobathymetric lows.
6. Conclusions
Interpretation of the MNSH ION Survey (a high-resolution, pre-stack
depth migrated 3D seismic reection dataset provided by TGS) unveiled
a localised fault network (striking WNW-ESE and NNE-SSW) within the
Zechstein subcrop in the east of the Orchard Platform. This study ana-
lysed the inuence of the fault network on the Zechstein stratigraphic
succession which allowed for a reassessment of an unusual, honeycomb-
like network of highs and lows found in the Z3 Plattendolomit Fm.
The subcrop fault network encouraged Z2 carbonate growth which
eventually controlled the distribution of palaeobathymetric highs in the
Z3 sea oor. After the construction of the Z2 Orchard Platform, sea-level
lowstand initiated evaporation in the Anglo-Polish Basin. During evap-
oration, intra-Platform lagoons were periodically replenished with ma-
rine water and resultantly parts of the surface of the Orchard Platform
were levelled by Z2 Stassfurt Halite Fm. salt lakes. Sudden reooding of
the Zechstein basin system initiated Z3 sea-level highstand where sea-
levels were higher than at the Z2 highstand. The Z2 Orchard Platform
was drowned as there was no time for carbonate growth to catch up with
sea-level rise. The palaeobathymetric highs were sufcient to bring the
sea oor back into the photic zone, allowing for the Z3 Plattendolomit
Fm. to recover in a honeycomb-like network of ridges and buildups of
colonial algae that mimic the underlying fault network. Minor defor-
mation in the Z2 Stassfurt Halite Fm. also incited Z3 Plattendolomit Fm.
growth and as such vast ridges of carbonate traverse the Z2 salt lakes.
The development of the Triassic overburden caused a loading effect on
the Zechstein stratigraphy. The weak Z3 Leine Halite Fm. was suscep-
tible to deformation and therefore spread laterally due to a uniform
overburden development. This resultant pinching caused the topsets of
the honeycomb buildups to pop up, therefore overemphasising their
dimensions in seismic data leading to misinterpretation as karst features.
By rening our palaeoenvironmental and stratigraphic understand-
ing of the Orchard Platform this study should help to de-risk frontier
exploration in the Southern North Sea by providing stratigraphic insight
whilst also explaining the occurrence of the honeycomb buildup
phenomenon.
Data availability
The seismic reection dataset related to this article belongs to TGS,
the details of which can be found using the North Sea lters on the TGS
website [https://www.tgs.com/seismic/multi-client/europe/north-se
a]. The petrophysical dataset related to this article can be found in the
National Data Repository (NDR) and is available at [https://ndr.nstaut
hority.co.uk/], an open-source online data repository hosted by the
North Sea Transition Authority.
CRediT authorship contribution statement
Thomas D. Houghton: Writing – review & editing, Writing – orig-
inal draft, Methodology, Investigation, Formal analysis, Conceptuali-
zation. Joyce E. Neilson: Writing – review & editing, Supervision,
Project administration, Investigation, Funding acquisition, Conceptual-
ization. John R. Underhill: Writing – review & editing, Supervision,
Project administration, Conceptualization. Rachel E. Brackenridge:
Writing – review & editing, Supervision, Funding acquisition.
Declaration of competing interest
The authors declare the following nancial interests/personal re-
lationships which may be considered as potential competing interests:
Thomas Houghton reports nancial support was provided by Geo-
NetZero Centre for Doctoral Training.
Acknowledgments
The work contained in this publication was conducted during a PhD
study undertaken as part of the Centre for Doctoral Training (CDT) in
Geoscience and the Low Carbon Energy Transition and is fully funded by
NeoEnergy Upstream whose support is gratefully acknowledged. The
interpretations and analyses were undertaken in the Centre for Energy
Transition at the University of Aberdeen, the underpinning nancial and
computer support for which is gratefully acknowledged. We kindly
thank TGS for access to and permission to publish examples from their
proprietary data (TGS MNSH ION Survey) on which these in-
terpretations and analyses are made and we are grateful to SLB for
providing academic licences for their Petrel software which was used to
visualise and interrogate the seismic and petrophysical data. Finally, the
authors thank Tiago Alves (editor), Peter Gutteridge (reviewer), and an
unnamed reviewer for their helpful suggestions which greatly improved
the manuscript.
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