Uplift, deflation and marine onlap of a Jurassic rift dome, illustrated by a backstepping Middle–Upper Jurassic shelf‐to‐slope succession, Geographical Society Ø, East Greenland

“Doming–rifting–spreading” constitute the main phases of classical plate tectonics. The Middle–Upper Jurassic succession of East Greenland is interpreted as reflecting formation, deflation and onlap of a rift dome, similar to contemporaneous rift domes in the North Sea, Denmark and southern Sweden. The succession forms a northwards backstepping and onlapping early rift succession in the N–S oriented basin. Deposition ranged from coastal over shallow marine and slope sands, through offshore siltstones to deep basinal mudstones. An outlier on the island Geographical Society Ø fills an important gap in the documentation of the doming, deflation and progressive onlap. Bajocian deposition took place in a shallow marine shelf. This was followed in the late Oxfordian by slope and base‐of‐slope deposition. The backstepping succession onlaps progressively older rocks towards the north, illustrating the gradual deflation, erosion and onlap of the dome, constituting the early part of the doming–rifting–spreading phases of classical plate tectonics.

Society Ø north of Traill Ø, and farther north; (2) to demonstrate the associated increasing age of the onlapped rocks in the same direction; (3) to interpret the backstepping as resulting from uplift, deflation and onlap onto a Jurassic rift dome; (4) to relate the setting to contemporaneous rift domes and uplifted areas in the North Sea, Denmark and southern Sweden;and (5) to highlight this as an excellent example of the early phases of the doming-rifting-spreading succession of classic plate tectonic theory.

| S TR ATI G R APHY
The Jurassic succession exposed at Tvaerdal on Geographical Society Ø forms a new and essential element in the rift dome hypothesis presented in this study.It is referred to the upper Bajocianmiddle Callovian Pelion Formation and the lower-upper Oxfordian Olympen Formation (Figures 2-5).The Pelion Formation is widely distributed in East Greenland from Jameson Land in the south to Store Koldewey in the north (Figure 1).The Olympen Formation is known mainly from Jameson Land and Traill Ø where the top is middle Oxfordian.Our new data show that the formation extends farther north to Geographical Society Ø, where it reaches up into the upper Oxfordian (Figures 1, 2).

| S ED IMENTOLOGY
The Pelion Formation at Tvaerdal overlies Upper Triassic terrestrial sandstones and redbeds of the Ørsted Dal Formation and is unconformably overlain by middle Albian mudstones (Loc. 1 in Figure 1b, Figures 2-4).The preserved thickness is 48 m.The formation consists of mainly loose sands where sedimentary structures are difficult to identify.The base of the formation is marked by a pebble-to-cobble conglomerate, up to 30 cm thick, interpreted as marking the unconformable Triassic-Jurassic boundary.Coarseningupwards parasequences, several metres thick, are characteristic, but most units are structureless without any changes in grain size.
Trough cross-bedding, hummocky cross stratification, wave ripples, heterolithic bedding with mud draped ripples and pebble stringers occur at several levels.An up to 10 m thick clinoform set is exposed in the middle of the succession with a progradation direction roughly towards the SE away from the basin bounding fault.This is similar to other Middle Jurassic clinoform sets known from Jameson Land and Milne Land (Surlyk & Larsen, 2023).These clinoform sets are interpreted as reflecting creation of new rift-controlled accommodation space associated with progradation away from the coastline fed by offshore-directed downwelling currents during major storms.
Ammonites, belemnites, bivalves and serpulids are common together with rich trace fossil assemblages, similar to those described from the formation in Jameson Land (Heinberg & Birkelund, 1984).
The rarity of preserved sedimentary structures precludes a detailed environmental interpretation, but the succession is interpreted as representing a shallow marine shoreface to shelf setting similar to the situation in Jameson Land (Engkilde & Surlyk, 2003).
About 30 m of the Olympen Formation is present at Tvaerdal (Loc. 2 in Figure 1b), but the lower boundary is not exposed.It is overlain with erosional unconformity by middle Albian mudstones.
The facies are markedly different from those of the Pelion Formation and are dominated by laterally extensive, primary structureless, nongraded sandstone beds, up to about 5 m thick (Figure 5).Interbeds of coaly mudstone contain thin irregular sandstone injections similar to those characterizing the upper Oxfordian-lower Volgian mudstonedominated Hareelv Formation in southern Jameson Land (Surlyk et al., 2007).The sandstones were deposited by concentrated cohesionless high-density debris flows in a slope or base-of-slope environment and are in all respects similar to those characterizing the Athene Member, forming the lowest member in the Olympen Formation in Jameson Land (Bruhn & Surlyk, 2004;Larsen & Surlyk, 2003).
In Jameson Land the age of the Athene Member is late Callovian, whereas the Zeus Member forming the top part of the Olympen Formation is lower-middle Oxfordian.This member comprises mainly shallow marine, cross-bedded shelf sandstones, but correlative base-of-slope sandstone beds are known from the southern part of the outcrop area in Jameson Land.

Significance Statement
This study highlights the uplift, deflation and marine onlap in mid-Jurassic time of a rift dome in northern East  (Surlyk, 2003;Surlyk et al., 2007Surlyk et al., , 2021))

| BA S IN E VOLUTI ON
The Middle-Upper Jurassic of East Greenland forms a stepwise northward backstepping succession, ranging from coastal and shelf sandstones of the Pelion Formation at the base, topped by offshore siltstones of the Fossilbjerget Formation followed by slope and base-of-slope sandstones of the Olympen Formation to basinal mudstones of the Hareelv and Bernbjerg Formations.Farther north the base of the succession is formed by the progressively younger fluvial and paralic deposits of the Bristol Elv, Bastians Dal and Muslingebjerg Formations (Figure 6) (Surlyk et al., 2021).The base of the fully marine Pelion Formation likewise youngs towards the north.It is the oldest in southern Jameson Land, where the base belongs to the lower upper Bajocian Cranocephalites borealis zone (Callomon, 1993).The ammonite Arcticoceras cf.ishmae which straddles the lower-middle Bathonian boundary, occurs in the basal Pelion Formation in northern Wollaston Forland (P.A., unpublished data).The base of the Middle Jurassic shallow marine sandstones youngs farther northwards, as strata overlying the highest coal bed of the Muslingebjerg Formation on Hochstetter Forland contain dinoflagellate cysts suggesting the upper Callovian P. athleta ammonite zone (Piasecki & Stemmerik, 2004).
The boundary between the sandstones of the Pelion Formation and the overlying silty mudstones of the Fossilbjerget Formation in Jameson Land also shows a northward younging from the late Bajocian to the late Callovian (Alsen, 2015;Alsen & Surlyk, 2004;Callomon, 1993).The Fossilbjerget Formation has been traced northward from Jameson Land to Traill Ø (Alsen & Surlyk, 2004;Vosgerau, Alsen, et al., 2004) but seems to be absent on Geographical Society Ø.In northern East Greenland in the Wollaston Forland-Kuhn Ø area, correlatives of the shelf-slope break, slope and baseof-slope deposits of the Olympen Formation are represented by shelf sandstones and thin mudstones of the Jakobsstigen and Payer Dal Formations (Alsgaard et al., 2003;Surlyk et al., 2021;Surlyk & Clemmensen, 1983).The top of the Middle-lower Upper Jurassic sandstone wedge thus youngs from late Bajocian to the late Oxfordian from southern Jameson Land and northwards to Hochstetter Forland (Figure 2).
The base of the succession shows an associated northwards onlap onto progressively older rocks ranging from Middle Jurassic, Aalenian in Jameson Land (Surlyk et al., 2021), over Upper Triassic on Traill Ø (Vosgerau, Alsen, et al., 2004) and Geographical Society Ø (this study), to Lower Triassic at Hold with Hope (Vosgerau, Larsen, et al., 2004), and Upper Permian in south-west Wollaston Forland (Maync, 1947), to Neoproterozic crystalline basement in northern Wollaston Forland and Kuhn Ø (Kokfelt et al., 2013), to Palaeoproterozoic gneisses and granitoids at Hochstetter Forland and north-western Germania Land (Kokfelt et al., 2013) (Figure 6).This situation with progressively younger rocks onlapping progressively older rocks is similar to the well-known North Sea dome region (Underhill & Partington, 1993;Ziegler, 1982Ziegler, , 1990)).Contemporaneous uplifted areas or rift domes were developed in Denmark and southern Sweden (Bergelin et al., 2011;Nielsen, 2003).These domes show a similar stratigraphic development as the uplifted region of the present paper.An early tantalizing suggestion of the presence of possibly Jurassic domes on the western Norwegian continental margin was presented by Torske (1975).
A dome-influenced interpretation for the Middle-Upper Jurassic of East Greenland was proposed by Surlyk (1977Surlyk ( , 1978) ) but at that time the exact ages of both onlapping and onlapped rocks were poorly known.The position of the dome cannot be precisely located due to the limited knowledge of the geology of the potential areas.
However, a position in the Germania Land region at the northern termination of the Jurassic rift of East Greenland is considered most likely.
These domes are excellent examples of uplifting and subsequently deflating and onlapped rift domes and areas, representing the early phases of the doming-rifting-spreading succession of the Wilson Cycle of classical plate tectonic theory (Burke, 2011;Wilson, 1968;Wilson et al., 2019).

ACK N O WLE D G E M ENTS
Data were collected during GEUS expeditions in North-East Greenland under the auspices of the Petroleum Geological Studies, Greenland.The dome belongs to a 'family' of contemporaneous rift domes also known from the North Sea, Denmark, southern Sweden and possibly western Norway and is an excellent example of the first phases of the domingrifting-spreading cycle of classic plate tectonic theory.The upper part of the formation at Tvaerdal comprises two thick clinoform sets with somewhat diffuse foresets, showing palaeocurrents towards the south, and probably passing into structureless slope and base-of-slope sandstones farther south.The sets represent the shelf-slope break and position of the clinoform rollover (cf.Surlyk & Larsen, 2023).Downslope transitions from clinoform sets to structureless slope and base-of-slope sandstones are well known from the Volgian Raukelv and Sjaellandselv Formations in southernmost Jameson Land . The younger age of the top of the formation at Tvaerdal compared to Jameson Land and Traill Ø to the south reflects the backstepping nature of the Olympen Formation with shelf, slope and base-of-slope F I G U R E 1 (a) Simplified geological map showing the distribution of Permian-Cretaceous deposits in East Greenland.Modified from Alsen (2015) and Guarnieri et al. (2017) as regards Triassic faults.(b) Inset geological map shows the Tvaerdal localities on Geographical Society Ø. Position of Tvaerdal is indicated with a red box in (a).Numbers 1 and 2 show the locations of the studied sections.Modified from Alsen (2015).PDMF = Post-Devonian Main Fault.[Colour figure can be viewed at wileyonlinelibrary.com] settings in Jameson Land and only shelf-slope break and slope deposits at Tvaerdal.

F
Stratigraphic scheme showing the Middle-Upper Jurassic succession at Tvaerdal on Geographical Society Ø. Identification of the ammonite zones marked with asterisks are based on palynostratigraphy.[Colour figure can be viewed at wileyonlinelibrary.com]F I G U R E 3 The Middle Jurassic succession downfaulted towards the west against Carboniferous deposits along the Post-Devonian Main Fault (PDMF) of Vischer (1943).Locality 1, Tvaerdal, Geographical Society Ø. [Colour figure can be viewed at wileyonlinelibrary.com]F I G U R E 4 The Bajocian-lower Bathonian (Middle Jurassic) Pelion Formation, comprising shallow marine sandstones, overlying the Upper Triassic Ørsted Dal Formation represented by light-coloured sandstones with a red mudstone bed close to the top.The Pelion Formation is overlain by mid-Cretaceous mudstones with a strongly erosional unconformity.Locality 1, viewed towards the north-northwest.Tvaerdal, Geographical Society Ø. [Colour figure can be viewed at wileyonlinelibrary.com]F I G U R E 5 The upper Oxfordian (Upper Jurassic) Olympen Formation at locality 2. Note the structureless nature of the thick sandstone beds deposited from highconcentrated cohensionless debris flows on a submarine slope.The sandstones have subsequently undergone burial liquefaction and the boundaries, indicated by dashed lines were strongly deformed, and the mudstones were injected by sand at the top of the lower sandstone bed.Sandstone dykes are indicated with arrows.Person for scale.Tvaerdal, Geographical Society Ø. [Colour figure can be viewed at wileyonlinelibrary.com]F I G U R E 6 Schematic S-N section along the axis of the Jurassic rift basin, showing the northwards younging and backstepping nature of the Middle-Upper Jurassic sandstone wedge represented by the Pelion and Olympen Formations.Ba = Bastian Dal Fm, Be = Bernbjerg Fm, Br = Bristol Elv Fm, EBS = Eleonore Bay Supergroup, FC = Foldvik Creek Fm, Fo = Fossilbjerget Fm, Ha = Hareelv Fm, Ja = Jakobsstigen Fm, KS = Kap Stewart Gp, Mu = Muslingebjerg Fm, NK = Neill Klinter Gp, Ol = Olympen Fm, Pa = Palaeoproterozoic, PD = Payer Dal Fm, Pe = Pelion Fm, So = Sortehat Fm, WC = Wordie Creek Fm, ØD = Ørsted Dal Fm.Fm stands for Formation and Gp for Group.Distances between the outcrop areas indicated by latitudinal position and by a scale bar.Ages of the Jurassic stratigraphic units are given in Surlyk et al. (2021).[Colour figure can be viewed at wileyonlinelibrary.com] 5 | CON CLUS IONS 1.A Middle-Upper Jurassic outlier is described from Tvaerdal on the island Geographical Society Ø in East Greenland.2.About 30 m of shallow marine shelf sandstones belonging to the mainly Bathonian Pelion Formation form the lower part of the succession.They are followed by at least 30 m of upper Oxfordian shelf-slope break and slope sandstones of the Olympen Formation.3. The Olympen Formation at Tvaerdal adds a new and important fixpoint in the understanding of the northwards backstepping nature of Middle-Upper Jurassic sandstone wedge of East Greenland.4. The marked northwards younging of both base and top of the wedge and the progressively older age of the onlapped rocks are interpreted as reflecting the formation, deflation and onlap of an Aalenian or Bajocian, Middle Jurassic rift dome. 5.The evolution of the East Greenland rift dome is similar to the roughly contemporaneous rift domes and areas in the North Sea, Denmark and southern Sweden.6.These deflating rift domes constitute the first phases of the doming-rifting-spreading Wilson Cycle of classical plate tectonics.