Palaeoecology of syn-rift topography: A Late Jurassic footwall island on the Josephine Ridge, Central Graben, North Sea

    Palaeoecology of syn-rift topography: A Late Jurassic footwall island on the Josephine Ridge, Central Graben, North Sea Adam D. McArthur, David W. Jolley, Adrian J. Hartley, Stuart G. Archer, Hugo M. Lawrence PII: DOI: Reference:

S0031-0182(16)30225-5 doi: 10.1016/j.palaeo.2016.06.033 PALAEO 7881

To appear in:

Palaeogeography, Palaeoclimatology, Palaeoecology

Received date: Revised date: Accepted date:

18 April 2016 18 June 2016 21 June 2016

Please cite this article as: McArthur, Adam D., Jolley, David W., Hartley, Adrian J., Archer, Stuart G., Lawrence, Hugo M., Palaeoecology of syn-rift topography: A Late Jurassic footwall island on the Josephine Ridge, Central Graben, North Sea, Palaeogeography, Palaeoclimatology, Palaeoecology (2016), doi: 10.1016/j.palaeo.2016.06.033

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ACCEPTED MANUSCRIPT Palaeoecology of syn-rift topography: A Late Jurassic footwall island on the Josephine Ridge, Central Graben, North Sea

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Adam D. McArthur*1,3, David W. Jolley1, Adrian J. Hartley1, Stuart G. Archer1, Hugo M. Lawrence2

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1 Department of Geology & Petroleum Geology, Meston Building, Kings College,

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University of Aberdeen, AB24 3EU, UK *Corresponding author (e-mail: [email protected])

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2 ConocoPhillips (U.K.) Limited, Rubislaw House, Aberdeen

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Abstract

Understanding rift topography is essential for determining source areas, sediment pathways,

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and the type of sediment delivered to a rift basin; factors essential for interpreting

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petroleum systems in ancient rifts. Here we investigate Upper Jurassic sediments from the Josephine Ridge region of the Central Graben, North Sea, by integrating geophysical, petrophysical

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palynological,

and

sedimentological

datasets

to

analyse

the

palaeoenvironments of the Jade and Judy horsts, the tops of which are not preserved. Interpretation of geophysical and petrophysical data together with core descriptions show study wells to step progressively away from the Josephine Ridge into adjacent grabens. One hundred and five palynological samples from six wells range from the Oxfordian to the Lower Tithonian, spanning the syn-rift period of the Central Graben. Samples from the adjacent grabens and the Jade Horst are rich in dinoflagellate cysts and possess <20% terrestrial palynomorphs. Samples from the Judy Horst contain a wide range of terrestrial palynomorphs, dominated by lycopsid, fern and moss spores, representing c.50% of the recovered palynomorphs. Correspondence analysis of the assemblages imply Jade did not possess a terrestrial ecosystem; Judy samples define seven groupings of related miospores,

3 Authors presente address: Instituto de Geociências, Universidade Federal do Rio Grande do Sul, Porto Alegre, RS 91501-970, Brazil

ACCEPTED MANUSCRIPT all interpreted to represent very low lying, relatively early successional type environments. This implies subaerial exposure of the Judy Horst during the Late Jurassic, which is

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interpreted to have formed an isolated, low relief, footwall crest island. This study provides

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a new methodology for investigating rift topography, particularly in cases where the tops of horsts were subsequently removed by erosion. The Judy Island would have separated the

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Central Graben into its eastern and western arms earlier than previously predicted, in the

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Late Oxfordian, with consequences for distribution of shallow and deep-marine reservoir

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quality sediments.

Keywords: palynology; horst palaeoecology; Upper Jurassic; Central North Sea; source-to-

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Introduction

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1

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sink

The extent of footwall uplift during major extensional phases is a well-studied and important factor in rift basin development (Barr, 1987; Leeder and Gawthorpe, 1987; Leeder et al., 1991; Collier et al., 1992; Roberts et al., 1993). Developing syn-rift topography is a major control on the type, source, and distribution of sediment accumulation in basins (Leeder and Gawthorpe, 1987; Surlyk, 1989; Prosser, 1993; Gawthorpe et al., 1994; Gawthorpe et al., 1997; Ravnås and Steel, 1997; Leeder et al., 1998; Ravnås and Steel, 1998; Gupta et al., 1999; Gawthorpe and Leeder, 2000; Cowie et al., 2006; Leppard and Gawthorpe, 2006; Sømme et al., 2009; Jackson et al., 2011; Leeder, 2011; McArthur et al., 2013; Lewis et al., 2015; McArthur et al., 2016). During Late Jurassic rifting of the North Sea, extensional graben and horst structures developed across the Central Graben, with highly variable associated footwall uplift effecting sediment 2

ACCEPTED MANUSCRIPT supply and pathways (Rattey and Hayward, 1993; Erratt et al., 1999; Fraser et al., 2003; Jackson et al., 2011; McArthur et al., 2016). Several studies indicate that Late Jurassic

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uplift may have been sufficient to project the crests of the horst blocks out of the proto-

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North Sea (Yielding et al., 1992; Berger and Roberts, 1999; Nøttvedt et al., 2000), despite relatively high sea-level during the Kimmeridgian (Haq et al., 1987). However, to date

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there has been no practical way to determine the extent of horst exposure.

palaeoecological

reconstructions

of

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Palynological studies have commonly been employed for palaeoenvironmental and ancient

systems

(e.g.

Potonié,

1967;

van

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Konijnenburgh-van Cittert, 1971; Filatoff, 1975; Heusser, 1979; Suc, 1984; Raine et al., 1988; van der Kaars, 1991; Boulter and Windle, 1993; Balme, 1995; Tyson, 1995; van der

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Kaars and Dam, 1995; Batten, 1996; Hubbard and Boulter, 1997; Abbink, 1998; Abbink et

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al., 2001; Twitchett et al., 2001; Crouch and Visscher, 2003; Shang and Zavada, 2003;

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Abbink et al., 2004; Moss et al., 2005; Traverse, 2007; Umetsu and Sato, 2007; Gary et al., 2009; Schrank, 2010; Wang and Zhang, 2010; Césari and Colombi, 2011; Daly et al., 2011; Stukins et al., 2013; Slater and Wellman, 2015; Zhang et al., 2015; Lindström et al., 2016). Palynology, which is often representative of locally produced, but not in-situ material, is particularly important for the study of palaeo-highs, the tops of which were subsequently eroded, and therefore do not preserve any information regarding their palaeoenvironments, e.g. uplifted footwall highs. Here we hypothesise that the palynomorphs preserved in adjacent depocentres can be used to interpret the nature of the palaeo-highs. As such we provide a case study of how palynology can be integrated with other datasets to determine the extent of footwall uplift on ancient horst blocks. This study aims to integrate a subsurface dataset of seismic reflection data, wireline well logs, sedimentological core descriptions and palynology to describe the nature of 3

ACCEPTED MANUSCRIPT ancient horsts, the palaeo-crests of which are not preserved. In particular we aim to 1) determine if evidence for relative elevation of a horst can be determined from palynological

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assemblages in sediments adjacent to a horst; 2) use the dataset to reconstruct the

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palaeoenvironments and palaeoecologies of the Josephine Ridge in the Late Jurassic; 3) consider the implications of footwall islands for source-to-sink modelling of rift basins. As

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such, analysis of Upper Jurassic marine sediments adjacent to the Josephine Ridge has been

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conducted, from Quadrant 30 of the UKCS, in the Central North Sea (Fig. 1), where understanding reservoir distribution is crucial for exploration strategies in a mature

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hydrocarbon province. Despite evidence for extensive footwall uplift (Keller et al., 2005), Zechstein salt halokinesis and post-Jurassic fault movements render attempts to determine

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exact amounts of Late Jurassic uplift extremely complex and inaccurate in this area, with

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previous studies implying the ridge was submerged throughout the Jurassic rift phase

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(Zeigler, 1990; Rattey and Hayward, 1993; Erratt et al., 1999; Jeremiah and Nicholson, 1999; Fraser et al., 2003; Sansom, 2010).

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Geological setting

The NW-SE trending Central Graben (Fig. 1) has experienced several failed phases of rifting (Coward et al., 2003). Continental rifting in the Permian to Triassic resulted in deposition of sediments, including the aeolian Rotliegend Group and Zechstein Salt Supergroup (Glennie et al., 2003), followed by the fluvial and lacustrine Triassic Skagerrak Formation (Goldsmith et al., 1995; Lines and Auld, 2004; Jones et al., 2005; Keller et al., 2005). Distribution of Skagerrak sediments was controlled by the initial stage of halokinesis and salt withdrawal (Goldsmith et al., 2003). Early to Middle Jurassic thermal doming in the North Sea, centred to the north of the Josephine Ridge, resulted in substantial 4

ACCEPTED MANUSCRIPT erosion of Lower Jurassic and Triassic sediment from the study area (Fig. 1; Underhill and Partington, 1993; Coward et al., 2003). The resulting erosion removed strata down to the

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Middle Triassic Anisian succession in the Judy area, but only to Upper Triassic Carnian age

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deposits on the Jade structure (McArthur et al., 2016). Subsidence resumed in the Bajocian resulting in deposition of the Pentland Formation, which was succeeded by the intercalated

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shallow marine Fulmar Formation and open marine Heather Mudstone in the Oxfordian

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(Fraser et al., 2003; McArthur et al., 2016).

The second stage of North Sea rifting occurred during the Late Jurassic, and resulted in

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the formation of horst and graben rift structures in the Central Graben (Rattey and Hayward, 1993). Significant rotation and uplift of fault blocks was accommodated by

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substantial subsidence of grabens and salt movement (Coward et al., 2003). These grabens

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were infilled with syn-rift stratigraphy, which exhibit growth adjacent to major faults

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(McArthur et al., 2016). Rifting, combined with global sea-level rise in the Kimmeridgian (Hallam, 2001) resulted in deposition of the Kimmeridge Clay Formation in the graben areas, whilst highs such as the Jade and Judy horsts saw further significant erosion (McArthur et al., 2016).

The Jade and Judy horsts are situated on the Josephine ridge (Fig. 1), a south-easterly extension of the Forties Montrose High, which separates the east and west sub-basins of the Central Graben (Fig. 1). The Josephine ridge comprises a complex series of NW plunging horst blocks, formed during the Late Jurassic rifting (Keller et al., 2005). Faults bounding the ridge primarily trend NW–SE, but are offset in places by NE–SW striking faults (Fig. 2 and 3). Horsts are bounded by a series of high angle normal faults with throws in excess of 600 m (Fig. 2 and 3), which affect the pre-Cretaceous strata.

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ACCEPTED MANUSCRIPT The dominant feature on the Josephine Ridge is the Judy Horst, which is located in the northern portion of the ridge, in UKCS Block 30/7a (Fig. 1 and 2). The Judy Horst is a

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NW-plunging structure, with a complex fault array (Fig. 2) approximately 35 km long by

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10 km at its widest point. Major NNW–SSE and WNW–ESE trending Jurassic extensional faults bound the structure, with throws exceeding 600 m, whilst the horst is also divided by

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WNW and NNW trending faults (Fig. 2). The Judy Horst is composed of deeply eroded

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Triassic sediment, capped by the Joanne Sandstone Member (Fraser et al., 2003). Erosion during the Late Jurassic resulted in deposition of a fringe of coarse sediments and mass-

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transport deposits extending < 1 km into the adjacent graben (Fig. 2; McArthur et al., 2016); these are interbedded with finer sediments of the Heather and Kimmeridge

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mudstones (Fig. 4). Wireline log signatures (Fig. 4) and core logging identified a c.142 m

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section of aberrant Triassic strata sitting within Jurassic marine mudstone in well 30/7a-P1,

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which is 600 m from the horst (Fig. 4). This is interpreted to represent an olistolith, reworked into the syn-rift stratigraphy (McArthur et al., 2016). Moving away from the horst a series of conglomerates and coarse sandstones are interbedded with mudstones in well 30/7a-8, located 800 m from the horst (Fig. 4). These are interpreted to represent minor hangingwall fans prograding from the horst (McArthur et al., 2016). Only mudstone is present in the Upper Jurassic section 1100 m away from the horst in well 30/7a-P7 (Fig. 4). Therefore, the wells 30/7a-P1, 30/7a-8 and 30/7a-P7 demonstrate a proximal to distal transect away from the Judy Horst (Fig. 4). The Jade Horst is situated on the northern point of the Josephine Ridge (Fig. 1), comprising a narrow structure 3.25 km long by 1 km wide (Fig. 3). Faults bounding the Jade structure trend NW-SE but are offset by a large NE-SW trending faults with throws exceeding 400 m (Fig. 3). The Jade Horst is grounded on Permian Rotliegend deposits due 6

ACCEPTED MANUSCRIPT to Jurassic extension and salt withdrawal (Jones et al., 2005). As rifting progressed, evacuation of the remaining salt from the flanks permitted relative subsidence of the

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northeast and southwest compartments, upon which marine mudstones onlapped the

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structure. The horst is capped by a thin section of Triassic Jonathan Mudstone overlying older strata (Fig. 3). The adjacent grabens were infilled with marine mudstones (Fig. 5),

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interpreted to represent the open marine Heather and deep-marine Kimmeridge Clay

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formations (Fig. 5). Rotation of the Jade Horst resulted in uplift of the northwest section (Fig. 3) of the horst and crestal collapse generating an olistolith composed of Triassic strata,

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which rests upon Upper Jurassic mudstone to the east of Jade (Fig. 3; McArthur et al., 2016).

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Basinal wells 30/2c-J9 and 30/13-4 demonstrate high gamma ray sections (Fig. 5),

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hundreds of meters thick, interpreted as marine mudstones, interspersed with low gamma

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ray, blocky, sandstone units interpreted as turbidites (McArthur et al., 2016). These were analysed and sampled for control sections in the deeper areas of the adjacent grabens, where much thicker Jurassic intervals are preserved, when compared to the sections proximal to the horsts (Fig. 5).

In the Late Jurassic the study area lay at a palaeolatitude of 37o N (Hudson and Trewin, 2002) with a warm climate; winter temperatures were approximately 5 - 15o C and summer temperatures increased to 15 – 30o C (Rees et al., 2000). The nearest continental shelf was located at least 80 km to the SE (Fraser et al., 2003), represented by thick deltaic and paralic sediments (Fig. 1). On the opposite side of the Central Graben, a broad shelf also developed, represented by the shallow marine Ula Formation (Stewart, 1993; Fraser et al., 2003). The nearest substantial upland areas were over 100 km to the west on the Scottish landmass (Bradshaw et al., 1992). 7

ACCEPTED MANUSCRIPT Previous palynological studies on Upper Jurassic sediments of the Central North Sea have focused on the biostratigraphic applications of dinoflagellate cysts (e.g. Partington

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et al., 1993) as with onshore sections of the Kimmeridge Clay (Riding and Thomas, 1988;

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Thomas, 1988). More detailed studies of Upper Jurassic palynomorphs have been conducted elsewhere in the North Sea rift system, e.g. in the Viking Graben (van der Zwan,

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1990). Most relevant are studies from the southern North Sea by Abbink (1998) and

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Abbink et al. (2001; 2004), who identified broad low-lying regions 300 km to the south of

Kimmeridgian (Abbink et al., 2001).

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our study area, with a diverse flora, implying a warm subtropical palaeoclimate in the Early

Continued transgression in the Tithonian led to a progressive drowning of the Central

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Graben and rifting of the Central Graben ceased in the Early Cretaceous, with thermal

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subsidence continuing throughout the Cretaceous and the Cenozoic (Zanella and Coward,

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2003; Keller et al., 2005). The Base Cretaceous Unconformity separates the Upper Jurassic from Cretaceous deposits (Kyrkjebø et al., 2004), which represent post-rift strata (Fig. 1). The Lower Cretaceous Valhall Formation onlapped residual structural highs and the Upper Cretaceous Chalk draped the region before deposition of Cenozoic strata, with the relevant Upper Jurassic intervals now being at depths in excess of 3 km. This already structurally complex region was further distorted by the post-Jurassic emplacement of the Joanne salt pillow on the western margin of the Josephine Ridge, (Lines and Auld, 2004), with final movement occurring in the Oligo-Miocene (Ashton et al., 1998).

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ACCEPTED MANUSCRIPT 3 3.1

Material and Methods Subsurface dataset

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Seismic reflection data over the Josephine Ridge, wireline log data from six wells,

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core from four wells and one hundred and five palynological samples comprise the study material. Seismic lines are depth converted, with depth given in True Vertical Depth

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(TVD), and were interpreted using Landmark´s Seisworks software. Interpretations were

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primarily made to understand the relationship of syn-rift strata in adjacent grabens to the Jade and Judy horsts (Fig. 2 and 3). Wireline well-logs from six wells (30/2c-J7; 30/7a-8;

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30/7a-P1; 30/7a-P7; 30/2c-J9: 30/13-4) adjacent to the Jade and Judy horsts (Fig. 2 and 3) were interpreted based on standard log response of gamma ray, resistivity, acoustic

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impedance, bulk density and neutron porosity (Rider, 1996). Cores from wells 30/2c-J7;

3.2

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sediments.

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30/7a-8; 30/7a-P1 and 30/13-4 were used to examine the lithofacies of the syn-rift

Palynological sampling, processing and analysis One hundred and five samples were collected from the Upper Jurassic strata from

cuttings and cored intervals (Fig. 4 and Fig. 5). Samples were taken from the base of the Upper Jurassic interval to the Base Cretaceous Unconformity, which truncates all sections (Fig. 4 and 5). Sampling was restricted to mudstone intervals to avoid lithological variation, although sandstone and conglomerate deposits adjacent to the Judy Horst contain macroscopic plant fragments. Samples were processed in a standard fashion after Wood et al. (1996), with the addition of Schultze reagent to remove excess amorphous matter and a minimum of 200 palynomorphs were counted per slide using transmitted light microscopy. In common with Kimmeridgian assemblages of the North Sea, the palynomorph preservation and quality is typically poor to moderate, particularly given the depths the 9

ACCEPTED MANUSCRIPT samples were recovered from (all >3 km). Nonetheless, most samples provided palynomorphs, with only 10 barren samples. The impact of transportation and preservation

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of the palynomorphs in this marine setting is not considered to be a factor; palynomorph

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assemblages from proximal marine settings are considered to directly reflect changes in the local terrestrial vegetation (Heusser, 1979; Tyson, 1995; Batten, 1996; Traverse, 2007).

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Empirical studies demonstrate that relative abundances of miospores are related to the

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distribution of plant communities (Suc, 1984; van der Kaars, 1991; van der Kaars and Dam, 1995; Abbink et al., 2001; Twitchett et al., 2001; Crouch and Visscher, 2003; Abbink et al.,

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2004; Moss et al., 2005; Umetsu and Sato, 2007).

Re-worked Triassic and caved Cenozoic taxa were commonly found in the cuttings

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samples. However, these were identified and not included in the count of palynomorphs.

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Triassic spores are of a higher thermal alteration index due to their pre-Late Jurassic burial

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and for the most part can be distinguished by their archaic forms, whereas Cenozoic taxa can be identified from their pale colouration and younger, dominantly angiosperm forms.

3.3

Correspondence analysis Correspondence analysis is a metric ordination technique, based upon reciprocal

averaging of eigenvalues (Hill, 1973), and is used for displaying multi-dimensional data on two axes that have the most influence on dataset variation. Eigenvalues are given for each sample, corresponding to their degree of variation along an axis, therefore providing a measure of a given axis in terms of its importance for ecological variation (Kovach, 1993). The MVSP program (Kovach, 2002) was used to run the analysis, the output of which is tables of eigenvalues demonstrating the variation of each axis, which can be plotted as cross-plot of the two axes exhibiting the largest variance. From these cross-plots, clusters of 10

ACCEPTED MANUSCRIPT palynomorph taxa can be identified, allowing ecological trends to be investigated. Correspondence analysis was chosen over other multivariate statistical analysis, given its

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reputation with ecological and palaeoecological datasets (Austin, 2005), particularly in

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palynological based palaeoecological studies (Courtinat and David, 1984; Kovach and Batten, 1994; Jolley and Whitham, 2004; Jolley et al., 2005; Daly et al., 2011; Stukins et

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4.1

Results

Palynoflora assemblages and variations in biodiversity

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4

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al., 2013).

The palynomorph association of the Josephine Ridge bears many similarities to those

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from the Upper Jurassic deposits of NW Europe (Riding and Thomas, 1988; Tyson, 1989;

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Abbink, 1998), containing many long ranging Mesozoic taxa (e.g. Cycadopites follicularis,

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Gleicheniidites spp. and Lycopodiumsporites spp.). Samples are dominated by dinoflagellate cysts (Fig. 4 and 5), with varying representation by trilete spores, pollen grains and acritarchs, totalling representatives of 101 genera; very few alete pollen grains were observed. The botanical, ecological and climatic affinities for the common miospores are described in Table 1.

Dinoflagellate cysts were used to provide a biostratigraphic framework for the sections (Fig. 4 and 5). The ratio of sporomorphs to dinoflagellate cysts is seen to vary markedly between wells (Fig. 4 and 5), with samples in the Judy area from 30/7a-P1 comprising up to 60% miospores, reducing with distance from the horst through wells 30/7a-8 and 30/7a-P7 (Fig. 2) to an average of 20% miospores (Fig. 4). Jade samples (30/2c-J7) display a more restricted terrestrial palynomorph association, with <10% of palynomorphs below the olistolith comprising terrestrial material, increasing slightly to 11

ACCEPTED MANUSCRIPT c.20% in samples above the olistolith (Fig. 5). Basinal wells 30/2c-J9 and 30/13-4 record low ratios of miospores with terrestrial material ranging from 1.4 to 19.5% (Fig. 5).

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Given the low numbers of miospores in the Jade and basinal samples, palynoflora

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analysis focused on samples from Judy and its miospore trends are highlighted in figure 6; for a detailed well by well breakdown of miospores refer to the supplementary material.

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Samples of uppermost Oxfordian age display an abundance of Lycopodiopsida

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spores (e.g. Lycopodiumsporites spp.) initially representing c.50% of the assemblage (Fig. 6); with lesser representation by pteridophytes (e.g. Deltoidospora spp.), bryophytes (e.g.

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Antulsporites spp.) and Equisetales spores (e.g. Calamospora spp.). The numbers of lycopsid and fern spores declines towards the Oxfordian - Kimmeridgian boundary, with

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increasing moss spores up to this boundary (Fig. 6). Pollen from Cycadales and

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Bennettitales (e.g. Cycadopites follicularis) are very rare, increasing to c.10% in the Upper

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Oxfordian rosenkrantzi Zone (Fig. 6). Moving into the Lower Kimmeridgian sections, the dominance of fern and lycopsid spores continued (Fig. 6). Relatively stable numbers of fern spores, approximating 30% of the association are observed in the Lower Kimmeridgian baylei – cymodoce deposits (Fig. 6), with high numbers of Todisporites spp. and Baculatisporites spp. during this period. The relative abundance of moss and Equisetales spores declined in the Lower Kimmeridgian deposits (Fig. 6), whilst pollen from Cycadales and Bennettitales were relatively stable. Numbers of gymnosperm type pollen are observed to have been consistently low, representing no more than 3% of the miospore association in any one interval in the Lower Kimmeridgian (Fig. 6). A change in the association is recorded from samples at the mutabilis – eudoxus boundary. Lycopsid spores increase their domination, gymnosperm type pollen attain a 12

ACCEPTED MANUSCRIPT maximum of 9% of the association, whilst bryophyte and Equisetales spores, and pollen from Cycadales and Bennettitales reduce in abundance (Fig. 6). The gymnosperms,

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Cycadales, and fern miospores all diminish in the Upper Kimmeridgian sections, and no

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pollen representative of Bennettitales was recorded in sections above the eudoxus Zone (Fig. 6). However, the number of spores representing the lycopsids and bryophytes are seen

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to increase, with a peak in moss spores in samples from the uppermost Kimmeridgian (Fig.

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6).

Lycopodiopsida spores peak in Lower Tithonian elegans Zone samples; fern and

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Equisetales spores are seen to make a slight recovery in numbers, whilst gymnosperm pollen becomes established at the top of the section, peaking in the scitulus Zone interval at

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22% of the association (Fig. 6). No Cycad pollen was recorded in the Tithonian sections

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6).

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and the abundance of moss spores continues to decline towards the top of the interval (Fig.

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ACCEPTED MANUSCRIPT 4.2

Ecological association of palynofloras Although detailed ecological interpretations based upon palynomorphs are fraught with

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uncertainties regarding Jurassic flora (Chaloner and McElwain, 1997), it is possible to

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interpret broad palaeoenvironmental patterns from the miospores (Table 1) by utilising the

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Jurassic sporomorph ecogroup model of Abbink et al. (2004). The majority of the miospores described from the Josephine Ridge have affinities with highly water dependant plants of coastal or lowland type environments (Table 1).

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The ecological interpretations from the sporomorph ecogroup model can be tested by

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multivariate statistical analysis of the palynomorph dataset. This analysis was conducted on wells proximal to the Jade and Judy horsts (Fig. 4 and 5), which record a statistically viable

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count of terrestrial miospores. Distal wells 30/2c-J9 and 30/13-4 did not record enough

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miospores to constitute valid statistical analysis. All samples from across the proximal well suite were normalised to a percentage, tested individually, and incorporated into master data

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sets for the Judy and Jade areas (Fig. 7). Species which represented less than 0.1% of the entire association were not included in this analysis; this included all pollen produced by

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upland forest type gymnosperms, e.g. Alisporites spp. and Cerebropollenites mesozoica. Given their very low abundances (overall <1%) it was not appropriate to include the saccate gymnosperms in this analysis of local palynomorphs, as their very low numbers strongly suggest they were not locally sourced. The parent plants which produce saccate pollen are known overproducers, and their pollen has the ability to be transported by wind over hundreds of kilometres (Vakhrameev, 1991). For the Judy assemblages (30/7a-8, 30/7a-P1 and 30/7a-P7) the data exhibit separation into seven principle groups (Fig. 7). Eigenvalues for the primary axes (Axis 1: 0.374 and Axis 2: 0.197) of the dataset show that axis one (the x-axis) is exerting the principle control on the distribution and is likely a function of moisture availability. This observation is an

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ACCEPTED MANUSCRIPT empirical observation, with miospores possessing the lowest values (left hand side), e.g. Exesipollenites sp., known to represent amongst the most drought resistant vegetation of the

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association (Abbink, 1998), whereas palynoflora that exhibit the highest values, e.g.

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Lycopodiumsporites sp. were produced by vegetation which required high levels of moisture

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(Vakhrameev, 1991). Axis two (y-axis) is exerting less control than axis one and is poorly defined, but appears to be a function of ecosystem disturbance. This interpretation is based on the observation that the palynomorphs associated with vegetation more tolerant of

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disturbance, hence those found in the most frequently disturbed environments have the lowest

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values, at the base of the chart (Fig. 7). Those species representing vegetation that requires a stable environment with the least disturbance have the highest values, found towards the top

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of the chart (Fig. 7). Confidence is high in the Judy ecological interpretation, as the

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components of the associations clearly relate to their botanical affinity, with miospores belonging to similar types of vegetation plotting together.

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All clusters define hygrophytic to hygro-mesophytic type vegetation groups, indicative of lowland and coastal ecologies (sensu Abbink et al., 2004). Clusters range through the most coastal

grouping,

containing

pollen

with

affinities

to

arborescent

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developed

Cheirolepidiaceae; to less well defined lagoonal and back marsh type groups with miospores from early colonizing type vegetation. Importantly, all groups represent relatively lowland vegetation (Abbink, 1998), and although some miospores have been related to plants with riverbank affinities, e.g. Deltoidospora sp. (Vakhrameev, 1991), based on the size of the Judy Horst (c.35 x 10 km), it is unlikely that any major river systems were developed, with only minor stream systems inferred. Variation in abundance of each ecogrouping is presented in figure 8, along with the case scores from the correspondence analysis; however groupings interpreted to represent mud flats and fern marsh ecologies dominate by volume of miospores (Fig. 8).

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ACCEPTED MANUSCRIPT Representing the second local high on the Josephine Ridge, samples from the Jade Horst were also complied for statistical investigation (Fig. 7). Eigenvalues were low for all

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axes, indicating no clear trends, and no groupings were identified (Fig. 7). The Jade

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assemblages appear to represent random groupings, with the distinct possibility of marine

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mixing generating these false groupings. Discussion

No Jurassic strata are preserved on top of the Jade and Judy horsts to determine their

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palaeoenvironment during the Jurassic; therefore suitable proxies must be identified to

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reconstruct the horsts. That the Josephine Ridge is surrounded by marine sediments may imply that the Ridge itself was submerged. However, attempts to calculate footwall uplift

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rates and therefore determine the elevation of the horsts, using traditional methods (e.g. Barr,

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1987; Yielding, 1990; Roberts et al., 1993) are severely hampered by the complex structural setting of the Josephine Ridge, which has seen post-Jurassic subsidence, Paleogene inversion

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and local salt movement (Fig. 2). These uncertainties render mathematical calculations of footwall uplift extremely unreliable. As such we may turn to the local palynomorph records

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to understand the Late Jurassic palaeoecological settings on the Josephine Ridge. Although samples are from marine settings, as defined by the dinoflagellate cyst rich mudstone from which samples were taken, the proximity to the horsts (Fig. 2 and 3); the fact that the proportion of marine to terrestrial material increases towards the Judy Horst (Fig. 4); and statistical investigations of the miospores (Fig. 7) implies that the palynomorphs recovered represent true palaeoecosystems. 5.1

Josephine Ridge Palaeoecology The use of pollen as an indicator of uplift is limited (e.g. Song et al., 2010), however if

it is accepted that the majority of the recovered palynomorphs represent local material, then palaeoecological interpretations may be inferred. The associations recovered from the Jade

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ACCEPTED MANUSCRIPT and Judy horsts display some similarities, particularly the diverse range of dinoflagellate cysts and confirm that the adjacent grabens were fully marine. However, it is the terrestrial

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miospore assemblages that are most valuable in determining the palaeoecology of the horsts.

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5.1.1 Jade and basinal grabens

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Examination of the Jade samples reveals a very restricted miospore association, representing <20% of the total palynomorphs, similar to the assemblages in grabens either side of the Josephine Ridge (Fig. 5). This lack of miospores could be a factor of poor

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preservation; however the samples contain abundant dinoflagellate cysts. Statistical analysis

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of the miospores recovered from the Jade region indicates that these palynomorphs do not represent true ecological associations (Fig. 7). Silt to clay sized miospores have the potential,

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in limited numbers, to be transported hundreds of kilometres (Traverse, 2007), hence the

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terrestrial palynomorphs observed in the Jade region. Therefore, it is implied that the Jade Horst was not subaerially exposed during the Late Oxfordian or earliest Kimmeridgian, with

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the few occurrences of pollen and spores being transported in from a distal setting. The abundant dinoflagellate cysts in Jade and basinal samples indicate these areas represented an

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open marine environment. 5.1.2 Judy

Samples from the Judy area present a different picture, with a diverse range of miospores, which are seen to increase in abundance towards the Judy Horst, to a maximum of 60% of the palynomorph association (Fig. 4). This is interpreted to represent the fact that lower volumes of miospores will be transported to more distal marine settings. The impact of transportation and preservation of the palynomorphs in this marginal marine setting is not considered to be a factor and statistical investigation of the Judy miospore association (Fig. 7) implies that a subaerial ecosystem existed on the Judy Horst during the Late Jurassic (Fig. 9).

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ACCEPTED MANUSCRIPT The presence of this terrestrial vegetation indicates that footwall uplift of the Judy Horst was sufficient to maintain subaerial conditions during the Late Jurassic. The presence

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of Oxfordian Fulmar Sandstone around the Judy Horst (Fig. 4) implies that initial rift

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subsidence may not have significantly lowered the Judy Horst, with the possibility that the

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horst formed an intra-basinal high at the onset of Jurassic rifting. The ecological analysis of the palynomorphs identified a range of palaeoenvironments (Fig. 7), which implies a wet, low-lying setting for the Judy Horst (Fig. 9), which certainly never attained significant

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elevation, as indicated by the lack of any upland type ecogroups (Fig. 7). Bisaccate pollen

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grains, which are typical of pollen produced by upland forest gymnosperms (Abbink et al., 2004), represent <1% of the encountered palynomorphs and are not interpreted to represent

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locally produced material.

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Variations in the abundance of the Judy Island ecogroups through time were observed (Fig. 8). In particular, the Oxfordian samples show increasing abundances of the coastal and

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fern marsh groupings and diminishing mud flats. Conversely, Kimmeridgian samples show declining coastal and marsh groupings, and a peak in the mud flats ecogroup (Fig. 8). The

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change in dominance from relatively stable coastal and marsh groupings to the disturbance resistant mud flats was contemporaneous with the change in rift style, from the initial stage to the rift climax stage recognised by McArthur et al. (2016). This is also recorded in the correspondence analysis case scores, which show a decline in axis 2 scores during the rift climax stage (Fig. 8), being a further indicator that vegetation susceptible to disturbance was reduced during this time. The transition from the initial stage of rifting to rift climax implies that major faults had broken to surface and were producing sufficient topography to induce gravitational collapse of the horst. Therefore, the observed ecological variations may be related to the degree of tectonic activity on the island, variably creating or destroying habitats.

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ACCEPTED MANUSCRIPT Although not a precise means of determining the exact elevation, the ability to determine whether rift structures remained submerged, as opposed to being elevated above

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sea-level and thus acting as barriers to sediment distribution is clearly of interest for source-

Implications of horst exposure

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5.2

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to-sink modelling of a basin.

Subaerial exposure of the Judy Horst has implications for the Late Jurassic development of the rift structure and strata of the Central Graben. This is the first report of

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fault block rotation and subsequent footwall uplift forming an island on the Josephine Ridge

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during Late Jurassic rifting. Isolated Upper Jurassic islands have been reported close to the western margin of the Central Graben (Howell et al., 1996), but the only examples in distal

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areas of the North Sea are from the Viking Graben, where exposure was implied from

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reworking of footwall strata (e.g. Barr, 1987; Badley et al., 1988; Yielding, 1990; Roberts et al., 1993; Ravnås and Steel, 1998; Berger and Roberts, 1999; Nøttvedt et al., 2000).

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However, shallow marine reworking may also occur (Nøttvedt et al., 2000). The lack of Upper Jurassic strata on top of the Jade and Judy fault blocks implies

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footwall uplift outpaced subsidence; however, this is not direct evidence of subaerial exposure. On all but the largest fault blocks with rapid footwall uplift, e.g. the Snorre Fault Block, which is inferred to have become subaerial during the Late Jurassic (Yielding et al., 1992; Nøttvedt et al., 2000; Welbon et al., 2007), erosion rates would have outstripped uplift and therefore islands would not have formed (Yielding et al., 1992). This was the case for many of the relatively smaller fault blocks in the South Viking Graben such as the Brent, Ninian and Statfjord tilted fault blocks, interpreted by Yielding et al. (1992) to have remained submerged, and the majority of Central Graben fault blocks (Howell et al., 1996). In the majority of these cases, fault block rotation and uplift was insufficient to project the crests above sea-level, as is the model proposed for the Jade Horst. Fault blocks with limited

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ACCEPTED MANUSCRIPT rotation, typically <9o cumulative tilt (Nøttvedt et al., 2000) would have been more susceptible to sea-level masking the effects of footwall uplift, particularly in the Late Jurassic

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where sea-levels remained high (Haq et al., 1987), and horsts would likely have remained

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submerged.

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That the Judy Horst is considered to have formed an intra-basinal high at the onset of rifting has implications for source to sink modelling of the Central Graben during the Late Jurassic. Previous models have indicated that the Josephine Ridge area was submerged

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during the Oxfordian and Kimmeridgian (Zeigler, 1990; Rattey and Hayward, 1993; Jeremiah

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and Nicholson, 1999; Fraser et al., 2003), with modelling of Upper Oxfordian submarine fan systems prograding from the Fulmar shelf and interpreted to cross-cut the Josephine Ridge

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(Jeremiah and Nicholson, 1999; Fraser et al., 2003). If, as we propose, the Judy Horst was

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subaerially exposed, forming a topographic high, then it appears unlikely that sediment would have been able to cross either arm of the Central Graben, to be deposited on the opposing side

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of the Josephine Ridge. Instead, at least two distinct sediment supply systems may have developed and onlapped the Josephine Ridge, with sediment derived from the Fulmar shelf

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being deposited into the western limb of the Central Graben (McArthur et al., 2016). Smaller structures such as the Jade Horst may have diverted or focused submarine gravity flows through developing rift topography (e.g. Jackson et al., 2011). Although subaerial exposure of the Judy Horst may identify it as a source of reservoir quality sediment, the estimated size of the Judy Island at c.350 km2 would have been insufficient to source large scale sedimentary systems with reservoir potential. Even had the entire Josephine Ridge become exposed (c.2000 km2), this would still have been insufficient to source reservoirs located in the adjacent grabens and other sources must be considered. Drainage systems on equivalent modern-day footwalls produce only localised scree slopes

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ACCEPTED MANUSCRIPT and small rivers resulting in restricted deltas, inadequate to feed reservoir scale systems (e.g.

Conclusions

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6

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Leeder et al., 1991).

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An integrated geophysical, palynological, petrophysical and sedimentological study of Upper Jurassic marine sediments, utilising seismic data and samples from six wells in the Josephine Ridge region of the Central North Sea was conducted for analysis of rift

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topography. Geophysical, petrophysical and sedimentological interpretations show wells to

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be placed in grabens either side of the Jade and Judy horsts, the crests of which are not preserved. One hundred and five samples were taken from mudstones that onlap the horsts,

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spanning the Upper Oxfordian to the Lower Tithonian, deposited during the syn-rift period of

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the Central Graben.

A wide range of terrestrial palynomorphs were examined, being particularly abundant in

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wells adjacent to the Judy Horst, though the percentage of terrestrial to marine palynomorphs decreased from 60% adjacent to the horst to <20% at 1 km distance. Samples bounding the

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Jade Horst were even more restricted, with <20% terrestrial palynomorphs throughout. Samples are dominated by lycopsid, fern and moss spores, with very few alete pollen grains recorded.

Correspondence analysis of the Jade and Judy horsts miospore assemblages implies the Jade Horst did not possess a terrestrial ecosystem in the Late Jurassic. Judy samples identified seven groupings, all representative of low lying, relatively early successional environments. These ecological groupings indicate that the Judy Horst was subaerially exposed in the Late Jurassic. The Judy Horst is envisaged to have formed an isolated footwall island and never achieved significant elevations, or delivered significant volumes of sediment to adjacent grabens.

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ACCEPTED MANUSCRIPT Subaerial exposure of the Judy Horst has implications for the Upper Jurassic Central Graben rift structure and stratal development. The Judy Island, present from the Late

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Oxfordian onwards, would have separated the fledgling Central Graben into its eastern and

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western limbs earlier than previously predicted. This has consequences for sediment source

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and distribution within the Central Graben, as reservoir sandstone deposition would have been isolated to either the eastern or western limbs and not able to cross the entire width of

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the graben.

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7. Acknowledgements

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We thank ConocoPhillips (U.K.) Limited for providing the study data and financial support, along with co-ventures in the study area BG Group, ENI UK Ltd, Chevron and OMV UK Ltd for permission to release data. Nick Schofield, Ben Kilhams and Rob Daly are thanked for discussions, and Fiona Thompson for laboratory assistance. Two anonymous reviewers and Editor David Bottjer are thanked for their comments.

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ACCEPTED MANUSCRIPT Figure captions

Fig. 1. A) Structural trends of the Central Graben with location map of the study area, and

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inset showing the Jade and Judy fields with seismic lines a-a´ for figure 2 and b-b´ for figure

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3, after Jones et al. (2005). B) Schematic stratigraphic column of the Triassic to Cretaceous

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sediments of the Josephine Ridge region, highlighting formations encountered in this study.

Fig. 2. Structural map for depth to Base Cretaceous Unconformity (BCU) over the Judy Horst after Keller et al. (2005) and seismic cross section A-A’ of the Judy Horst and southern

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terrace, within which interpreted syn-rift Jurassic strata are highlighted red and the Judy

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olistolith is highlighted blue. Seismic data courtesy of CGG Veritas.

Fig. 3. Structural map for depth to BCU for Jade Horst, after Jones et al. (2005) and seismic cross section B-B’ of the Jade Horst and eastern terrace, within which the syn-rift Jurassic

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strata are highlighted red and the Jade olistolith is highlighted blue. Seismic data courtesy of

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CGG Veritas.

Fig. 4. Well logs of the studied Upper Jurassic intervals adjacent to the Judy Horst hung on

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the Base Cretaceous Unconformity, with percentage of dinoflagellate cysts and pollen per sample, and the biostratigraphic zonation of the wells.

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Fig. 5. Well log of the studied Upper Jurassic interval adjacent to the Jade Horst (30/2c-J7) and basinal wells, with percentage of dinoflagellate cysts and pollen per sample, and the biostratigraphic zonation of the wells. Note 30/13-4 and 30/2c-J9 are at a different scale to accommodate these much thicker basinal Upper Jurassic sections.

Fig. 6. Proportion of miospores per biozone collated from 30 7a-8, 30/7a-P1 and 30/7a-P7, arranged by their botanical affinity. Please refer to the supplementary material for a detailed breakdown of miospores recovered from each well.

Fig. 7. A) Correspondence analysis cross-plot of the Judy (30/7a-7a-8, 30/7a-P1 and 30/7aP7) palynoflora dataset. B) Correspondence analysis cross-plot of the Jade (30/2c-J7) palynoflora dataset.

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ACCEPTED MANUSCRIPT Fig. 8. Judy Horst ecological groupings proportions. Rift stages after McArthur et al. (2016) are reflected by changes in groups susceptible to disturbance. Taphonomic influence is

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negated by the plotting of correspondence analysis case scores.

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Fig. 9. Palaeoenvironmental reconstruction of Judy Horst terrestrial ecologies based upon

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Figure 8

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ACCEPTED MANUSCRIPT Table 1. Common Upper Jurassic miospores from the Josephine Ridge area and their ecological implications. Miospore

Botanical affinity

Environment

Climatic

Authority

inference Pteridosperms

mangrove?

warm & dry

Balme 1995

Antulsporites spp.

Bryophytes

bogs, marshes

wet

Hubbard & Boulter 1997

Araucariacites spp.

Pinales, Araucariaceae

dry coastal

cool & dry

lowland

Calamospora mesozoica

Equisetales

riverbanks

wet & warm

Balme 1995

wet & warm &

Hubbard & Boulter 1997

Pinales, Araucariaceae

dry coastal

cool & dry

Boulter & Windle 1993

Camarozonosporites spp.

Lycopodiopsida

diverse but wet

wet

Hubbard & Boulter 1997

Cibotiumspora spp.

Pteridophyta, Pteridaceae

lowland

wet & warm

Wang & Zhang 2010

Classopollis spp.

Pinales, Cheirolepidiaceae

dry coastal

warm & dry

Balme 1995

Cycadopites follicularis

Cycadale

riverbanks

warm

Filatoff 1975

Deltoidospora spp.

Pteridophyta, Cyatheaceae

lowland

wet & warm

Abbink 1998

Densoisporites spp.

Lycopodiopsida,

coastal

wet

Raine et al., 1988

lowland

wet & warm

Balme 1995

Dictyophyllidites spp.

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Selaginellales

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Callialasporites spp.

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marshes

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Pteridophyta, Osmundaceae

van Konijnenburgh- van Cittert 1971

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Baculatisporites spp.

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Alisporites thomasii

Pteridophyta, Dipteridaceae

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/ Matoniaceae Bennettitales

coastal

warm

Abbink 1998

Foveosporites spp.

Lycopodiopsida,

coastal

wet

Balme 1995

lowland

wet & warm

Harris 1981

lowland

wet & warm

Hubbard & Boulter 1997

lowland

wet & warm

Hubbard & Boulter 1997

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Exesipollenites tumulus

Selaginellales

Gleicheniidites spp.

Pteridophyta,

Gleicheniaceae Pteridophyta, Schizaeaceae

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Klukisporites spp. Laevigatosporites

Pteridophyta

vulgaris

Polypodiaceae

Leptolepidites spp.

Lycopodiopsida

diverse but wet

wet

Abbink 1998

Lycopodiumsporites spp.

Lycopodiopsida

diverse but wet

wet

Hubbard & Boulter 1997

Monosulcites spp.

Bennettitales &

riverbanks

warm

Boulter & Windle 1993

Ginkgoaceae Neoraistrickia spp.

Lycopodiopsida

diverse but wet

wet

Hubbard & Boulter 1997

Staplinisporites spp.

Lycopodiopsida

diverse but wet

wet

Hubbard & Boulter 1997

Stereisporites spp.

Bryophytes

bogs, marshes

wet

Filatoff 1975

Todisporites spp.

Pteridophyta, Osmundaceae

lowland

wet & warm

Abbink 1998

Verrucosisporites spp.

Pteridophyta, Osmundaceae

lowland

wet & warm

Balme 1995

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Highlights

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Integrated palynological, geophysical, petrophysical and sedimentological study of the Upper Jurassic section of the Josephine Ridge area of the Central Graben, UK Continental Shelf. Palynomorphs were used to investigate the Late Jurassic palaeoecologies of the Jade and Judy horsts, the tops of which are not preserved. Samples from mudstones adjacent to the Jade Horst are rich in dinoflagellate cysts; samples adjacent to the Judy Horst show a rich assemblage of terrestrial miospores, which decline in abundance moving away from the high. Ecological investigations reveal the Judy Horst was subaerially exposed during the Late Jurassic, forming a low lying footwall island. This provides a new methodology for investigating tectonically induced topography, where palaeo-highs are not preserved. Implications for sediment source and distribution in syn-rift basins are presented.

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