The data for the present study were derived from four fully cored boreholes (labelled Dh1 to Dh4, Fig. 1C), drilled near the central Spitsbergen town of Longyearbyen under a UNIS project (Braathen et al. 2012), and from a corresponding road-cut outcrop section, about 2.5 km long, between the town and its airport (Fig. 1C). The well cores show the lower to middle part of the Dalkjegla Member of the Carolinefjellet Formation (Fig. 1D), whereas the outcrop shows the member’s middle to upper part. The cores were studied by detailed sedimentological logging (scale 1:20), with a special focus on biogenic features. Documentation included photographs. The core samples, stored at the UNIS in Longyearbyen, were non-oriented, hence palaeocurrent directions could only be measured in the outcrop section.
The descriptive sedimentological terminology is after Folk (1980), Harms et al. (1982) and Collinson et al. (2006). The term lithofacies refers to the basic types of sedimentary deposits distinguished on the macroscopic basis of their bulk characteristics (Harms et al. 1975; Walker 1984a). The term facies association denotes an assemblage of spatially and genetically related lithofacies, thought to represent a particular zone (sub-environment) of the depositional system. The distinction of shoreface, offshore transition and offshore zones is based on sedimentary facies and pertains to the prevalent depths of fairweather and storm wave bases (Reading and Collinson 1996). The simplified four-grade scale for sediment bioturbation intensity used in summary figures refers to the bioturbation index (BI) of Taylor and Goldring (1993) as follows: no bioturbation (BI = 0), low (BI = 1), moderate (BI = 2 to 3) and high bioturbation (BI = 4 to 6).
Sedimentary Facies
The following lithofacies have been recognized as the basic building blocks of the sedimentary succession in the present study:
C – basal conglomerate, massive to diffusely planar stratified;
Sp – sandstone with planar parallel stratification;
Shs – sandstone with hummocky and/or swaley stratification;
Sr – sandstone with wave and/or combined-flow ripple cross-lamination;
Sm – massive mottled sandstone homogenized by bioturbation;
H – heterolithic facies of thinly interlayered sandstone and mudshale;
Hm – massive silty to sandy mudstone or muddy sandstone homogenized by bioturbation;
Ml – planar parallel laminated silty mudshale;
Mm – homogenous (non-laminated) mudshale.
The sedimentary facies are briefly described and genetically interpreted in Table 1, with an example well-core log in Fig. 3 and lithofacies shown in Figs. 4–6. For a more detailed depiction of lithofacies, see Grundvåg et al. (2021).
Table 1
Lithofacies of the Dalkjegla Member of the Carolinefjellet Fortmation in the study area
| Facies | Description | Interpretation |
| C | Conglomerate — Poorly sorted, massive to diffusely plane-parallel stratified, granule to fine-pebble conglomerate composed of subrounded to well-rounded clasts, forming a laterally discontinuous solitary layer ≤ 10 cm thick at the formation base. These broad basal conglomerate patches have a sharp, erosional base and sharp top, show weak normal grading. The conglomerate is locally capped with facies Sr and generally covered by mudshales It overlies mudshales or fluvio-deltaic sandstones of the topmost Helvetiafjellet Fm (Figs. 3, 5A, 6B, D). Occasional thin mudclast conglomerate, shell-hash lag or pebbly sandstone underlie beds of facies Sp, Shs and Sm (Figs. 3, 5D, 6B, D, G). | This patchy conglomerate layer is thought to be a basal transgressive lag (Frebold 1930, 1931) derived by erosional stripping from mouth bars of the retreating Helvetiafjellet delta and spread laterally into adjacent muddy areas by storm action (Hwang and Heller 2002; Cattaneo and Steel, 2003). The patchy occurrence of this basal conglomerate at widely separated localities (Birkenmajer 1964) may reflect diachroneity of the end-Barremian transgression (Gjelberg and Steel 1995). |
| Sp | Sandstone with planar parallel stratification — Well-sorted, fine-grained to coarse-grained arenitic sandstone, forming beds 2 to 150 cm thick, with occasional stringers/lenses of granule gravel and scattered small pebbles or mud clasts. Sets of planar parallel strata up to a few decimetres thick, separated by subhorizontal or gently undulating erosional surfaces. Typically associated with facies Shs and Sr (Figs. 3, 5D, 6D). | Shoreface-derived sand spread seawards by storm-generated currents and deposited in the upper flow regime (Harms et al. 1982; Arnott and Southard 1990; Arnott 1993b) or by the action of waves with high orbital velocities (Clifton et al. 1971; Komar and Miller 1975; Clifton 1976). |
| Shs | Sandstone with hummocky or swaley stratification — Well-sorted, very fine-grained to medium-grained and rarely coarse-grained arenitic sandstones, commonly with scattered small granules and sporadic pebbles in the basal part of swales. Hummocky and swaley strata sets 10–30 cm thick, with a wavelength of 1–5 m, draped with thin facies Sr or covered by mudshale facies Ml. Composite units up to 1.5 m thick, with intrasets of facies Sp, Sr or Sm and with a preferential westward (WNW) stacking of mounded bed sets (Figs. 3, 4B-F, 5B-C, 6B, D). | Deposition of storm-derived sand in combined-flow conditions, with bedforms ranging from pod-shaped scour-and-fill swales to oval accretionary hummock domes (Bourgeois 1980; Dott and Bourgeois 1982; Duke 1985; Arnott and Southard 1990; Duke et al. 1991; Dumas et al. 2005; Dumas and Arnott 2006). |
| Sr | Sandstone with wave and combined-flow ripple cross-lamination — Well-sorted to moderately sorted, very fine-grained to medium-grained and rarely coarser arenitic sandstone with cross-lamination indicating symmetrical to asymmetrical 2D ripples or dome-shaped 3D ripples (‘micro-hummocks’ of Kreisa 1981). Sand units 3‒5 cm thick (Figs. 3, 5C, 6B, E, H, J). | Deposition of storm-derived sand as ripples by oscillatory waves with noderate orbital velocities (Komar and Miller 1975; Clifton and Dingler 1984) or as 3D ripples by combined flow (Leckie 1988; Cheel and Leckie 1992; Yokokawa et al. 1995; Dumas et al. 2005; Perillo et al. 2014). |
| Sm | Massive sandstone — Well-sorted or moderately sorted, very fine- to medium-grained sandstone, locally with scattered small granules and sporadic pebbles or mudclasts, forming units 10 to 100 cm thick that often cap sandstones Shs/Sr or alternate with them. The massive sandstone is commonly mottled and richer in mud at the upward transition to muddy facies Mm, Ml or Hm (Figs. 3, 6C). | Pervasively bioturbated top parts of originally stratified sand beds or sand-rich heterolithic units. Sharp-based massive lower parts of sand units may be due to a non-tractional dumping of sand from storm-generated current or represent local seafloor liquefaction (Leckie 1988; Jelby et al. 2020). |
| H | Heterolithic deposit — Units of thinly interlayered massive to laminated mudshale and ripple cross-laminated sandstone, very fine- to fine-grained, sporadically coarser with scattered granules and small pebbles. Layers are 0.5 to 5 cm thick. Sandstone layers are often weakly graded, with sharp and slightly erosional bases and with gradational or sharp tops; the thinnest layers show pinch-and-swell geometry (Figs. 3, 4A, 6F-I). | Deposition of ‘background’ mud (see facies Ml/Mm) punctuated by frequent incursions of wave-worked sand attributed to relatively weak (seasonal?) storms. Lenticular pinch-and-swell layers represent formation of sand-starved wave ripples (De Raaf et al. 1977; Allen 1982). |
| Hm | Massive silty/sandy mudstone or muddy sandstone – Thin (< 5 cm) to thick (> 4 m) units of massive, poorly sorted, dark grey sediment composed of thoroughly intermixed mud and sand, with irregular speckles of sand and silt. Primary depositional structure obliterated by animal burrows, often superimposed (Figs. 3, 6I, 7I, 10D). | Mud-rich deposit (originally facies H and/or Ml) completely bioturbated by benthic fauna (Reineck and Singh, 1975; Taylor and Goldring 1993; Seilacher 2007). |
| Ml | Laminated mudshale — Blackish to dark-grey mudshale thinly banded with lighter-grey silt and/or silty mud. The individual silty bands range from single laminae (≤ 0.1 cm thick) to layers up to 0.5 cm in thickness, with sharp to gradational flat boundaries. Common animal burrows (Figs. 3, 6F). | Pulsating ‘background’ deposition of alternating clayey and silty hemipelagic suspension with a rhythmic shedding from storm-derived suspension plumes (cf. Kerr 1991; Nemec 1995) and/or action of tidal currents (Reineck and Singh 1975). |
| Mm | Massive mudshale – Blackish or brownish dark-grey mudshale forming non-laminated, homogenous beds up to 15 cm thick. Animal burrows absent to common. The brownish shale variety is variously sideritized and often shows sideritic ‘cannon-ball’ concretions (Figs. 3, 6G-H). | ‘Background’ mud deposition by intense fallout of hemipelagic suspension or by local intra-shelf flow of fluid mud (Krajewski and Luks 2003; Baas et al. 2008; Ichaso and Dalrymple 2009). |
The lateral discontinuity of the basal conglomerate layer (lithofacies C), interpreted as a transgressive lag, is probably due to the erosional stripping and lateral reshuffling by sea waves of the coarse sediment from accessible fluvial channel belts of the topmost Helvetiafjellet Formation (cf. Nemec 1992). Sandstone beds range in thickness from 1 to 80 cm and have sharp bases and tops. The thinnest beds consist of lithofacies Sr, with the ripple cross-laminae sets commonly flattened and/or loaded by compaction (Fig. 6F‒I). Thicker beds consist of lithofacies Sp and/or Sr, whereas the thickest ones show mounded segments with lithofacies Shs (Figs. 4B‒E, 5B‒D). The internal structure of hummocks ranges from roughly symmetrical solitary sets of dome-shaped parallel strata to multiple sets separated by convex-up truncations (Fig. 4B), and to sporadic unidirectional cross-strata sets underlain by thin lithofacies Sp and draped by lithofacies Sp and/or Sr (Fig. 4A). Structure asymmetry (anisotropy) indicates transport towards the WNW or NW. Amalgamated composite sandstone beds reach locally 2 m in thickness (Figs. 3, 4). The tops of non-amalgamated bed, overlain by mudshale, show well-preserved oscillatory wave vortex ripples or combined-flow ripples, often with worm trails (Figs. 6F, I, 7B). All these features support Nøttvedt and Kreisa’s (1987) original interpretation of the sandstone beds as tempestites (Table 1; cf. Dott and Bourgeois 1982; Dumas et al. 2005). Bed soles show occasional grooves, small flutes and prod marks indicating flow direction broadly towards the WSW, but often varying by up to 30° at the same bed sole (Birkenmajer 1966). Cross-lamination of combined-flow ripples indicates sand transport in directions ranging between SW and WNW. Asymmetrical wave ripples indicate transport mainly towards the NE or ESE, which may reflect variable wind-modulation of the water oscillatory movement towards the contemporaneous irregular shoreline. Local post-depositional remobilization of sand by fluidization is evidenced by ptygmatic injection dikes, a few centimetres thick, extending above sandstone bed tops (lithofacies Sd; Fig. 6H) and possibly representing seismites.
The sandstones are predominantly well sorted, fine-grained to medium-grained and quartz-rich sublithic to subarkosic arenites (Maher et al. 2004), and also the rare small scattered pebbles and basal patchy conglomerate layer consist mainly of rounded quartz and chert clasts (Fig. 6D‒E). The textural and mineralogical maturity supports the notion of a wave-worked sediment derived by storms from nearshore zone. However, some of the gravel clasts are merely subrounded. The sand fraction also shows highly varied grain roundness and an admixture of relatively weak mineral grains, such as plagioclase, microcline, chlorite and biotite (Maher et al. 2004). Sediment supply probably involved recycling of older deposits by coastal erosion and local fluvial delivery, with a alongshore drift of sediment, its mixing and variable maturation. Sediment provenance included a contemporaneous volcanic source (Nysæther 1966), linked to the HALIP area in the NE part of the Lomonosov Ridge (Ziegler 1988; Tarduno 1998; Maher 2001; Maher et al. 2004; Mutrux et al. 2008).
Carbonate clasts are rare, including fragments of pelecypod shells, fine-grained contemporaneous hardground limestones and eroded intraformational concretions of microspar calcite or siderite (Maher et al. 2004). In addition to resedimented iron ooids in the lower part the formation, some coalified plant detritus and scattered petrified fragments of driftwood are sporadically found (Mutrux et al. 2008). Early-diagenetic glauconite occurs and increases in abundance upwards in the succession (Maher et al. 2004), which may indicate a decreasing bulk rate of sediment accumulation. Small concretions of bacterial framboidal pyrite tend to be associated with clay pellets and animal burrows. Ball-shaped and strata-bound sideritic ironstone concretions in mudshale beds indicate an early subsurface cementation of uncompacted sediment by non-ferroan calcite in the upper part of sulphite-reduction zone, enriched in framboidal pyrite at the local sites of enhanced bacterial decomposition of organic matter (Krajewski and Luks 2003).
Trace Fossils
Trace fossils are common in the studied succession, slightly more abundant in wells Dh1 and Dh2 than in wells Dh3 and Dh4 (Figs. 1C, 3). Only the basal conglomerate (lithofacies C), the overlying black mudshale unit (lithofacies Ml/Mm) and the majority of thick sandstone beds (lithofacies Sp and Shs) are nearly devoid of bioturbation structures (Figs. 6B, D and 8B). The distinct basal unit of lithofacies Ml/Mm shows merely sporadic structures reminiscent of thread-like burrow fills, manifested on the core surface as small spots of lighter-shade very fine-grained sand, 1‒2 mm in diameter. In contrast, all units of lithofacies Sm are highly bioturbated (BI = 5 to 6; Fig. 7C).
The BI varies from 0 to 5 on a bed thickness scale and on the scale of bed packages several metres thick. urrows are virtually absent in the lower parts of sandstone beds thicker than 5 cm and are increasingly more common in their top parts, at the contact with the overlying mudshale (Fig. 6A, C), which means ‘lam-scram’ sequences sensu Bromley (1990). Burrows are also relatively rare or absent in the heterolithic deposits of lithofacies H and in sideritized mudshales (Fig. 6B, F). On a larger stratigraphic scale, burrows are most abundant in the interval from 30 to 55 m above the formation base, whereas the higher part of the Dalkjegla Member shows marked fluctuations in bioturbation intensity (Fig. 3).
The burrows indicate a range of seafloor animal ethological activities, mainly feeding, dwelling, grazing, resting and crawling. Most bioturbation structures visible in non-slabbed core samples are small portions of burrow systems of unclear taxonomic affiliation. Many of them were produced by sediment scrambling, often multiple, or by scrambling of a soupy-state sediment substrate, which renders them taxonomically non-classifiable. In general, an exact taxonomic classification of trace fossils at ichnospecies level was seldom possible, but 38 ichnotaxa were inferred in the sedimentary succession (Figs. 7‒11), with three undetermined and 14 uncertain (labelled with question marks). A systematic ichnological description of the trace fossils is given in Table 2.
Table 2
Taxonomy of bioturbation structures in the Dalkjegla Mb of the Carolinefjellet Fm in the study area, with ichnogenera ordered alphabetically
| Ichnogenus/ichnospecies | Description (incl. orientation, branching, shape, fill, lining and preservation) | Ethology | Inferred producer | Remarks | References |
| Arenicolites isp. (Figs. 8G, 10C) | U-shaped, vertical, tubular, smooth-walled structure 3 mm in diameter, distance between limbs up to 30 mm. | Domichnion, fodinichnion | Worm-like animal | Observed as several poorly exposed specimens on core surface in one sandstone bed and on a bedding surface in outcrop. | MacEachern and Pemberton (1992) |
| Asterosoma isp. (Fig. 9B–E) | Branched(?), bifurcated(?), radial(?), spindle-form horizontal to oblique endichnial burrow recorded in vertical section as semicircular to elliptical structure, 5‒30 mm across, showing concentric sand and mud laminae packed about a central tube. The thickest and most numerous laminae occur in the burrow bottom part. | Fodinichnion | ?Worm-like animal | Many specimens observed in different cross-sections on core surface. The spindles are solitary or crowded. Specimens in Fig. 9B, D, E may represent vertical sections of the lower, non-vertical segments of Scalichnus isp. | MacEachern et al. (2007a), Gani et al. (2007), Knaust (2015) |
| ?Aulichnites isp. (Fig. 7A) | Positive, bilobate, unlined, unbranched, actively filled epirelief recorded on bedding surfaces as straight to sinuous, bedding-parallel, unornamented, 3–5 mm wide. Median furrow narrow in small forms and wide in large specimens. | Repichnion | Gastropod | Observed exclusively in outcrop, poorly preserved. Small forms show some similarity to Gyrochorte, but are much smaller. | Frey and Howard (1990) |
| ?Chondrites targionii (Fig. 8H) | Branched, straight endorelief recorded in vertical section as clusters of light-grey circular to elliptical spots and veins ~ 1–1.5 mm in diameter, built of silt. | Chemichnion, agrichnion | ?Worm-like animal | Clustered spots and veinlets observed on core surface in several horizons in the top part of succession. | Fu (1991), Seilacher (2007) |
| Conichnus isp. A (Fig. 10E) | Amphora-shaped, vertical positive hyporelief recorded in vertical section as sand-filled body 10 mm high, 7 mm in diameter, acute basal apex. Burrow outline as in Conichnus papillatus, but the latter is much larger and shows a distinct bump on the apex. | Domichnion? | Sea anemone, ?bivalve | Single specimen observed on core surface. Possible affiliation with the ichnogenus Lockeia. | Mänill (1966), Pemberton et al. (1988), Pacześna (2010) |
| ?Conichnus isp. B (Fig. 10F) | Conical, vertical positive hyporelief recorded in vertical section as sand-filled body 10–20 mm high, 15–30 mm wide at its upper extremity. It tapers downwards at 40–70º smooth as sharply rounded basal apex. The infill shows inhomogeneity reminiscent of burrow draping. | Domichnion, cubichnion | ?Bivalve | Recorded on core surface as two specimens with different tapering angles (40º and 70º). | Frey and Howard (1981) |
| ?Conichnus isp. C (Fig. 11) | Conical, vertical, positive hyporelief recorded in vertical section as siderite-filled body, 65 mm high, 40 mm wide at its upper extremity, with smooth, rounded basal apex. Sideritized infill reminiscent of smaller burrows. | Domichnion, cubichnion | Unknown | Recorded on core surface as one specimen, which is much higher than isp. A and B, and tapers downwards at an angle of 25º. | Frey and Howard (1981) |
| ?Cylindrichnus concentricus aff. (Fig. 11B, C) | Straight to bow-shaped, horizontal to oblique endorelief, cylindrical in section perpendicular to elongation, 5‒8 mm in diameter, concentrically layered inside. The layered structure marked by alternating darker and lighter layers. | Domichnion | Terebellid polychaete | Recorded on core surface as many specimens in vertical and oblique sections. Smaller than specimens described by Belaústegui and Gibert (2013). | Belaústegui and Gibert (2013) |
| Diplocraterion isp. A (Fig. 8G) | U-shaped, vertical sand-filled structure recorded on bedding surface as two joined, vertical, unlined, sand-built cylinders (dumbbell shape), 8 mm in diameter, 7 mm apart. Spreite between the arms poorly discernible, of retrusive type. | Domichnion | Corophiid amphipod | Paired, dumbbell-shaped sand-built cylinders suggestive of a U-shaped burrow. One specimen on bedding surface of a loose sandstone slab at outcrop foot. | Fillion and Pickerill (1990), Šimo and Olšavský (2007) |
| ?Diplocraterion isp. B (Fig. 9K) | U-shaped, vertical, tubular, sand-filled structure 4 mm in diameter with spreite between limbs. Limbs up to 6 mm apart in vertical section parallel to burrow extension. In perpendicular section, seen as a column of vertically stacked menisci underlain by rounded sand body 4 mm in diameter. | Domichnion | Corophiid amphipod | Observed as three specimens on core surface, in wavy to lenticularly bedded heterolithic facies H. Spreite indistinct in one specimen. | Fillion and Pickerill (1990) |
| Gyrochorte comosa (Fig. 7B) | Positive, bilobate, unlined, unbranched, actively filled epirelief, recorded on bedding surfaces as ridges (lobes) 4–5 mm wide, showing plaited ornamentation, winding, intersecting itself. | Fodinichnion | Endobenthic worm-like animal | Ornamentation poorly preserved. Observed in two thin sandstone slabs with oscillatory rippled top and no other burrows. | Gibert and Benner (2002), Cabrera et al. (2008) |
| ?Gyrolithes isp. (Fig. 8K) | Spiral, unbranched endorelief recorded in vertical section as cluster of sand-filled, vertically stacked burrows, showing elliptical cross-section, 3 mm in diameter. | Domichnion, fodinichnion | Decapod crustacean | Two specimens recorded on core surface in mudstone as cluster of 4 sand-filled, vertically stacked burrows. The structure suggests to represent one coil. | Netto et al. (2007), Buatois et al. (2005), Lettley et al. (2007) |
| ?Helminthopsis isp. (Fig. 7C) | Positive, winding, unlined, unbranched hyporelief and endorelief recorded in vertical section as densely distributed, black, lenticular spots, 1 mm in diameter and as horizontal to oblique veins, up to 10 mm long and 0.5 mm thick. Winding black strings on bedding surfaces. Locally with light halo. | Fodinichnion | Endobenthic worm-like animal | Recorded on core surface, frequently as a common trace fossil, except for the lower part of succession. | Fillion and Pickerill (1990), Han and Pickerill (1995), Coates and McEachern (2007) |
| ?Lingulichnus isp. (Fig. 11D, E) | Straight to curved in the lower part, vertical endorelief recorded as sand-filled shafts, usually surrounded by fluid-state deformed sediment. The fill occasionally shows subtle annulation | Fugichnion, equilibrichnion | Lingulid Brachiopod | Several specimens in core samples. The lack of distinct aureole of fluidal sediment resembles burrows interpreted as lingulid pedicle traces. | Zonneveld and Pemberton (2003), Zonneveld et al. (2007) |
| Ophiomorpha cf. rudis (Fig. 9F, G) | Straight to slightly curved, branched endorelief, positive hyporelief recorded as circular to elliptical elongate sand-filled bodies in vertical section and as cylindrical, gently winding sand-filled tunnels, 4‒6 mm in diameter, irregularly lined with flame-like deformed mud pellets. Lining preserved fragmentarily, with granular texture. | Domichnion, fodinichnion | Crustacean | Observed on core surface as several bedding-oblique to horizontal specimens, and as single bedding- parallel specimen in a loose sandstone slab at outcrop foot. | Howard and Frey (1984), Gani et al. (2007), Uchman (2009) |
| Palaeophycus heberti (Fig. 7I, J) | Gently curved, unbranched endorelief and epirelief with elliptical shape on core surface, thickly grain-lined, 3–10 mm in diameter. Lining 0.9‒1.5 mm thick, made of structureless light-coloured sand. Cross-sectional views indicate that the trace fossil has a form of horizontal and subhorizontal, lined cylinder. Some specimens show lining thicker than the tunnel. | Fodinichnion | Endobenthic worm-like animal | The trace fossil seems to occur solitarily, and this feature ‒ together with the simple lining ‒ renders P. heberti different from Schaubcylindrichnus. | Pemberton and Frey (1982), Howard and Frey (1984), Nara (2006) |
| Palaeophycus ?sulcatus (Fig. 8C) | Bedding-parallel, straight to slightly curved endichnial and epichnial tubular burrow, 7 mm in diameter, sculpted by small, interwoven longitudinal to slightly oblique ridges. | Fodinichnion | Endobenthic ?worm-like animal | Recorded in a loose sandstone slab at outcrop foot. | Pemberton and Frey (1982) |
| Palaeophycus tubularis (Fig. 8C, F, G) | Bedding-parallel, straight to slightly curved epichnial tubular burrow, 7 mm in diameter, sculpted by small, interwoven longitudinal to slightly oblique ridges. | Fodinichnion | Endobenthic ?worm-like animal | Recorded as short segments on bedding plane of loose sandstone slabs at outcrop foot and on core surface. | Pemberton and Frey (1982) |
| Palaeophycus isp. (Fig. 8D, E) | Bedding-parallel, straight to slightly curved endichnial and epichnial tubular burrow 4 mm in diameter, mud lined, made of sand, observed on bedding plane. | Fodinichnion | Endobenthic ?worm-like animal | Several specimens recorded on core surface. | Pemberton and Frey (1982) |
| ?Phoebichnus isp. (Figs. 9J, 10B) | Straight, cylindrical endorelief 5–12 mm in diameter, representing complex stellate system, circular to elliptical in vertical section, marked with sand-built centre 3‒4 mm in diameter, surrounded by mud lining 1‒2 mm thick. | Fodinichnion | Endobenthic?echiuran worm | Found on core surface as several bedding-oblique to horizontal specimens. Relatively thick mud lining indicates Phoebichnus affinity. | Bromley and Asgaard (1972), Kotake (2003), McEachern et al. (2007a) |
| ?Phycodes isp. (Fig. 9I) | Bundle of cylindrical endichnial burrows, circular to elliptical and lenticular in vertical section, made of homogeneous sand and showing slightly irregular outline, 7‒12 mm in diameter. | Fodinichnion | Endobenthicworm-like animal | Found as several bedding-parallel specimens on core surface. Bundled occurrence suggests Phycodes affinity. | Frey and Pemberton (1985), Fillion and Pickerill (1990) |
| Phycosiphon incertum (Fig. 9) | Complex, lobate, U-shaped, unbranched epirelief and endorelief recorded in vertical cross-section as elliptical to elongated, vermiform black spots surrounded by a pale-grey halo (frogspawn texture). The black spots are 1‒1.5 mm in diameter and the mantle is 1‒2 mm thick. | Fodinichnion | Endobenthic worm-like animal | Observed in lithofacies H and Hm. No visible spreiten. Occurrence of U-shaped lobes is inferred from frequent occurrence of elongate spots up to 3 mm long. Occurs gregariously in many horizons. | Wetzel and Bromley (1994), Bednarz and McIlroy (2009, 2012), McEachern et al. (2007a, c, d) |
| ?Phycosiphon isp. (Fig. 8I, J) | Horizontal to oblique, curved endorelief marked in vertical section as dark-grey to black, highly curved streaks and spots up to 1 mm thick surrounded by a faint halo. | Chemichnion, fodinichnion | Endobenthicworm-like animal | Recorded as crowds on core surface, represents compactionally flattened mud-filled tunnels. Differs from Phycosiphon incertum by less distinct, diffuse, flame-like halo similar to ichnogenus Multina. | Bednarz and McIlroy (2009), McEachern et al. (2007a, c, d), Kotlarczyk and Uchman (2012) |
| Planolites beverleyensis (Fig. 7A) | Straight to irregularly curved, unbranched endorelief, epirelief and hyporelief seen on bedding planes as cylindrical and in vertical section as circular, 5–6 mm in diameter, made of homogeneous sand, with smooth to slightly irregular walls. | Fodinichnion | Endobenthic worm-like animal | Observed both in cross-section on core surface and on bedding planes in outcrop. | Pemberton and Frey (1982), Uchman (1995) |
| Planolites montanus (Fig. 7A) | Gently irregularly curved, rarely branched, smooth endorelief, epirelief and hyporelief recorded on bedding planes as tubular and in vertical section as circular, 1.5–2 mm in diameter, made of sand. | Fodinichnion | Endobenthic worm-like animal | Recorded both in cross-section on core surface and on bedding planes. Locally quite common on bedding planes in sandstone lithofacies. | Pemberton and Frey (1982), Fillion and Pickerill (1990) |
| Planolites isp. (Fig. 7A, H) | Gently irregularly curved, unbranched, unlined endorelief, epirelief and hyporelief recorded in vertical section as circular and elliptical spots, 2.5‒5 mm in diameter, marked by sand in mudstone layers. Cross-sections show a horizontal or subhorizontal, curved, unlined cylinder infilled with homogeneous sand. | Fodinichnion | Endobenthic worm-like animal | Recorded both in cross-section on core surface and on bedding planes. | Pemberton and Frey (1982) |
| Rhizocorallium isp. (Fig. 7D–F) | U-shaped in planar view, actively filled endorelief recorded in vertical section as a meniscated stripe 5 mm thick, indicative of spreite structure. Spreite arrangement protrusive or retrusive. | Fodinichnion | Polychaete | Recorded as several specimens on core surface. | Fielding et al. (2007), Seilacher (2007) |
| Rosselia isp. (Figs. 8E, 10D) | Chalice-shaped, unbranched, vertical endorelief seen in vertical section as vertically or obliquely elongate, horn-shaped to conical structure 25‒40 mm in diameter, 30–50 mm high, filled with faintly laminated mud. Laminae arrangement reminiscent of concentric sheaths. | Fodinichnion | Unknown | Recorded as a few specimens on core surface. | Miller and Aalto (2008), Coates and MacEachern. (2007), Hansen and MacEachern (2007), Davison and MacEachern (2007) |
| Schaubcylindrichnus coronus (Fig. 8B) | Straight, oblique to bedding, unbranched endorelief seen in vertical section as clusters of tubular burrows, 3 mm in diameter, showing relatively thick, distinctive light-coloured lining (0.5–0.9 mm thick). | Domichnion | Endobenthicsessile ?polychaet | Recorded as many specimens on core surface only. The trace similar to S. freyi in the type of lining but its tubes are much larger and show only bedding-oblique alignment. Clustering distinguishes it from Palaephycus heberti. | Frey and Howard (1981), Nara (2006 |
| Schaubcylindrichnus freyi (Fig. 8A) | Straight to gently curved, horizontal to oblique, unbranched endorelief seen in vertical section as clusters of well-lined and usually flattened tubes, 2–5 mm in diameter. Light-coloured wall lining 0.5‒1 mm thick. | Domichnion | Endobenthicsessile ?polychaet | Recorded as many specimens on core surface. S. freyi may represent the lower part of the S. burrowing system where the individual tubes tend to converge. | Miller (1995) |
| Siphonichnus isp. (Fig. 10A, B) | Straight to slightly curved, vertical endorelief seen on core surface as columnar, cylindrical sand body 10 mm wide, showing faint meniscate backfill structure accentuated by delicate, irregular rippling of walls. | Fugichnion, equilibrichnion | Endobenthic bivalve | Recorded as several evident specimens on core surface and bedding plane. | Stanistreet et al. (1980), Zonneveld and Gingras (2013), Knaust (2015) |
| Skolithos linearis (Fig. 11F) | Straight to slightly curved, vertical to subvertical endichnial shafts 5–7 mm in diameter, filled with homogeneous sand, show wall lining 1–2 mm thick, slightly darker than the shaft infill. | Domichnion | Annelid, phoronid | Observed on core surface, with longest specimen up to 6 cm. Resembles stem tunnels of Rosselia socialis (cf. Miller and Aalto 2008). | Haldeman (1840), Alpert (1974), McEachern et al. (2007a), Schlirf and Uchman (2005) |
| Teichichnus rectus (Fig. 11G) | Columnar to lobate vertical endorelief with stacked concave-up spreite, 5–20 mm wide, up to 30 mm high and ≥ 40 mm long. Long, wavy, down-bowed laminae tending to merge upwards at their ends. Oblique sections show shorter, truncated laminae. | Fodinichnion | Crustacean ?worm-like animal | Recorded as many specimens on core surface. | Seilacher (1955, 2007), Coates and MacEachern. (2007) |
| Thalassinoides suevicus (Figs. 8C, 9H) | Cylindrical, unlined boxwork endorelief, positive hyporelief, 10‒20 mm in diameter, slightly enlarged at points of branching, filled with sand. Smooth walls and T- or Y-shaped ramifications. | Domichnion, fodinichnion | Crustacean | Several specimens on bedding surface of loose sandstone slabs at outcrop foot. | Frey et al. (1984), Frey (1990) |
| ?Thalassinoides isp. (Fig. 9I) | Cyllindrical, unlined to delicately mud-lined endorelief, positive hyporelief, 10–20 mm in diameter, filled with sand, showing Y-shaped branching. | Domichnion, fodinichnion | Crustacean | The fill is occasionally laminated, with thickest laminae at the bottom. Ichnotaxon noted on bedding surface of several sandstone slabs at outcrop foot. | Frey et al. (1984), Bromley and Uchman (2003), MacEachern et al. (2005) |
| Undetermined ichnotaxa | |
| Cup-shaped isp. (Fig. 11H, I) | Endorelief with a copular lower outline, pointing downwards centrally and having sharp, uneven margin. Structure 0.5‒1.5 cm deep and 1.0‒3.5 cm wide in its broadest part, with irregularly streaked, down-bent sandy infill. | Cubichnion, equilibrichnion | Sea ?anemone, ?bivalve | Structure seen as many specimens on vertical core surface. | None |
| Funnel-shaped isp. (Figs. 7F, 13I) | Epirelief with sheath-shaped lower outline, pointing downwards centrally, 7‒10 mm deep and 10‒15 mm wide in its broadest part. Irregularly laminated infill with sharply down-bent laminae . | Fodinichnion, equilibrichnion | Sea ?anemone | Structure seen as many specimens on vertical core surface. | None |
| U-shaped isp. (Fig. 11H) | Endorelief with irregular U-shaped lower outline, pointed downwards centrally. Spreite-type infill and a fringed margin. Structure 4 cm deep and ~ 3.5 cm wide. Margins marked by muddy shreds protruding outwards from the trace body. | Domichnion, equilibrichnion | Sea ?anemone | Apart from its fringed margin, the structure resembles Teichichnus. Observed one specimen on core surface. May represent downward extension of the cup-shaped isp. | None |
The occurrence and stratigraphic distribution of particular ichnotaxa and trace-fossil assemblages is generally related to lithofacies. For example, the ichnotaxa found in the thick sandstone beds of lithofacies Sp, Shs and Sr are different from those in lithofacies Sm or Hm. These differences entail variation in ichnofabric. An ichnofabric dominated by Phycosiphon-like burrows (Phycosiphon incertum, ?Phycosiphon isp., Helminthopsis isp.) is characteristic of lithofacies Sm (Figs. 7C, I, J, 8I, J) and locally Mm/Ml, and of the bioturbated divisions of lithofacies H (Fig. 7H). An ichnofabric dominated by Asterosoma, locally accompanied by Teichichnus, Thalassinoides and plug-shaped equilibrium structures (cf. Bromley and Uchman 2003), characterizes the top parts of the thick sandstone beds of lithofacies Sp, Shs and Sr, subsequently covered with the mudshales of lithofacies M (Fig. 9B, D, F). Bedding surfaces in the outcrop show also ?Aulichnites isp., Gyrochorte comosa, Palaeophycus sulcatus, ?Phycodes isp. and Thalassinoides suevicus (Fig. 7G, H). A package of sandstone lithofacies 20 to 26 m above the base of the formation in well cores (Fig. 3) shows an ichnofabric dominated by Skolithos, Palaeophycus isp. and Arenicolithes burrows (Figs. 10C, 11F).
The studied succession as a whole bears a mixed Cruziana–Skolithos ichnofacies (cf. Seilacher 1964, 1967, 2007; Bromley 1990; MacEachern et al. 2007a). The trace fossil suite in the above-mentioned sandstone package in well profiles represents the Skolithos ichnofacies, with the highly burrowed (scrambled) sediment indicating a distal variety of this ichnofacies. Trace fossils in the remaining part of the succession represent the Cruziana ichnofacies, which varies between the following two suites:
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A proximal to archetypal variety of the Cruziana ichnofacies (MacEachern et al. 2007a, c) dominated by Asterosoma isp., Cylindrichnus isp., ?Phoebichnus isp. and Teichichnus isp., often accompanied by Schaubcylindrichnus coronus, Rosselia isp., Palaeophycus heberti, Thalassinoides ?div. isp., Rhizocorallium (?Taenidium), Diplocraterion div. isp., Conichnus div. isp., plug-shaped equilibrium structures and undetermined cup-shaped burrows (Table 2).
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A distal variety of the Cruziana ichnofacies (MacEachern et al. 2007a, c) dominated by Phycosiphon-like trace fossils (Phycosiphon incertum, ?Phycosiphon isp., ?Helminthopsis isp.) with some Planolites isp., Diplocraterion div. isp., plug-shaped equilibrium structures, ?Lingulichnus isp., Siphonichnus isp. and rarely some other ichnotaxa.
The first suite occurs in the top parts of medium to thick sandstone beds overlain by mudshale and in some packages of the heterolithic deposits of lithofacies H. The second suite occurs in lithofacies Sm and in some beds of lithofacies H.
Notably, both the Skolithos and Cruziana ichnofacies here show quite specific composition in comparison to that commonly reported from similar sublittoral deposits in other regions (cf. Buatois and Angriman 1991; MacEachern and Pemberton 1992). The Skolithos ichnofacies differs from its ‘classic variety’ by nearly lacking such trace fossils as Ophiomorpha, Bergaueria, Taenidium and Macaronichnus (cf. MacEachern et al. 2007a, 2012; Buatois and Mangano 2011), whereas the Cruziana ichnofacies lacks its most characteristic arthropod tracks and Zoophycos burrows (cf. Buatois and Mangano 2011). Moreover, the facies-crossing ichnogenus Chondrites, so commonly reported from such deposits, has not been evidenced in the present case. Its absence is particularly remarkable in the lower part of the succession, in the blackish to brownish dark-grey mudshales of lithofacies Mm overlying the non-bioturbated basal lithofacies Ml, where other trace fossils begin to appear, such as Phycosiphon-like burrows and Schaubcylindrichnus freyi.
Lithofacies Associations
The lithofacies (Table 1) have been recognized to form four main associations, labelled FA1 to FA4, which range from sand-rich to muddy and are vertically alternating with one another in the stratigraphic succession (Fig. 3). The extensive outcrop section of gently-inclined deposits and the two pairs of closely-spaced wells (Fig. 1C) show also considerable lateral changes over the distances of a few hundred metres, with some lithofacies pinching out and with one lithofacies association passing laterally into another.
The lithofacies associations and their ichnofauna assemblages are described and interpreted below. The lithofacies associations are thought to represent different inner to outer shelf zones and for simplicity are given interpretive environmental labels, but their descriptions are separated from interpretations in the text. The stratigraphic alternation of lithofacies associations (Fig. 3) is attributed to the lateral shifting of shelf zones in response to relative sea-level fluctuations accompanied by morphodynamic changes of the shoreface–shelf profile (cf. Hampson 2000; Hampson and Storms 2003; Grundvåg et al. 2021).
FA1: Lower shoreface deposits
The deposits of lithofacies association FA1 occur several times in the stratigraphic succession (Fig. 3), forming packages up to 6 m thick, enveloped by the mud-richer association FA2 and passing laterally into the latter. Their boundaries are mainly gradational, marked by a rapid upward change in the thickness and relative proportion of sandstone and mudstone beds, but the basal contact of FA1 in some cases is sharp and recognizably erosional.
Description – This lithofacies assemblage (Figs. 3, 4) consists mainly of lithofacies Sp and Shs, with drapes or intercalations of lithofacies Sr and subordinate thin interbeds of lithofacies Sm, Hm and/or Mm, rarely Ml. Lithofacies Shs volumetrically dominates, as the sandstone beds with hummocky stratification are the thickest. Beds of sandstone lithofacies tend to be amalgamated by erosion, whereby the interbeds of muddy lithofacies are commonly truncated, discontinuous or virtually removed. Mudclasts, up to 7 cm in length, occur scattered along internal erosional surfaces.
Sandstone beds in the outcrop are generally extensive sheets, but their geometry varies from tabular to irregularly mounded or lenticular (Fig. 3; see also Grundvåg et al. 2021). Some beds show a uniform thickness over lateral distances of several tens of metres, before thinning markedly or pinching out within a few metres. Other sandstone beds, particularly the thickest ones, show irregular mounds in the form of simple low-amplitude domes (isotropic to anisotropic hummocks) or compound, broader and thicker (≤ 1.5 m) domes with a clinoformal lateral stacking of the successive units of lithofacies Shs and subordinate Sr, mainly towards the W or WSW (Fig. 3). The amalgamated clinoforms are convex-upwards to sigmoidal, inclined at 5 to 20°. Most of the sand mounds have erosional bases with an irregular low relief (< 10 cm), but reaching a relief of up to 50 cm where forming a sharp lower boundary of FA1 package (Fig. 3). There are also solitary channel-like scours (Fig. 3F), up to 1.5 m deep and a few tens of metres wide, trending towards the W or WSW and filled with vertically accreted packages of lithofacies H or obliquely inclined (10‒15°) packages of lithofacies Sp and Sr (see Birkenmajer 1966, Figs. 3, 4).
Animal burrows in lithofacies association FA1 are relatively uncommon and limited mainly to infrequent bioturbated horizons, generally corresponding to the mudshale-covered top parts of sandstone beds or their amalgamated packages. The degree of bioturbation thus varies from none to high (BI = 0 to 6) on the thickness scale of a bed or a bed package. The following ichnotaxa have been recognized: Arenicolithes isp., Asterosoma isp., Paleophycus tubularis, Palaeophycus isp., ?Phoebichnus isp., Planolites isp., Rosselia isp., Schaubcylindrichnus coronus, Skolithos isp. and Teichichnus rectus, with accompanying ?Helminthopsis-like traces and sporadic Ophiomorpha cf. rudis and Diplocraterion isp. (Table 2).
The FA1 packages are generally hosting a proximal variety of the Cruziana ichnofacies (cf. MacEachern et al. 2007a), dominated by either Asterosoma or undefined scrambled ichnofabric overprinted by other taxa as deeper tiers. Only one sandstone package, about 1 m thick and located 20 to 26 m above the base of Carolinefjellet Formation in the wells (Fig. 12), hosts a distal variety of the Skolithos ichnofacies (cf. MacEachern et al. 2007a) dominated by suspension-feeder traces and comprising Skolithos isp., Arenicolithes isp., Palaeophycus tubularis, Palaeophycus isp. and sporadic Ophiomorpha and Diplocraterion (Figs. 10C, 11F).
Interpretation – The assemblage of sandstone lithofacies Sp, Shs and Sr indicates the action of littoral waves with variable orbital velocities, punctuated by storm combined-flow events (Table 1). The clinoformal stacking of amalgamated sandstone units suggests broad subaqueous sand bars formed by series of high frequency storms, with the geostrophic currents deviated from shoreline by the Coriolis effect (Walker 1984b) and with the initial hummocks instigating subsequent deposition in their hydraulic shadow. Midtgaard (1996) described similar sandbodies, accreted seawards at ~ 70° away from shoreline, from the Lower Cretaceous mid-shelf deposits in western Greenland. The channel features, limited to FA1, were originally attributed to tidal currents (Birkenmajer 1966), but this lithofacies association represents sedimentation dominated by waves and storm-generated currents. These features are likely storm-current elongate scours (cf. Bentley et al. 2002, Fig. 19) or bypass troughs formed by storm-boosted rip currents (cf. Gruszczyński et al. 1993; Stone et al. 1995; Mathers and Zalasiewicz, 1996), filled in directly by the storm and/or subsequent fairweather sedimentation.
The scarcity of burrows in sandstone beds implies seafloor conditions of high and quasi-perennial sediment mobility, and hence deposition mainly above the fairweather wave base. The numerous internal truncation and amalgamation surfaces imply that a considerable part of fairweather sand (lithofacies Sp and Sr) was probably removed by erosive storms and transferred seawards. The interbeds of muddy facies and isolated bioturbation horizons represent episodes when the wave base stayed above the seafloor. The evidence as a whole indicates deposition between the maximum and mean depth of fairweather wave base, which means a lower shoreface zone (Clifton 1981; Brenchley 1985; Tillman 1985; Reading and Collinson 1996; Hampson 2000).
Deposition below the mean fairweather wave base is consistent with the occurrence of Cruziana ichnofacies and with the bioturbation horizons indicating distinct time-windows for benthic colonisation (Pollard et al. 1993; Goldring et al. 2004, 2007). The solitary sandstone package with Skolithos ichnofacies (Fig. 12) indicates a highly mobile sandy substrate and low sedimentation rate (cf. MacEachern et al. 2007a; MacEachern and Bann 2008), which would suggest deposition around the mean fairweather wave base in a middle shoreface zone and imply the maximum fall of relative sea level recorded within the sedimentary succession. This episode might either signify the greatest shoreline advance or represent brief opportunistic colonization of lower shoreface by Skolithos ichnofauna following an exceptionally high delivery of sand by a series of strong storms (e.g., Vossler and Pemberton 1988, 1989; Pemberton et al. 1992b, 2012; Pemberton and MacEachern 1997). In either case, the sparsity of bioturbation in this sandstone package would reflect the short duration of inter-storm benthic colonisation windows (MacEachern and Pemberton 1992; Dashtgard et al. 2012; Pemberton et al. 2012).
FA2: Proximal offshore-transition deposits
Lithofacies association FA2 shows a comparable thickness proportion of alternating sandy and muddy deposits, forming packages about 1 to 4 m thick. They underlie and overlie lithofacies association FA1 or occur in isolation from the latter (Fig. 3). The outcrop section also shows FA1 passing laterally into FA2 by the lateral thinning or pinch-out of sandstone beds and an increase in the relative proportion of muddy deposits.
Description – This lithofacies assemblage (Figs. 3, 4) consists of the heterolithic deposits of lithofacies H and mudshale lithofacies Mm interspersed with discrete sheet-like sandstone beds showing various combinations of lithofacies Sp, Shs and Sr (Table 1; Fig. 5B‒D). Subordinate are beds of lithofacies Sm or intervals of Hm. The muddy and sandy beds are up to 50 cm thick, but mainly 10 to 20 cm. Sandstone beds have mainly flat bases, sharp to slightly erosional, whereas their tops in the outcrop section show broad undulations on a lateral scale of several to a few tens of metres. The undulations render some of the thinner beds discontinuous, split into irregular broad lenses (probably 3D patches). Erosional amalgamation of sandstone beds is uncommon and only local.
Bioturbation in lithofacies association FA2 is more common than in FA1 and ranges from isolated to clustered burrows, but occurs mainly in the muddy facies and reaches its highest intensity in distinct horizons of unknown lateral extent (observation from well cores). The trace fossils include Asterosoma isp., Paleophycus tubularis, Palaeophycus isp., ?Cylindrichnus concentricus, Teichichnus rectus, Thalassinoides isp., Rosselia isp., Conichnus div. isp., Planolites isp., Arencolites isp., ?Helminthopsis isp., ?Phycosiphon incertum and various plug-shaped equilibrium structures, with sporadic Diplocraterion ?div. isp., Rhizocorallium isp. (?Taenidium isp.), ?Lingulichnus isp., Siphonichnus isp., ?Phoebichnus isp., ?Phycodes isp. and ?Chondrites targionii (Table 2). In addition, Planolites montanus, P. beverleyensis, Palaeophycus sulcatus, Gyrochorte comosa and ?Aulichnites isp. were found on sandstone bedding surfaces in the outcrop (Fig. 9A, B, G).
The bioturbation intensity in FA2 varies from none to high (BI = 0 to 6). Most common are stratigraphic intervals with no or little bioturbation (BI = 1), hosting isolated burrows (Figs. 6A, E, F, H, I, 7D). Similarly varied is ichnofabric, dominated by horizontal burrows. The rare strongly bioturbated horizons show a scrambled fabric intersected by Asterosoma and/or Teichichnus, rarely other ichnotaxa. The less bioturbated deposits show a full range of isolated burrows, rarely intersecting one another.
Interpretation – Compared to FA1, this lithofacies association contains thinner sandstone beds and a higher proportion of muddy facies, which suggests a more distal depositional zone relative to palaeoshoreline. The discrete sandstone sheets with wave-generated and combined-flow structures indicate sand emplacement by brief events, considered storms. The thicker beds of muddy facies and greater abundance of bioturbation indicate considerably longer periods of sand-starved seafloor conditions below the fairweather wave base. Such conditions characterize the lowermost shoreface to upper offshore-transition zone, at a water depth range from around the maximum fairweather wave base to the mean storm wave base, where sand is delivered and spread mainly by storms (Howard & Reineck 1981; Kreisa 1981; Reading and Collinson 1996; Hampson 2000). The lenticularity of sandstone beds on a lateral scale of up to a few hundred metres (Nøttvedt and Kreisa 1987; Grundvåg et al. 2021) probably reflects a patchy style of the spatial distribution of sand by storms (cf. Bentley et al. 2002; Keen et al. 2004).
The ichnofauna assemblage in FA2 indicates environmental conditions hospitable to a wide range of benthic animals, but also reflects the ecological stress imposed by episodic sand emplacement (cf. Pemberton et al. 1992b; Pemberton and MacEachern 1997; MacEachern et al. 2007b; Bann et al. 2008; MacEachern and Bann 2008). The predominance of horizonal burrows indicates a deposit-feeding benthic fauna (cf. Pemberton et al. 2012), which implies relatively long periods of low-energy bottom water conditions. The scarcity of suspension-feeding fauna indicates a quick burial of the episodically emplaced sand layers by ubiquitous mud, which disfavoured seafloor colonization by organisms preferring a sandy substrate (Pemberton et al. 2012). The event sedimentation and variable time-windows for benthic colonisation may explain the varying bioturbation degree of the deposits (Pemberton et al. 1992b, 2012).
FA3: Distal offshore-transition deposits
Lithofacies association FA3 contains a much higher thickness proportion of muddy lithofacies, with sandstone beds generally thinner and finer-grained. These mud-dominated deposits form packages up to ~ 20 m thick, both underlying and overlying lithofacies association FA2 (Fig. 3). The contacts of these two associations are conformable and transitional, marked by changes in the relative proportion of sandy and muddy lithofacies. In the outcrop section, FA2 is also passing laterally into FA3 within a few hundred metres.
Description – The assemblage FA3 (Fig. 3) consists mainly of lithofacies Mm, Ml and Hm, interspersed with thin sandstone sheets of lithofacies Sr (Table 1, Fig. 6F, I). The sandstone beds are very fine- to fine-grained and locally up to 20 cm thick, but are mainly thinner than 5 cm and commonly discontinuous, composed of isolated lenses with a lateral extent of 10 cm to several metres. The boundaries of sandstone beds are sharp, but the bases are seldom recognizably erosional and the tops show well-preserved ripple forms. Ripple crests are trending mainly SE, and the ripple cross-laminae sets in some beds show mud drapes, occasionally rich in carbonaceous plant detritus.
FA3 in its lowest stratigraphic occurrence (Fig. 12), particularly in its basal part, is distinctly less burrowed and shows a much lower diversity of trace fossils than in the higher occurrences. Moderate to high bioturbation occurs in thickness intervals of 1 m to a few metres, particularly in lithofacies Hm. There are isolated horizons of intense burrowing as well as random solitary burrows. High bioturbation (BI = 5 to 6) prevails in the FA3 package at 40 to 55 m above the base of the formation.
Trace fossils include Helminthopsis isp. and some other Phycosiphon-like ichnotaxa, Asterosoma isp., Planolites isp., Thalassinoides isp., Teichichnus rectus, Palaeophycus isp., Siphonichnus isp., ?Cylindrichnus concentricus and Rosselia isp., accompanied by sporadic Diplocraterion ?div. isp., ?Lingulichnus isp. and undetermined cup-shaped, funnel-shaped and fringed U-shaped burrows (Table 2). Ichnofabric is dominated by Phycosiphon-like burrows, locally with Asterosoma, Planolites, Thalassinoides and Teichichnus. Asterosoma is characteristic of the sand-richer parts of FA3 packages. The lowermost stratigraphic package of FA3 (Fig. 12), particularly at its transition from the underlying FA4, shows only ?Phycosiphon isp., Schaubcylindrichnus freyi and some sand-filled thin pipes reminiscent of Planolites. Overall, the FA3 deposits bear a trace-fossil assemblage of the Cruziana ichnofacies, mainly its distal variety (cf. Savrda et al. 2001; MacEachern et al. 2007a).
Interpretation – The differences in lithofacies and ichnofauna and the stratigraphic relationship between lithofacies association FA2 and mud-dominated FA3 indicate that the latter is a more distal seaward equivalent of the former. The isolated thin sandstone beds are thought to be distal tempestites, deposited at water depths where only the strongest storm-generated currents were delivering fine sand and where the sand was fully reworked by storm waves with mainly low near-bottom orbital velocities (Komar and Miller 1975; Clifton 1981; Clifton and Dingler 1984). Ripple mud drapes indicate brief rhythmic detachments of the wave base from the seafloor, which may reflect pulsating storms or the impact of tides on the wave base during storms lasting for several days. The pinch-and-swell geometry of thin sandstone beds, on the scale of ripples or small groups of ripples, indicates sand-starved seafloor conditions. The lenticularity of thicker beds, on a lateral scale of several tens to hundreds of metres (Nøttvedt and Kreisa 1987), reflects the inherently patchy pattern of spatial sand distribution by storms (cf. Bentley et al. 2002; Keen et al. 2004).
The evidence as a whole indicates deposition in the distal offshore-transition zone, below the mean storm-wave base (Howard and Reineck 1981; Reading and Collinson 1996; Hampson 2000). This interpretation is consistent with a distal Cruziana ichnofacies, dominated by traces of deposit-feeding, dwellings and grazing fauna. The general lack of suspension-feeders reflects the sparsity and infrequent availability of sandy substrate.
The poor assemblage of ichnofauna in the stratigraphically lowest package of FA3, at its transition from the underlying FA4, apparently reflects a benthic ecological change from an unhospitable (anoxic?) muddy environment to a more hospitable environment with episodic delivery of sand and improved seafloor oxygenation. A similar ichnological case was reported, for example, by Savrda et al. (2001) from the Pleistocene shelf of New Jersey.
FA4: Offshore deposits
Lithofacies association FA4 occurs only in the basal part of the succession (Figs. 3, 12), where these muddy deposits form a unit ranging in thickness from ~ 2 m in wells Dh1/Dh2 to 5 m in wells Dh3/Dh4 (Figs. 1C, 3) and to nearly 15 m in the southern outcrops more distal from the inferred palaeoshoreline (Midtkandal et al. 2016). This muddy unit is underlain by a solitary erosional conglomeratic sheet (wells Dh1 to Dh3) or locally by a correlative erosional bypass surface (well Dh4). The conglomeratic layer, although laterally discontinuous, is a regionally widespread feature (Birkenmajer 1966) considered in mapping to mark the base of the Carolinefjellet Formation (Parker 1967; Nagy 1970; Mørk et al. 1999). Similarly widespread is the basal mudshale unit, which apparently extends beyond the coastal outcrops in SE Spitsbergen (Nemec et al. 1988; Århus 1991).
Description – This lithofacies association consists chiefly of the blackish grey, parallel-laminated clayey to silty mudshales of lithofacies Ml (Fig. 6F) interspersed towards the top with sporadic thin (< 1 cm) and mainly discontinuous, pinch-and-swell sheets of very fine-grained sandstone lithofacies Sp and/or Sr. Ripple cross-laminae sets in lithofacies S4 occasionally show carbonaceous mud drapes. The underlying polymict conglomeratic sheet consists of poorly sorted lithofacies C, generally no more than 10 cm thick, alternating laterally between massive and faintly planar stratified (Table 1, Figs. 3, 5A, 12). The conglomerate layer contains intraformational mudclasts and has an uneven erosional base and a flat top locally covered with a thin discontinuous layer of lithofacies Sr.
Lithofacies association FA4, although the most muddy (Table 1), generally lacks bioturbation. Sporadic tiny sand spots (~ 1 mm in diameter) occur on the non-slabbed core surfaces, but it is unclear if these are some sand-filled very thin burrowing pipes or rather floating tiny aggregates (intraclasts) of very fine-grained sand.
Interpretation – The unit of muddy lithofacies Ml intercalated with sporadic very thin sheets of finest-grained sand indicates deposition in an offshore zone, where only some of the strongest storms would occasionally spread sparse sand (Howard and Reineck 1981; Reading and Collinson 1996; Hampson 2000). Non-bioturbated black mudshales may indicate either a prolonged episode of sand-starved anoxic seafloor conditions or a high-rate deposition of ubiquitous mud rich in organic carbon. The widespread occurrence of these muddy deposits directly above the gravel-lain erosional surface indicates an abrupt ultimate drowning of the retreating fluvio-deltaic system of the Helvetiafjellet Formation (Fig. 2C). Therefore, the discontinuous basal conglomerate sheet has been interpreted in the regional stratigraphy to be a transgressive lag, with the sand and gravel fraction derived by erosional reworking of the fluvio-deltaic substrate (Nemec et al. 1988; Gjelberg and Steel 1995).
The basal muddy unit (Fig. 12), recording the global episode OAE1a (Midtkandal et al. 2016, Fig. 3), was deposited in environmental continuity with the increasingly anoxic Svalbard lagoonal embayment of the topmost Helvetiafjellet Formation (Fig. 2C, D). The retreating broad Barremian delta of the Helvetiafjellet Formation (Gjelberg and Steel 1995) accumulated abundant organic-rich black mud (TOC 2‒5%) in its interdistributary bays and back-barrier lagoon (Fig. 2C; Nemec et al. 1988; Nemec 1992). The abrupt marine invasion probably resuspended the latest of these deposits and spread anoxia unhospitable to benthic fauna in the resulting Svalbard embayment (Fig. 2D), while increasing its bulk water energy, sparse sand delivery and aeration. The mud deposition at this stage may have involved resedimentation of bottom fluid mud, flowing in accordance to the local seafloor gradients (Allison et al. 2000; Traykovski et al. 2000; Sheremet et al. 2005; Ichaso and Dalrymple 2009). Large volumes of mobile seafloor mud might thus be gravitationally redeposited within the Svalbard embayment avoiding bioturbation (Mehta 1991; Trowbridge and Kineke 1994). The volumetric concentration of clay aggregates in fluid mud may reach 95% (Wells 1989), which allows the flow to carry silt particles and possibly tiny clumps of fine sand grains in its rigid-plug zone (Baas et al. 2009). Thin rhythmic sets of graded silty to clayey mud layers (Fig. 6F) may be due to a pulsating downward flux of sediment settling from storm-generated suspension plumes, caused by the depletion of their unstable density gradient by deposition-driven convection (Kerr 1991; Nemec 1995).