4.a. FA1: coarse conglomerate (alluvial fan)

These are thick sequences of reddish-brown conglomerate, tens to hundreds of metres thick (Figs 3a, 4a, 6), located at Jersey, Montmartin, Erquy and Bréhec. Stratified pebble conglomerates predominate. Chaotic cobble and boulder conglomerates are also common (Went et al. Reference Went, Andrews and Williams1988; Suire et al. Reference Suire, Dabard and Chauvel1991; Went, Reference Went2005, Reference Went2017). They are interpreted as streamflood and debris-flow deposits, respectively, associated with alluvial fans. Palaeocurrents from these deposits suggest local lateral drainage systems, feeding trunk rivers (e.g. Fig. 9).

4.b. FA2: coarse arkosic sandstone, locally pebbly (braided fluvial)

These are pinkish-red, very coarse-grained, feldspar-rich sandstones, locally pebbly or containing thin beds of pebble conglomerate (Figs 3b, 4b). Good examples of this facies association are present at Alderney, May sur Orne, Erquy-Cap Fréhel and Bréhec (Figs 5, 6). The sandstones are arranged in sheet-like beds with scarce conspicuous channel margins. They are typically trough cross-stratified and show unidirectional, basinward-directed palaeocurrents. Argillaceous beds are scarce, forming less than 1% of these facies; where present, they contain common sand-filled polygonal (desiccation) cracks. Thicker intervals of red mudstone at Erquy and Bréhec contain the enigmatic fossil or microbially induced sedimentary structure Arumberia (Glaessner & Walter, Reference Glaessner and Walter1975; Bland, Reference Bland1984; Davies et al. Reference Davies, Liu, Gibling and Miller2016; McMahon et al. Reference McMahon, Davies and Went2017). These are interpreted as fluvial strata and possess architectural characteristics typical of many prevegetation river deposits (Cotter, 1978). The strata are interpreted as mostly braided fluvial in origin, locally accompanied by the deposits of sinuous, possibly meandering, streams in low-gradient coastal plain settings. These deposits are discussed in detail by Todd & Went (Reference Went1991), Went (Reference Went2017) and Went & McMahon (Reference Went and McMahon2018).

4.c. FA3: feldspathic sandstone (deltaic, transgressive systems tract)

These are very-fine- to medium-grained, feldspathic sandstones showing either: (1) erosively based, sheet-like beds of medium-grained, trough cross-stratified sandstone; (2) tabular to wedge-shaped beds of very-fine- to fine-grained hummocky cross-stratified and wave-rippled sandstone (Fig. 4c); or (3) tabular to channel-form geometry beds of very-fine- to fine-grained, massive or parallel laminated sandstone (Fig. 3c). Trace fossils are locally common in the hummocky cross-stratified and wave-rippled sandstones and include Planolites, Diplichinites, Cruziana and Bergaueria. The massive or parallel laminated sandstones show load and water escape structures and are locally intercalated with intervals of more argillaceous strata showing evidence of syn-sedimentary downslope movement and deformation. These deposits are collectively interpreted as deltaic sandstones (Went, Reference Went2017, Reference Went2020). They represent a mixture of: (1) delta plain to delta-front distributary channel deposits (MacNaughton et al. Reference MacNaughton, Dalrymple and Narbonne1997); (2) wave-influenced delta-front strata (Yang et al. Reference Yang, Dalrymple and Chun2006); and (3) river-fed, delta-front turbidity current deposits (hyperpycnites) (Plink-Björklund & Steel, Reference Plink-Björklund and Steel2004). The trace fossils attest to a seabed colonized by sea anemones, worms and arthropods (Seilacher, Reference Seilacher1955; Pemberton & Frey, Reference Pemberton and Frey1982; Hiscott et al. Reference Hiscott, James and Pemberton1984; Mata et al. Reference Mata, Corsetti, Corsetti, Awramik and Bottjer2012). Good examples of this facies are found at La Hague, May sur Orne, Lessay and Bréhec (Figs 5, 6).

4.d. FA4: feldspathic sandstone and siltstone, locally with coarser lenses (lower delta front, highstand systems tract)

This facies association is exposed at a limited number of road cuttings and quarries at Pont Farcy and St Rémy, where it typically forms beds 0.5–1 m thick of structureless to parallel-laminated fine to very-fine-grained sandstone (Fig. 3d). At each locality, some more thinly bedded intervals occur that locally show ripple forms and the trace fossil Diplichinites. Some beds at the top of the section at Pont Farcy are reported to contain some coarser lenses (Doré, Reference Doré, Chantraine, Rolet, Santallier, Piqué and Kleppie1994). Diplichinites represent arthropod trackways and are consistent with deposition in marine water. The thick beds of structureless to faintly laminated sandstone resemble turbidity current deposits. They are tentatively interpreted as hyperpycnites (river-fed, shallow-water turbidity current deposits) of marine delta-front origin.

4.e. FA5: quartz arenite and protoquartzite (nearshore shelf)

These are pinkish white, texturally and mineralogically mature or submature, medium to very coarse-grained sandstones (Figs 3e, 4g). They are predominantly planar laminated and contain common intervals showing tabular or trough cross-stratification. These sandstones are best observed at Erquy, where they attest to deposition in shallow water under the action of powerful waves and currents. Some of the currents were evidently persistent, building extensive sandwaves, suggesting a possible tidal origin. They are interpreted as nearshore-marine deposits (Went, Reference Went2013).

4.f. FA6: green to locally purple, thin-bedded sandstone and siltstone (offshore shelf)

These are thin beds of typically greenish-grey, very-fine-grained sandstone and siltstone (Fig. 3f). Beds showing primary sedimentary structures predominate and include current and wave-formed ripples, localized hummocky cross-stratification, convolutions and synaeresis-style shrinkage cracks. Intervals showing Kinneyia-type wrinkle structures (Hagadorn & Bottjer, Reference Hagadorn and Bottjer1997; Porada & Bouougri, Reference Porada and Bouougri2007; Herminghaus et al. Reference Herminghaus, Thomas, Aliaskarisohi, Porada and Goehring2016) or trace fossils are also common. Trace fossils include Planolites, Teichichnus, Cruziana, Rusophycus, Diplichnites and Bergaueria (Fig. 4d, e). They locally contain scarce body fossils (e.g. hyoliths, bivalves (Fordilla) and gastropods (Circatheca)). They are interpreted as open-marine shelf sandstones and siltstones that were periodically subject to reworking by storms (Went, Reference Went2020). Kinneyia-type wrinkle structures suggest periods of algal matground development (Porada & Bouougri, Reference Porada and Bouougri2007), whereas bioturbated sedimentary strata point to a seabed widely colonized by worms, arthropods, sea anemones and rare shelly faunas (Pemberton & Frey, Reference Pemberton and Frey1982; Hiscott et al. Reference Hiscott, James and Pemberton1984; Yang et al. Reference Yang, Dalrymple and Chun2006; Mata et al. Reference Mata, Corsetti, Corsetti, Awramik and Bottjer2012; Moysiuk et al. Reference Moysiuk, Smith and Caron2017). Locally, the colour is purplish rather than greenish-grey. The purple colour may reflect a shift in rock chemistry, possibly to one containing more oxidized manganese-phosphate (Johnson et al. Reference Johnson, Webb, Ma and Fischer2016).

4.g. FA7: magnesian limestone and dolostone (nearshore-marine carbonate platform)

These are stacked beds 30–60 cm thick of massive, commonly pinkish-coloured high-Mg limestone and dolostone (Fig. 3g). The beds are massive, exhibit crinkly lamination and locally contain stromatolites. They tend to occur towards the base of the study interval (Fig. 2). These are interpreted as open-marine carbonate platform deposits that have been dolomitized. The upwards transition from conglomerate to high-Mg limestone and dolomite points to marine transgression. The preferential development of dolomite deep in the succession suggests the possibility that the dolomitization process occurred close to shore. Facies belts move landwards during transgression and it may have been the case that, whereas dolomitization occurred nearer to shore, unaltered limestones were preserved in more basinwards locations, and hence are present higher in the transgressive systems tract (TST) of the study section (Fig. 2). This is common in carbonate platforms, with various models available to explain the systematic distribution of dolomite and limestone (Rahimi et al. Reference Rahimi, Adabi, Aghanabati, Majidifard and Jamali2016). A terrestrial source of high-Mg waters is favoured in this case, which promoted dolomitization of limestone close to shore and/or close to the unconformity. Dolomitization is interpreted to have occurred during early diagenesis and shallow burial as high-Mg meteoric pore waters mixed with those of marine phreatic origin (Humphrey & Quinn, Reference Humphrey and Quinn1989; Rahimi et al. Reference Rahimi, Adabi, Aghanabati, Majidifard and Jamali2016).

4.h. FA8: detrital and stromatolitic limestone (offshore marine carbonate platform)

These are intervals of limestone, 5–10 m thick, that are either thinly bedded (10–20 cm thick beds) and detrital, or which show mounded bedding with cauliflower-like lamination formed by domal to columnar stromatolites (Fig. 3h). The detrital limestones are horizontally or locally cross-bedded and commonly oolitic (Went, Reference Went2020). Less commonly, the detrital limestones contain bioclasts of archaeocyathans (Doré, Reference Doré1972, Reference Doré, Chantraine, Rolet, Santallier, Piqué and Kleppie1994). The limestones are interpreted as shallow-water, open-marine, high-energy offshore detrital shoals and stromatolite reefs, akin to those seen in modern-day Highborne Cay, Bahamas (Chafetz, Reference Chafetz1973; Reid et al. Reference Reid, Visscher, Decho, Stolz, Bebout, Dupraz, Macintyre, Paerl, Pinckney, Prufert-Bebout and Steppe2000; Dupraz & Visscher, Reference Dupraz and Visscher2005; Purkis & Harris, Reference Purkis and Harris2017). Intervals of increased siliciclastic influx periodically resulted in a cessation of limestone deposition and accumulation of siltstone and very-fine-grained sandstone (Fig. 5).

4.i. FA9: rhyolite (extrusive volcanic)

Extrusive volcanic rocks are present in the Pay de la Loire outside the main study area (Fig. 8). They are mostly acidic, dominated by pyroclastic deposits of welded and non-welded ashflow tuffs, but with a minor component of andesitic lavas (Doré, Reference Doré, Chantraine, Rolet, Santallier, Piqué and Kleppie1994).

For the first eight sedimentary facies associations, six main environments of deposition are identified. These are alluvial fan, braided fluvial, deltaic, nearshore-marine, offshore siliciclastic marine, carbonate nearshore and carbonate offshore. For the purposes of preparing maps of gross depositional environment, alluvial fan and braided fluvial environments are grouped together, as are deltaic and nearshore-marine environments, and the two carbonate environments. This reduces the number of discriminated environments presented in the maps of gross depositional environment to four: (1) alluvial; (2) deltaic/nearshore; (3) siliciclastic offshore; and (4) carbonate platform (Fig. 9a, b).

5.a. Basin limits

The thickness of the Cambrian strata through Lower Normandy varies greatly and systematically, defining two major basin fills (the North and Middle Normandy basins). Figure 7 shows the facies architecture of the basin fills in cross-section with the key outcrops located in true geographic position, rather than evenly spaced as in Figure 2. The North and Middle Normandy basins are separated by a NE-trending ridge termed the Cotentin Swell, which shows a condensed lower Cambrian succession as seen at Montmartin (Fig. 2). The south of the Middle Normandy basin is defined by an emergent area known as Mancellia. A third basin is found further south in the Pays de la Loire (Doré, Reference Doré, Chantraine, Rolet, Santallier, Piqué and Kleppie1994). A detailed evaluation of the Cambrian strata in this latter basin lies outside the scope of this study. However, a tentative correlation to strata contained within it is proposed in Figure 8.

5.b. Sequence stratigraphy

The definition of the sequences in this study broadly follows that of the Exxon model (Van Wagoner et al. Reference Van Wagoner, Mitchum, Campion and Rahmanian1990). Specifically, the base of each sequence is defined by an erosional unconformity or its correlative conformity. The Exxon method was preferred over the genetic sequence stratigraphy (Galloway, Reference Galloway1989) or transgressive–regressive sequence stratigraphy (Embry, Reference Embry2002) methods, since erosional unconformities overlain by coarse siliciclastic strata are conspicuous in all but one basin centre locality where the equivalent correlative conformity had to be identified. It should be noted that these are tectonically active basin fills and the sequences developed reflect a combination of tectonic subsidence, sea-level change and sedimentation rate (Rattey & Hayward, 1993). Typically, subsidence is rapid relative to sedimentation rate, resulting in thick, deepening upwards (retrogradational) facies transitions (Figs 5, 6). The concept of systems tracts is still valid. Lowstand systems tract (LST) deposits reflect lowered base level, basin margin erosion and a basinwards shift in facies. A marine transgressive surface marks the point where LSTs are over-run by rising sea levels and facies belts move landwards. In these sequences, following the initial marine transgression, a thick interval of marine strata accumulates, which gradually onlaps the basin margin (TST). Where more or less complete sequences are preserved, a reduction in the rate of sea-level rise is marked by a return to progradation at the basin margin and sandstone deposition in the basin (highstand systems tract or HST). Lowstand, transgressive and highstand systems tract deposits may each be identified. However, the TST is much thicker than that seen in passive margin settings, typically dominating the sequence. The HST is commonly truncated (Fig. 7).

5.c. Sequence 1

The lowermost Cambrian deposits in both the North and Middle Normandy basins are typically conglomerates or coarse, pebbly arkosic sandstones (FA1 and 2). These coarse siliciclastic deposits of the LST vary in thickness but are generally more conspicuous at the basin margin, for example at Alderney/La Hague or Erquy, where continental or marginal marine environments of deposition are indicated (Figs 2, 6). The coarse sediment size suggests a period of uplift and erosion associated with a period of relative sea-level lowstand.

In the North Normandy basin, the coarse basal deposits are succeeded by thick intervals of green to locally purple, very-fine-grained, bioturbated sandstones and siltstones (FA6) in basin centre locations (Fig. 3f). These fine-grained sediments locally contain hyolith and chancellorid fossils, consistent with a Stage 2 Cambrian age. At Lessay, scarce beds of granular sandstone locally occur interbedded with green, thinly bedded, very-fine-grained sandstone and siltstone (Fig. 4h). These anomalously coarse beds are interpreted as storm deposits, and the sediment is inferred to have been transported from a shoreline that lay in close proximity to Lessay. Although the deposits of such an environment are not preserved, the evidence of coarse alluvial deposits in Jersey suggest that associated trunk drainage systems most likely existed that would have dispersed E-wards toward the coast. The reworking of the resulting shoreline or delta by storms may explain the scarce interbeds of granular sandstone in otherwise very-fine-grained sandstone and siltstone. In other words, the anomalous coarse layers may represent a tenuous link to the bedload drainage systems in Jersey.

At Carteret, fossiliferous limestone (FA8) occurs interbedded with green sandstone and siltstone towards the top of the sequence. These beds contain archaeocyathans (sponge-like reef-builders) and the trilobite Bigotina, indicative of deposition during Stage 3 of the Cambrian Period (Pilola, 1993). The limestones are interpreted to reflect deposition in offshore carbonate banks at a time of near-maximum transgression (Brasier, Reference Brasier1980), that is, early in the HST when siliciclastic supply to the basin was limited. Sandstones lying above the limestones near Carteret may reflect the local preservation of deposits associated with the HST in the basin centre or, alternatively, the deposits of the falling stage systems tract (FSST) or LST of the overlying sequence (Fig. 7a).

Thick successions of feldspathic sandstone or conglomerate are locally prominent in basin margin settings, for example at La Hague (Figs 2, 6). Here 600 m of bioturbated sandstone of deltaic origin (FA3) fines upwards as a gradually retrogradational parasequence set, suggestive of deposition during a steady rise in relative sea level following the initial marine transgression. Sequence 1 is 500–2100 m thick in the North Normandy basin, where it forms most of the Cambrian section.

In the middle Normandy basin, Sequence 1 is thinner (300–700 m thick) and forms the lower one-third of the total Cambrian section (Figs 2, 5). Carbonate rocks (FA7 and 8) are common in the TST in the east of the basin throughout Sequence 1, where they alternate with intervals of green sandstone and siltstone. They are typically magnesium-rich limestones and dolostones in the lower parts of the TST and Mg-poor limestones in the upper part. The restriction of coarse siliciclastic sediment to the west of the middle Normandy basin, together with the presence of limestones in the east, suggests siliciclastic input was predominantly from the west. This is similar in terms of direction to that in the North Normandy basin. However, in terms of volume, the siliciclastic input was evidently much less than that in the northern basin.

5.d. Sequence 2

Thick intervals of coarse-grained, arkosic sandstone characterize the basal units of this sequence in the Middle Normandy Basin at May sur Orne, near Caen (Fig. 5a). The sandstones at this locality fine upwards over 625 m. The lower 300 m are trough cross-stratified, coarse-grained sandstones of probable braided fluvial origin (FA2, Fig. 3b). They are overlain by massive to parallel laminated, fine-grained sandstones of delta-front origin (FA3, Figs 3c, 5a). Further west, in the Zone Bocain at St Rémy, feldspathic sandstones (FA2 and 3) fine upwards into green sandstones and siltstones (FA6). The latter contain sparse bivalve (Fordilla) and gastropod (Circtheca) faunas. These faunas confirm an open-marine environment in the centre of the basin of early to early middle (Stages 3–5) Cambrian age (Yu, Reference Yu2002; Elicki & Gursu, Reference Elicki and Gursu2009). The successions at May sur Orne, Urville and St Rémy suggests a retrogradational stacking associated with rising relative sea level. The green sandstones and siltstones are also present at Pont-Farcy where they are succeeded by an upwards coarsening succession of siltstones, sandstones and arkoses with coarser lenses (FA4; Figs 2, 3d, 5b). Pont Farcy is the one locality where the boundary between Sequence 1 and 2 is identified via a correlative conformity. It is picked within an interval of green sandstones and siltstones (FA6) at a level which conceivably represents the point of maximum flooding at the top of Sequence 1. Coarser siliciclastic deposits of FA4 are also found at the top of the succession at St Rémy, and suggest the progradation of a siliciclastic wedge during more slowly rising relative sea levels of the HST.

A 300-m-thick succession of sandstone of fluvio-deltaic origin characterizes Sequence 2 at Lessay. Palaeocurrents point to flow from the south. The occurrence of thick Sequence 2 sandstones north of the Cotentin swell is local to Lessay and may in part reflect preservation in the hanging wall of a horst block. However, the rate of thickness and facies change is such that a palaeovalley fill interpretation is suggested, the valley having been cut during the falling stage with the fill occurring during rising sea levels in the lowstand and transgressive systems tracts (Fig. 7a).

5.e. Correlation to the south Normandy basin and maps of gross depositional environment

The facies distributions and thickness of strata in the basins allows for the development of maps of palaeoenvironment (Fig. 9). To broaden the scope of the maps, a tentative correlation of the Cambrian rocks of the north and middle Normandy basins into the south Normandy basin in shown in Figure 8. The siltstone and stromatolite-bearing carbonate succession at Couvron–Charnie is comparable to the siltstones and carbonates formation at May sur Orne and, on a lithostratigraphic basis, can reasonably be correlated with it. The overlying St Suzanne and Sille formations contain the brachiopods Leptobolus and Tomasina, respectively (Doré, Reference Doré, Chantraine, Rolet, Santallier, Piqué and Kleppie1994). Leptobolus is from the Obolellida family, which is restricted in age to the lower and middle Cambrian (Gould & Calloway, Reference Gould and Calloway1980). A correlation of the rocks of the St Suzanne Formation with those of the Caumont Formation (LST and TST of Sequence 2) in May sur Orne therefore seems reasonable. Correlation is complicated by the presence of extrusive volcanic rock. Nevertheless, broad palaeoenvironmental zones can be identified and show similarities with the more northerly basins. Basin margin facies are alluvial and may transition to the marine environment via deltas. Basinward facies are either siliciclastic in Sequence 2, or a mixture of siliciclastic and carbonate rocks in Sequence 1. They exhibit stromatolites, trace fossils and wave-formed sedimentary structures, and are consistent with a shallow-marine environment of deposition.

5.f. Basin evolution

The thickness and facies changes observed in the sequence stratigraphic correlation (Figs 7, 8) suggest the lower Cambrian strata accumulated under a tectonic control. Coarse alluvial fan deposits at Bréhec, Erquy, Montmartin and Jersey point to accumulation adjacent to active faults. Palaeocurrents in these units suggest an E–W orientation to these faults, which acted to define the margins to the basins in landwards locations. At Montmartin the deposits accumulated at the northern margin of the middle Normandy basin, south of the Cotentin swell, a horst block that was a positive feature throughout the early Cambrian Period but which was partly drowned at the end of Sequence 1 times (Fig. 9a). The other alluvial fan deposits occur at the westwards limits to the northern and middle Normandy basins. The deposits of major river systems into which the alluvial fans dispersed show palaeocurrents, indicating drainage parallel to the basin margin fault systems (Went & Andrews, Reference Went and Andrews1991). In Sequence 1, the prevailing and typically gentle, basin floor slope was consistently to the east. Carbonates accumulated periodically in the easterly parts of the basin, distant from the source of siliciclastic influx (Figs 7, 9). In Sequence 2, fault-controlled subsidence increased, resulting in a thicker basin fill. Fluvial sand input continued from the west and was joined by a significant new supply from the east, NE of Caen (Fig. 7). Increased siliciclastic input effectively ended carbonate deposition. Basin centre deposits were composed of sandstone and siltstone. There is an incomplete picture of sedimentation during Sequence 2 in the northern basin. Sandstone-rich fluvio-deltaic strata were deposited in the far south around Lessay. Further north, any deposits were removed by uplift and erosion during late Cambrian time, prior to a renewed phase of subsidence and deposition during early Ordovician time.