Comment: Carbonate production and reef building under ferruginous seawater conditions in the Cambrian rift branches of the Avalon Zone, Newfoundland by J.J. Álvaro and A. Mills, Sedimentology (71, 1245–1269)

Avalonia is a giant terrane (ca 2500 km long) in the middle Palaeozoic Acadian–Caledonian orogen (i.e. northern Appalachian and Caledonian mountains). Its significance is that fault-bounded exotic/suspect terranes are key parts of many collisional mountain belts (e.g. Jones et al., 1983; Howell & Howell, 1995), and a correct understanding of their provenance, palaeogeography and tectonic evolution is the basis for reconstructing the ‘pre-orogenic history’ of a mountain belt. Avalonia, now divided with opening of the Atlantic Ocean, features a large (ca 40 million) population in north-east coastal North America and southern Britain–northern Germany. Avalonia has received a number of alternative reconstructions since the mid-20th century (reviews in Murphy et al., 2019, 2023), with available evidence used to support its reconstruction as a unified ribbon microcontinent (Landing, 1996; Landing et al., 2022). Stratigraphy and sedimentary rocks define Avalonia by its distinctive terminal Ediacaran–Ordovician cover sequence dominated by siliciclastics, minor marine carbonates and local volcanics (Rast et al., 1976; e.g. Landing et al., 2022). The Avalonian cover sequence (ACS) unconformably overlies a basement collage (Keppie, 1985; Nance, 1990) of Precambrian arc, continental, oceanic and mélange blocks, increasingly regarded as of Baltic, not Gondwanan, origin (Landing et al., 2022; Beranek et al., 2023; Landing & Geyer, 2024). This Comment emphasizes that the ACS is unique among earliest Phanerozoic palaeocontinent cover sequences by its very shallow–peritidal, cool-water carbonates deposited on a persistently stratified platform in a strike–slip regime. The thin-bedded carbonates have an outsize role in understanding the Ediacaran–Ordovician history of Avalonia based on regional study of their detailed lithostratigraphy and sequence stratigraphy, sedimentology, biostratigraphy and U–Pb zircon geochronology since the late 1970s (e.g. Landing, 1996; Landing et al., 2022, 2023a, 2023b, and references therein). The authors' perspective is that Álvaro & Mills' (2024) report largely repeats Álvaro's (2021) review and conclusions on carbonate deposition at all localities he earlier described and on hydrothermal stockwork mineralization. In particular, there is no citation (nc) of numerous reports on the occurrence and significance of metre to multi-metre shoaling-up, mudstone–carbonate alternations (e.g. Landing & Benus, 1988a, 1988b; Myrow & Landing, 1992; Landing & Westrop, 1998, 2004; all nc) nor of limestones that form regional, thin transgressive systems tract (TST) and highstand systems tract (HST) markers that bracket undiscussed type 1 Avalonian depositional sequence (Ads) unconformities (i.e. Fig. 1, Ads 1–11). These 11 Ads are correlatable between North American and British Avalonia (Landing, 1996; Rees et al., 2014, nc; Landing et al., 2022, 2023a, 2023b, 2023c, with references). Many results of regional geological syntheses counter the Álvaro & Mills (2024; Álvaro, 2021) synthesis. Álvaro & Mills (2024; Álvaro, 2021) assert a half-graben (i.e. rift) tectonic regime that would mandate wedge-shaped lithosomes (Álvaro et al., 2023). However, a rift model is contradicted by an ACS architecture with remarkable, east–west lateral persistence (to 120 km; Fig. 1) that spans purported ‘half-grabens’ (Álvaro & Mills, 2024, fig. 8; Álvaro, 2021, fig. 9). Evidence for ‘half-grabens’ is not available in outcrop or stratigraphically, and there are only minor, lateral depth-related lithofacies changes or diachroneity of lithostratigraphic units in south-east Newfoundland. Indeed, thin (to 12 m) carbonates (Fosters Point Formation, lower Fossil Brook Member) and siliciclastic mudstones (for example, Manuels River Formation) and their bounding Ads unconformities (Fig. 1) bridge as many as nine purported ‘half-grabens’ in south-east Newfoundland (e.g. Landing, 1996; Landing et al., 2022, 2023a, 2023b; Landing & Geyer, 2024). The sole exception of lateral lithological changes is a more organic-rich and grey Fosters Point and lower Brigus Formation in near-shore, onlap facies in south-east Conception Bay (Landing & Westrop, 1998; Fig. 1). An Avalonian rift/half graben model is further countered by the long-known, fault-bounded, NNE-striking depocentres in south-east Newfoundland (i.e. Hutchinson, 1962; Landing & Benus, 1988a, 1988b, nc; Landing, 1996). These three major, terminal Ediacaran–Cambrian depocentres are not represented by the Álvaro & Mills (2024; also Álvaro, 2021; Álvaro et al., 2023) palaeogeographical cross-sections. The earliest depocentre is the Burin Peninsula basin in the west (Fig. 1). It comprises the Avalonian marginal platform (Landing & Benus, 1988a, 1988b, nc; references in Landing et al., 2022). It has the oldest ACS units (Rencontre–Random formations; Fig. 1) and features a peritidal limestone at the Ads 1–2 contact at Little Dantzic Cove (Landing et al., 1988, nc; Myrow & Landing, 1992, nc; Álvaro, 2021; Álvaro & Mills, 2024). Smith & Hiscott (1984, nc) emphasized that their analysis of Rencontre–Random deposition documented the characteristic lateral migration of depocentres of a strike–slip, not rift, regime (e.g. Steel & Gloppen, 1980). To the south-east on the Avalonian inner platform, Hutchinson (1962) recognized two post-Random, NNE-striking depocentres also not present in Álvaro & Mills' (2024; Álvaro, 2021) palaeogeographical reconstructions. Transtensional extension and subsidence of the Burin Basin was followed by transpressional uplift of the Burin Peninsula region and erosion of the Random Formation (Landing et al., 2022; Fig. 1, base and right column of figure). This epeirogenic activity was followed by renewed transtension and formation of the fault-bounded ‘Placentia–west Trinity axis’. This second formed depocentre extended east–west for ca 70 km from the eastern Burin Peninsula and across much of Fortune and Trinity bays. It bridges at least five purported ‘half-grabens’ in the Álvaro & Mills (2024, fig. 8) reconstruction. The Placentia–west Trinity axis (Landing & Benus, 1988a, 1988b, nc; Landing et al., 1988, nc; Landing et al., 2017 with references) has the thickest successions of the locally sub-trilobitic Bonavista Group with bedded carbonates of the Petley, West Centre Cove, Cuslett and Fosters Point formations (Fig. 1). These bedded carbonates form less than 2% of the mudstone-dominated succession in the middle of the axis and are laterally continuous units, not ‘wedge-shaped’ units supposedly characteristic of a ‘rift regime’ (see Landing & Benus, 1988b, figs 35, 36, nc). The oldest carbonate-rich units (Petley and West Centre Cove) are in the middle of the axis and the succession onlaps and thins into east Trinity and Fortune bays (Fig. 1). The thin (to 12 m) Fosters Point blankets all of the axis and is the lowest post-Random ACS unit in east Fortune and Conception bays. Hutchinson (1962) documented a third NNE-striking depocentre that opened further east (St. Mary's–east Trinity axis of Landing & Benus, 1988a, 1988b, nc), with the upper Cuslett and Fosters Point formations onlapping out of the Placentia–west Trinity axis as the oldest post-Random units (Fig. 1). This depocentre has the thickest successions of the Brigus and Chamberlain's Brook formations (Landing & Benus, 1988b, nc; Landing, 1996; Fletcher, 2006; Landing et al., 2022; Fig. 1, Ads 4A–7). The next depositional event featured the end of easterly basin migration with western onlap of the Brigus and Chamberlain's Brook formations with subsidence of the Burin Peninsula. This depocentre migration (i.e. east and then west) characterizes strike–slip and transform, not rift, basins (Steel & Gloppen, 1980; Noda, 2013). Trans-south-east Newfoundland limestone deposition of the lower (ca 3 m) Fossil Brook Member (Landing et al., 2023a; Fig. 1, Ads 7) is unaffected by any differential movement of the purported ‘nine half-grabens’ illustrated from Conception Bay to the Burin Peninsula by Álvaro & Mills (2024, fig. 8). Simply stated, the ‘half grabens’ and rift regime did not exist. The tectonic environment of south-east Newfoundland and elsewhere in Avalonia drove marine onlap with submergence by transtensional epeirogenic (not eustatic or ‘rift’) activity. The initial onlap sediments locally fill north-east/south-west conjugate fissures in the NNE-striking depositional basins (Landing & Benus, 1988a, 1988b, at Sunnyside, nc; Landing, 1992, fig. 3, at Bacon Cove nc; Landing & Westrop, 1998, at Chapel Cove and Duffs). These onlap limestones show the minor sulphide–sulphate boxwork mineralization in the Bonavista and Burin peninsulas and eastern Trinity Bay noted by Álvaro et al. (2022; repeated in Álvaro & Mills, 2024). In summary, after transtensional subsidence and Rencontre–Random deposition, a very ‘non-rift’ epeirogenic event (i.e. transpressional uplift) took place in the Burin Peninsula region with erosion of much of the Random that had also onlapped east into Placentia Bay (Landing & Benus, 1988b, nc; Fig. 1). This was followed by transtensional events that sequentially opened the Placentia–east Trinity and St. Mary's–west Trinity axes to the east and included later transtensional subsidence of the Burin region to the west. Comparable and coeval epreirogenic activity occurred elsewhere on the Avalonian marginal platform (i.e. south-east New Brunswick, Antigonish area, Nova Scotia; Mira belt, Cape Breton Island; South Wales) and inner platform (Rhode Island–eastern Massachusetts; Cradle Brook, south-east New Brunswick; Bonavista–Avalon peninsulas, south-east Newfoundland; North Wales; English Midlands) (Landing, 1996; Landing et al., 2022; Fig. 1). Álvaro et al. (2023) disputed deposition of the ACS in a strike–slip/transform regime because of a seemingly ‘too low’ depositional rate through the terminal Ediacaran–Ordovician. This must be put into context – most published data and reviews assert ‘paradigmatic’ high sediment accumulation rates (ca 3 mm/year) in transform regimes. However, high depositional rates along transforms are not invariable. Indeed, basins along the North Anatolian transform fault show local sediment accumulation rates of less than 1.0 mm/year (Şengör et al., 2005). The origin of Avalonia along a transform has been compared with the accumulation of a collage of Cretaceous and older continental and oceanic blocks on the modern North Scotia ridge transform (Landing et al., 2022). The sediment accumulation rate on the middle and western North Scotia transform since the middle Cretaceous varies from essentially nothing to only 0.04 mm/year in areas with 4.0 km of sediment (Lemenkova, 2021, fig. 3). Higher accumulation rates (0.7 mm/year) adjacent to the Avalonian transform can be estimated for the 2.2 km thick Rencontre–Random interval (e.g. Smith & Hiscott, 1984) over ca 30 Ma (i.e. 550 to 520 Ma; Fig. 1). Recent U–Pb volcanic zircon dates through the upper Lower–middle Cambrian of south-east Newfoundland show that the upper Brigus through to middle Chamberlain's Book formations bracket a surprisingly brief interval of time (Landing et al., 2023b) with possible depositional rates of these mudstone-dominated formations in excess of several mm/year. Álvaro & Mills (2024) and Álvaro (2021) assert ‘tilted blocks’, ‘tilting’, rift shoulders', ‘rift branches’, ‘horsts’ and ‘carbonate factories’ on ‘rift shoulders’ without providing any evidence of such syndepositional features. Álvaro & Mills (2024) cite Nance et al.'s (2012) report of an early ‘network of rift branches’ in justifying a rift model. However, Nance et al. (2012) actually summarized Murphy & Nance's (1989) proposal of a San Andreas-style transform fault in Avalonia's origin, which agrees with the strike–slip and transform model of Landing (1996) and Landing et al. (2022, 2023a, 2023b, 2023c). Exactly where carbonates on ‘rift shoulders’ is in south-east Newfoundland because many limestone units (for example, Fosters lower Brigus Formation, lower Fossil Fig. 1, Ads 3, 7) are and onlap the of the Random Formation or basement and for to 120 km across south-east Newfoundland (e.g. Landing et al., 2022). and erosion of the basement very minor and and which show at the Fosters Formation and St. Cove Member is not by of any ‘rift (see Landing & Benus, nc; Landing, nc; Landing & Westrop, The only with to movement by strike–slip (ca m) features the at the base of the Manuels River Cambrian succession (Landing & Westrop, ACS carbonates in not the (Landing & Benus, 1988a, 1988b; Landing et al., Landing, 1992; all nc). Álvaro & Mills (2024) later facies with of the thin limestone at the of their Fig. 1) at Little Dantzic Cove with of carbonate and of the Random However, the Random is ca in the and the Member is the facies that the limestone deposition (i.e. Myrow & Landing, nc). bedded mudstone–carbonate alternations are characteristic of the Cambrian of the Avalonian platform in south-east Newfoundland and form (Landing & Benus, 1988b; Landing, 1992; Myrow & Landing, 1992; all nc). with the thickest (to 12 m) limestone in south-east Newfoundland – the Fosters Point Formation of Landing & 1988b, nc; Fig. 1, Ads 3). This is the of Álvaro (2021) and Álvaro et al., 2022, figs and the of Álvaro & Mills Landing & nc) Cuslett Formation (Fig. 1, lower Ads has three alternations followed by a the Fosters These are 1 to of Landing et al. 8; Landing & Benus, 1988b, Landing, 1992; Landing & Westrop, 1998, all nc). The Cuslett three alternations and unit are in Trinity Bay as with the older and West Centre Cove are not as key Cambrian in Álvaro & Mills' (2024, fig. Landing & nc) the Bonavista Formation to as a number of carbonate-rich formations and are Álvaro & Mills (2024, figs 8) the of Hutchinson Avalonian alternations & Landing, 1992, nc; Landing & Westrop, nc; Landing et al., successions from grey or through to with of increasingly numerous and a from into the which increasingly and followed by of with thin and with and on the This is a facies (e.g. Landing et al., 1988, nc; Landing, 1992, nc), a also used by Álvaro & Mills The Little Cove limestone (Fig. 1, Ads 1–2 is an onlap unit (Landing et al., 1988, Landing et al., nc; Landing et al., Landing & Westrop, 1998, nc). The of has a distinctive the upper of the limestone changes from a facies into an marine with a not noted by Álvaro & Mills, 2024, or Álvaro, Álvaro & Mills' (2024, figs 8) must be by a number of limestones that bracket Ads Álvaro & Mills (2024) evidence for a and report a Point of Hutchinson The of a the by Landing, 1996; Landing & Westrop, 1998; Landing et al., has been with a lower interval the Fosters Point Formation Ads by an with and and local and and an limestone the base of the Brigus Formation Fig. 1). The of a (Landing, 1992, nc) is by a ca 520 Ma on the Fosters Point Formation and ca Ma on the of the Brigus Formation (Landing & Landing et al., 2023c). that with must show a lower 3 to a (to ca 12 and a thin (to 1 carbonate to the base of the Brigus limestones must also be at the Ads contact (Fig. 1) that the Brigus Formation into lower St. and upper Cove (Landing & Westrop, 1998; Landing et al., Fig. 1). However, evidence for regional depositional sequence is not of their and the Brigus is in Álvaro & Mills (2024, figs 8; Álvaro, 2021, fig. 9). In and an of the Cambrian Chamberlain's Brook Formation (Álvaro & Mills, 2024, figs 8; also Álvaro, 2021, fig. Álvaro et al., fig. to be as by Landing et al. and Landing & The Chamberlain's Brook is of three Ads a lower Cove middle and upper Fossil Brook with an unconformably Manuels River Formation 8) (e.g. Landing, 1996; Landing et al., 2022, 2023a; Fig. 1). The onlap facies of the thin Fossil Brook Member (ca m) show thin at the base of Ads from southern New to south-east Newfoundland (e.g. Landing et al., 2023a, with references). Avalonian Cambrian limestones from coeval West limestones and of Indeed, Álvaro & Mills' (2024) of the limestones or is in south-east Newfoundland and elsewhere in Avalonia because there are no in all limestones the Fossil Brook Member (Fig. 1). Indeed, Avalonian are An is in the Cambrian in the Cambrian of (i.e. is dominated by & In North only the middle Cambrian Fossil Brook Member of Álvaro & Mills, 2024, fig. by Landing & Westrop, 1998; review in Landing et al., 2023a; Fig. 1) has limestones in Avalonia from to and of limestone and siliciclastic are in to facies (e.g. Landing et al., Landing, 1992; Myrow & Landing, 1992; Landing & Westrop, all nc). Álvaro & Mills (2024; Álvaro, 2021) that Avalonian and West Cambrian limestones formed Álvaro et al., or and regimes. However, the facies characteristic of West carbonates or of and e.g. Álvaro et al., are in Avalonia (e.g. Landing et al., Landing et al., 2022). Álvaro & Mills (2024; Álvaro, 2021) that such facies are also in more areas in and are in the purported facies of south-east Newfoundland. This the that Avalonian bedded limestones are at or in and the and (e.g. Landing et al., in purported West (Álvaro & Mills, not a depositional for Avalonian to a with or without as by its on the Cambrian of the & or of et al. that in Avalonia a However, the occurrence of modern in of modern New a occurrence of (Landing & 2024). in Avalonia and West is that are in Avalonia and locally in West (e.g. & & Landing, of is noted not illustrated in Álvaro & Mills from in have a (see in Avalonia, an which seemingly a of Avalonia by et al.'s The Avalonian to in (to by These are the with and and the possible with a along its from siliciclastic to peritidal limestones where are in through the through time (Landing et al., Landing & Westrop, 2004; nc). at the of the West Centre Cove Formation has been detailed (Landing & Benus, 1988a, 1988b, nc; Landing et al., 1988, Landing et al., Landing, 1992, Landing, Landing & Westrop, 1998, nc; Álvaro & Mills, 2024). In in Avalonian that the are (e.g. Landing & Benus, 1988a, 1988b; Landing et al., nc; Smith Point The most comprises the of the Fosters Point Formation right the Ads This across much of south-east Newfoundland; in the Mira Cape Breton Island; and the in (Landing et al., 2022, figs and nc; Fig. 1 The of between Avalonia and West and brief of Cambrian interval (i.e. Álvaro et al., the to Ma interval (Landing et al., be as evidence for on the two This is by the palaeogeographical of Avalonia and West (Landing & Westrop, nc; Landing & and with present in the earlier Cambrian and later Cambrian (Landing et al., 2022). only are Avalonian and West bedded limestones the Avalonian limestones to be the only high Cambrian The most for Cambrian Avalonia of and of a in a marine facies without any in south-east New (Landing & fig. Landing et al., 2023c, fig. Fig. 1). the the in the are and not evidence of Álvaro et al., 2022). Indeed, has been for over in sediments & as a of et al., and as et al., The of across Avalonian south-east Newfoundland in of minor boxwork in carbonates is (i.e. Álvaro, 2021; Álvaro & Mills, 2024). Indeed, the alternations have been used to a basin (Landing et al., Landing et al., Myrow & Landing, 1992, nc; Landing & Westrop, nc). This facies that the (i.e. in the late in Newfoundland and elsewhere in Avalonia (e.g. Landing & Westrop, Landing et al., as a to across the Avalonian platform to of in carbonates is was on the Avalonian as by and alternations in the most facies of the Cuslett Formation at in the northern Bonavista Peninsula (Landing & Benus, 1988a, Landing et al., 1988, nc; Landing & nc; Landing et al., is in many reports (e.g. & to the origin of facies and is by on marine Indeed, Álvaro & Mills (2024, fig. model across ca 120 of the Avalonian However, no modern or of across very (for example, with the and between of and onlap would to such regional In the purported evidence of in many reports (for example, or organic-rich sedimentary not of lower In the of Avalonia local lower to epeirogenic very to the in the Cambrian (Landing et al., 2023a, any of into facies is an the model of It be further emphasized that lateral changes in lithofacies in Avalonian Cambrian basins & Landing, 1992, nc). It is possible at modern at Cove, eastern Trinity to from peritidal limestones into alternations of the Cuslett Formation several (Landing & Westrop, An alternative of the boxwork mineralization in Álvaro & Mills (2024; Álvaro, 2021) must the that the mineralization is in very onlap limestones that define the base of Avalonian depositional sequences (Fig. 1) – Avalonian depositional sequences are key of North American and British Avalonian depositional architecture Fig. 1, Ads that their reports not with sediment transtensional that to epeirogenic subsidence and marine onlap These would have the hydrothermal for the mineralization (i.e. Álvaro, 2021; Álvaro & Mills, coeval with epeirogenic subsidence and onlap of by the model. volcanics in south-east Newfoundland and Avalonian at the base and of Avalonian depositional are also thickest along the axes of the fault-bounded strike–slip Cambrian depocentres (Landing et al., 2022, 2023a; Fig. 1). these depocentres and epeirogenic regime are not of the tectonic by Álvaro & Mills Landing et al. three of Cambrian on St. Mary's–east Trinity axis in south-east Newfoundland. The earliest is a sequence of volcanics that and form the lower Manuels River Formation of Ads 8) at on the east of Trinity Bay (Fig. 1, volcanic The second is a Cove volcanics of Fletcher, 2006; Fig. 1, to the in the upper Manuels River Formation Ads 8) on the west of St. third volcanic on the St. Mary's–east Trinity axis is at the end of Trinity These Chapel volcanics of Fig. 1, have and been to the Manuels River Formation (for example, Cove in Mills & Álvaro, 2023; Álvaro & Mills, 2024). The of the Chapel in Álvaro & Mills, is actually have only a than that by Hutchinson, and the in the lower of the and Cove The has been and to the Formation Landing et al., 2023b; Fig. 1). limestone the and at Its formation is to be (Álvaro & Mills, with no evidence for (Landing and Westrop, is not have noted that are in on at It is possible that much of the carbonate from with and of at a (e.g. Landing et al., Landing & with to and mudstones that (Álvaro & Mills, was not followed by of the in a regime The available evidence is that an older of the Avalonian basement is by thick of the Group in Newfoundland and in the coeval Point Group in Bonavista and St. peninsulas (Landing & Geyer, 2024). The was deposited by a (e.g. Beranek et al., 2023). the terminal Formation in the Burin Peninsula (Fig. 1) has a m) of lower that into with easterly & Hiscott, The Random Formation an of sediment that basement was over the strike–slip that the Burin Basin (e.g. Smith & Hiscott, Myrow & Hiscott, minor and on such Ads as the of the Fosters Point Formation and St. Member (Fig. 1, blocks at the base of the Manuels River a local (Landing & Benus, Landing & Westrop, 1998; Landing et al., In and formation of sediments did not place in the and the of the Avalonian basement by the Group was in the Comment Avalonia as a ribbon microcontinent by a distinctive late Ediacaran–Ordovician cover sequence that unconformably overlies a basement collage of continental, and oceanic The cover depositional history and architecture are with a strike–slip, not rift, regime by activity on a transform fault to the modern North Scotia east–west succession of to nine coeval syndepositional in south-east Newfoundland be by data that mandate three strike–slip fault-bounded depocentres that and to the south-east in eastern Newfoundland. Avalonia is in palaeogeographical reconstructions as its succession the only of Cambrian limestones deposited on a persistently The limestones epeirogenic history because in alternations and also and onlap facies at depositional sequence and Cambrian Landing et al.'s proposal of Cambrian of and and on Avalonia and was by and Murphy and an for their The have no of to is not to as no data or in

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