Unconformity-bounded rift sequences in Terreneuvian–Miaolingian strata of the Caledonian Highlands, Atlantic Canada: Reply Open Access

Landing et al. begin their Comment by criticizing our geographic selection of study area in the Caledonian Highlands “limiting Avalonia to a small New Brunswick area that actually extended far SW and NE of Saint John and thus well beyond the Caledonian Highlands.” Obviously, the aims of our paper were restricted to our selected area of study, which included five stratigraphic sections and eight facies associations. Nothing in our paper suggests that our purpose was to synthetize the Cambrian paleogeography of all Avalonia.

The lithostratigraphic subdivision subsequently advocated by Landing et al. for the Caledonian Highlands and the entirety of Avalonia has serious problems. Many of the stratigraphic packages display “wedge-like stratal patterns” and pinch out on the scale of hectometers to kilometers: e.g., the slope-related breccia to conglomerate deposits marking the base of the Handren Hill and Mystery Lake members in the Caledonian Highlands (Pickerill and Tanoli, 1985; Álvaro et al., 2023), the conglomerates of the Rencontre Formation (Smith and Hiscott, 1984), and the reddish thin- to thick-bedded limestone beds and nodules grading laterally into nodular limestone nodules and layers embedded in shales that punctuate the Bonavista Group and the Smith Point and Brigus formations (e.g., Hutchinson, 1962; Fletcher, 2006). Many of their basal and inter-bedded erosional contacts (potential unconformities) cannot be followed laterally (for the Caledonian Highlands, see Pickerill and Tanoli, 1985; Tanoli and Pickerill, 1988, 1990). Specific examples are highlighted here:

• (1) Landing et al. consider that many marker beds, less than 10 m thick (they call them “lithosomes”), can be followed “from Rhode Island to Belgium.” They report, as an example, the Fosters Point Formation in the Avalon Zone of Newfoundland, Canada. This artificial unit was proposed by Landing and Benus (1987) to subdivide the Smith Point Formation (“the most useful horizon marker within the Cambrian sequence,” according to Hutchinson, 1962, p. 13–14) into a lower trilobite-free part (Fosters Point Formation) and an upper trilobite-bearing part (Cuslett Formation). The argument was not lithologic but based on the presence of a supposed “cryptic unconformity” separating them, which those other than Landing and Benus (1987) have not been able to recognize. However, the lithology of the Smith Point Formation does not occur in our study area of the Caledonian Highlands (even Landing, 2004, did not report it). So how can Landing et al. claim that this marker bed can be easily recognized from Rhode Island (USA) to Belgium? If we accept that any bed underlying the lowest occurrence of trilobites is useful for correlation, we can follow it worldwide simply by selecting the bed, no matter its lithology, which underlies the first occurrence of trilobites. But this artificial correlation is not based on lithology (the main feature of lithostratigraphic units), but on the fossil content (a biostratigraphic character). The rules of the International Commission on Stratigraphy preclude the procedure of selecting lithostratigraphic units by their fossil content, rather than by their lithology.

• (2) The Forest Hills Formation of Tanoli and Pickerill (1988) and the remaining Cambrian lithostratigraphic units currently used in the Caledonian Highlands are “unjustified” for the authors. Recently, Landing and Geyer (2023) made the same type of comments on another contribution focused on the Cambrian of Cape Breton Island, Nova Scotia, Canada (Barr et al., 2023). Their justification is simple: Landing et al. want to use a common lithostratigraphic nomenclature for the Cambrian of Avalonia, or, at least, for the largest possible portion of Avalonia (see figure 1 of Landing et al.), whether or not the same lithologies of the Avalon Zone are recognizable outside SE Newfoundland. They then claim that the main characteristic that allows distinction of Avalonia is the “cover sequence” that, by chance, shares a common (but forced) lithostratigraphic framework. This reasoning is circular. Based on the references provided, it appears that those who support this forced lithostratigraphic assignation are only Landing and collaborators.

• (3) The reddish shales and interbedded thin- to thick-bedded limestone beds and nodules that characterize the Bonavista Group and the Smith Point and Brigus formations in the Avalon Zone appear to be absent from the study area in the Caledonian Highlands (Álvaro et al., 2023). The case of the Random Formation is similar: this unit is absent in the Caledonian Highlands. However, Landing (2004) reported it overlying the Mystery Lake Member. How was it possible to suggest such a correlation and synonymy? By using biostratigraphy. The same author dated the lower part of the Mystery Lake Member in the Hanford Brook section as “middle to upper Watsonella crosbyi Zone” based on the presence of some poorly preserved microfossil specimens determined as Eccentrotheca kanesia and Aldanella attleborensis. However, after sampling 1 kg of these limestone layers and nodules, Álvaro et al. (2023, p. 1229) recognized that these specimens do not preserve the diagnostic characters that would allow recognition of both taxa, whereas other etched sclerites represent well-preserved occipital rings and/or axial rings of trilobites (Álvaro et al., 2023, figure 4). As a result, the presence of trilobite sclerites modifies the age of these levels to Cambrian Epoch 2 (so above the Random Formation), contradicting Landing’s (2004) litho- and biostratigraphic correlation of these levels. As a consequence, Landing’s correlation and synonymy of the Random Formation in the Caledonian Highlands has been discredited. A similar conclusion was reached from the organic-walled microfossils in the Mystery Lake Member (Palacios et al., 2011). The correlation method used by Landing et al. is inadequate.

• (4) Landing et al. state that a major unconformity occurs under the Fossil Brook Member, which is the lower part of the Forest Hills Formation, and add that “this unconformity is absent in Álvaro et al.” This is not the case. Both in the description of the facies association “hiatal shelled and microbial limestone” and our figures 3 and 9B are marked by a tectonic break at the base of the Forest Hills Formation, marking a transgressive surface that disappears laterally, as this limestone bed is absent in other outcrops. Based on this incorrect statement, Landing et al. write that “the Hanford Brook Formation and Fossil Brook Member are shown as laterally gradational and temporally equivalent.” In fact, and by definition, the lithostratigraphic contacts are necessarily diachronous and, based on the lateral disappearance of this limestone level, the transition of the Hanford Brook (siltstone-dominated) and Forest Hills (claystone-dominated) formations is necessarily transitional. For Landing et al., a gradational transition between two formations is apparently anathema, and an unconformity must always separate all the formations and members. If the supposed unconformity is not visible, they call it a “cryptic unconformity” and only they can find it. Again, these arguments are not scientifically sound.

• (5) The authors claim that the unconformity that marks the base of the Hanford Brook Formation at its stratotype can be followed from Rhode Island to the English Midlands. In fact, this unconformity cannot be followed beyond Hanford Brook in the Caledonian Highlands but can be broadly dated as marking the base of the Miaolingian (former Middle Cambrian). Apparently, any stratigraphic discontinuity close to the base of the classical “Paradoxides Beds” (Miaolingian) represents this discontinuity. Following Landing et al.’s questioned method of correlation, we could follow it throughout the Anti-Atlas (Brèche à Micmacca Member) and the Iberian Chains (Valdemiedes event), two regional events that also occur close to the base of the regional Miaolingian (Álvaro et al., 2021). We are led to conclude that Landing et al. are again using circular reasoning.

• (6) So how can Landing et al. recognize the same Cambrian lithostratigraphic units from Rhode Island to Belgium? Their figure 1 is a synthetic correlation showing how the Caledonian Highlands share the same lithostratigraphic subdivision and stratigraphic gaps as the Burin Peninsula in the province of Newfoundland and Labrador. This is imaginative, but not tenable. Fortunately, they have maintained the lithostratigraphic nomenclature of the Brabant Massif, where our Belgian and French ch’ti colleagues have traditionally subdivided their Cambrian “rifting megasequence” into six formations (e.g., Herbosch and Debacker, 2018) but without including the distinct upper Terreneuvian gap deliberately added by Landing et al. We also call attention to another correlation in their figure 1: South Wales (UK) shares the same lithostratigraphic nomenclature and gaps as the Burin Peninsula of the Avalon Zone. The lithostratigraphic subdivision of St. David’s and Hayscastle by Rushton et al. (2000) and Brenchley et al. (2006) illustrates the problem with this imaginative interpretation.

The fact that the Geological Surveys of New Brunswick (https://dnr-mrn.gnb.ca/Lexicon/Lexicon/Lexicon_Search.aspx), Nova Scotia (https://novascotia.ca/natr/meb/download/mg/map/htm/map_2000-001.asp), and Newfoundland and Labrador (https://geoatlas.gov.nl.ca/) do not follow the lithostratigraphic framework of Landing et al., and prefer maintaining former subdivisions based on Pickerill and Tanoli (1985) in the Caledonian Highlands, Hutchinson (1952) in Nova Scotia, and Hutchinson (1962) in the Avalon Zone of Newfoundland should give Landing et al. pause for thought.

At present, three separate biostratigraphic charts are used to subdivide Cambrian strata of western Avalonia, based on ichnofossils, shelly fossils (tommotiids and trilobites), and acritarchs. The Treptichnus pedum, Rusophycus avalonensis, and Cruziana tenella zones (MacNaughton and Narbonne, 1999) are valid for the Terreneuvian strata. However, the tommotiid-based zonation (Sunnaginia imbricata and Camenella baltica zones) is problematic, because both microfossils occur with trilobites of the overlying Callavia broeggeri Zone (Landing and Westrop, 1998; Fletcher, 2006): the stratigraphic ranges of the three taxa overlap. As a result, the finding after acid etching of S. imbricata does not necessarily imply that the limestone bed belongs to its homonymous biozone. This problem can be bypassed if the first occurrences of S. imbricata, C. baltica, and C. broeggeri are clearly established along a stratigraphic log, but not, as it is usually the case, if only some poorly preserved and scattered microfossil specimens are found. In their Comment, Landing et al. try to support the tommotiid-based zonation based on chemostratigraphic analyses from Morocco and Siberia. However, to support the Avalonian biozonation, these analyses should be done in western Avalonia, not in Morocco and Siberia, so as to avoid circular reasoning.

Landing et al. counter biostratigraphic implications of trilobite sclerites low in the Mystery Lake Member (Álvaro et al., 2023) by arguing that poorly skeletonized trilobites probably ranged deep into the Terreneuvian, without any bibliographic support. This argument, however, is a speculative hypothesis that contradicts the aims of the Working Group on Stage 3 Global Boundary Stratotype Sections and Points (GSSP) of the International Subcommission on Cambrian Stratigraphy that is taking into account the first occurrence of trilobites as a future GSSP marker bed (e.g., Peng and Babcock, 2011).

Landing et al. repeat an earlier claim (e.g., Landing et al., 2013) for diachroneity in “early Cambrian” acritarch zones. This claim is partly an outcome of their preferred correlation of acritarch-bearing sections in New Brunswick, in contrast to that of Palacios et al. (2011). Recent radiometric dating (Hamilton et al., 2023) further supports the acritarch-based correlation of these sections and no evidence exists for diachroneity of the “early Cambrian” acritarch zones in the Acado-Baltic region. The claimed contradictions in the appearance of acritarchs in Siberia, compared with that of the Acado-Baltic region, brought up by Landing et al. in their Comment, are unsupported and in need of evidence (cf. Grazhdankin et al., 2020).

Finally, Landing et al. claim that our correlations include “errors of 10 m.y. or greater in litho-, bio-, and chronostratigraphic correlations that compromise an understanding of basin evolution.” And we agree. Our correlation of the basal part of the Mystery Lake Member, based on the presence of trilobite sclerites, necessarily modifies the age of these limestone layers from Terreneuvian to post–Terreneuvian. The acritarch-based zonation published by Palacios et al. (2011) contradicts former correlations exclusively based on lithostratigraphic assumptions and, also based on acritarchs, the gap involved at the Glen Falls/Hanford Brook contact is beyond any chronostratigraphic control. The new U-Pb data yielded by interbedded tuffs (see Hamilton et al., 2023) support our correlation and definitively compromise former statements made by Landing et al.

We devoted a section in our previous paper to the issue of an “early Cambrian” glacial event. In the original report, this supposed glaciogenic level was described as 40 cm thick and lying “within mudstone and 5 cm above the top of the fluvial deposits” (Landing and MacGabhann, 2010, p. 180). Landing and MacGabhann (2010) stands out as a unique claim for “early Cambrian” glacial conditions (cf., Hearing et al., 2018), but, as outlined in Álvaro et al. (2023) and below, it does not hold up to scrutiny.

In their Comment, Landing et al. purport that the “location of the glacial diamictite above trough cross-bedded sandstone on the Ads 1–2 boundary are in Landing and Westrop (1998, figure 22) and Landing and MacGabhann (2010, figures 4–6C).” Contrary to the above statement, there is no reference to, or illustration of, diamictite in Landing and Westrop (1998, figure 22). Furthermore, these authors (p. 59) specifically refer to that same interval as “an important stratigraphic break indicated by conglomerate-filled channels in the interval 157.5–174.3 m,” which they go on to describe as “stream-laid gravels.” However, Landing and MacGabhann (2010, p. 179) stated that the “interval 173.95–174.35 m is a diamictite bed with oversized clasts in a muddy matrix near the base of the olive green mudstone.” At this same interval, we described a wedge-shaped, matrix-supported conglomerate bed near the base of the Mystery Lake Member that we interpreted as a debris flow. In their Comment, the authors admit that the photograph in figure 5C is in error, and as stated in our paper, we continue to maintain that figure 5B is also in error and is one of the reasons for the confusion surrounding the location of the “diamictite” bed.

Johnson et al. (2019) reported how the outcrop images purported to illustrate the diamictite-like level can be identified as occurring higher in the Mystery Lake Member, ~70 m and 110 m, respectively, above where the 40-cm-thick “diamictite” level was reported by Landing and MacGabhann (2010). The same beds were re-figured in our repository data to demonstrate that their images do not support their arguments. Hence, based on our observations, we cannot accept that Landing and MacGabhann’s (2010) figured log represents “a detailed Hanford Brook East section and location of the glacial diamictite.”

Their interpretation is also unconstrained as evidence for dropstone origin is not presented. We described in our paper two different concepts: “diamictite” (a descriptive term) and “glaciomarine deposit” (an interpretative term). The authors consider that their photos show “glaciomarine dropstones” without explaining which features allow identification of dropstone. Their figures show rounded to subrounded (not angular) clasts embedded in a shale matrix, with neither striated/faceted clasts nor laminar adaptations of the matrix under the clasts. In contrast, their figures appear to show ball-and-pillow structures representative of substrate fluidification. These structures are not restricted to a 40-cm-thick level, as stated by the authors, but occur throughout the Quaco Road and Mystery Lake members, and are concentrated in the proximity of the breccia and conglomerate channels controlled by slope pulses (Álvaro et al., 2023). The two new figures illustrated in their Comment deserve no further discussion: figure 2A is out of focus, and the quality of figure 2B is insufficient to show the purported features. In any case, the presence of rounded clasts in a matrix-supported shale does not demonstrate their assignation to a diamictite. But even accepting this single level as a diamictite, the authors should explain why this supposed diamictite level represents a glaciogenic and not a slope-related deposit. The authors simply write that it is a glacial deposit without providing evidence other than the introduction of exotic material (marble clasts) seemingly unknown elsewhere in the region. They did not take into account the record of neighboring synsedimentary faults such as those marking the Handren Hill Formation/Coldbrook Group contact along the same eastern Hanford Brook section. We do not know which rock lithologies were exposed as landscape during the deposition of the Ediacaran Coldbrook volcanosedimentary complex, but we cannot eliminate the possibility of the reworking and resedimentation of marble clasts derived from the limestone nodules and layers that mark the base of the Quaco Road Member (Álvaro et al., 2023, figure 2). We etched some of these sparry limestone layers, which yielded no microfossils.

Yet this interpretation of a glacial deposit has been one of the arguments repeated as a mantra to place Avalonia under subpolar conditions “far from coeval tropical successions in West Africa (southern Morocco) with carbonate platforms, local evaporites, and archaeocyath reefs.” In examining thin-sections, we discovered that similar evaporitic pseudomorphs occur in the lower part of the Forest Hills Formation in the Caledonian Highlands, as illustrated in our paper (figures 8H and 8I). These observations support the presence of evaporites in the Cambrian of Avalonia, despite the insistence of Landing et al. that “there are no Cambrian evaporites in Avalonia.” Together with the presence of carbonate strata in the Caledonian Highlands and more broadly in the Avalon Zone, and the presence of Terreneuvian–Cambrian Epoch 2 reefs and mud-mounds in western Avalonia (see descriptions in Landing et al., 2017), these data discredit these supposed climatically sensitive lithologies and facies used to separate “subpolar” Avalonia from the subtropical Moroccan margin of Gondwana. Landing et al. should also take into consideration paleomagnetic data, which place Avalonia in mid-latitudes during this time interval (e.g., McNamara et al., 2001; Hamilton and Murphy, 2004; Waldron et al., 2022), and blatantly close to Morocco (Wen et al., 2020).

In our paper, we included a paleogeographic reconstruction of the Cambrian (half-)graben infill interpreted for the Caledonian Highlands within a broader western Avalonian context. We added a tentative reconstruction including those “tectonostratigraphic units” (a descriptive, not genetic, term) that allows correlation. Of course, this model is not definitive but a proposal likely in need of improvement. Formerly, some of these tectonostratigraphic units have been referred to in the literature as “terranes,” although they do not necessarily represent the present-day concept of exotic microcontinents, but different tectonostratigraphic parts of the Avalonian Basin. We cannot understand why Landing et al. contemplate that we are considering our tectonostratigraphic units (former “terranes”) exotic microcontinents. It is clear in our text and figures that we include them in a succession of interconnected (half-)grabens.

Assuming that an Ordovician Avalonian microcontinent existed, what is the signature that allows identification of Avalonia as a single Ordovician entity? According to Landing et al., the answer is its stratigraphic cover. The character of its crustal basement (e.g., inherited Sm-Nd and Hf isotopic and zircon signatures) appears to be considered irrelevant information. Landing et al. summarize in their Comment that the evidence that “demonstrates” that Avalonia was a Cambrian microcontinent are the presence of: (1) Coleoloides mud-mounds, (2) abundant bentonites in the lower part of the Miaolingian, and (3) a massive quartzite level broadly marking the top of the Terreneuvian (Random Formation and laterally equivalent units). Obviously, we disagree.

Coleoloides (in fact, “Ladatheca” cylindrica) mud-mounds, and orthothecids preserved upright in siltstones, suggesting a mud-sticking life-style, are reported from the northern Bonavista Peninsula in Newfoundland (Landing, 1993) and probably England (Brasier and Hewitt, 1979). Although at present time, evidence for this behavior has not been described outside Avalonia, the taxa are cosmopolitan. The presence of abundant lower Miaolingian bentonites is a feature shared by many rift transects, such as the Moroccan Anti-Atlas and the Iberian Ossa–Morena Zone. And upper Terreneuvian sections display Random-style shoal complexes worldwide, such as the Bámbola and Embid formations in the Iberian Chains (NE Spain), the Azorejo Formation in the Central Iberian Zone, and the Marcory Formation in the Montagne Noire (southern France; Álvaro et al., 2021). Therefore, the “stratigraphic [Cambrian] cover” of western Avalonia, the Anti-Atlas and Iberian basements are broadly similar, but not their Ediacaran and Cambrian detrital zircon populations.

The definition of a terrane or continental margin should be based on a holistic approach, taking into account all available data, such as the basement, tectonic activity, geochemical affinity of the magmatic record and cover stratigraphy. A similar cover stratigraphic succession cannot unilaterally define the outline of any microcontinent/terrane drifting from one continent to another: the Nile and Mississippi deltas display common and coeval lithologies but are in different continents. If we imagine the Cambrian seas, the onset of Miaolingian kerogenous shales does not allow recognizing single terranes; otherwise, the Alum Shale of Baltica would be a lithostratigraphic synonymy of the Manuels River Formation of Avalonia. A terrane should be defined by its basement at the time the basement formed, and the cover, once the microcontinent block rifted, should display distinct variations explicable by several factors, such as climate, palaeolatitudinal position of its margins, accommodation space or siliciclastic input. For instance, carbonate production can be poisoned by siliciclastic input, and its presence versus absence is not an argument for distinguishing microcontinents. Lithological similarities (or lithostratigraphic assignments, see above) are a weak argument for geodynamic assessment. Landing et al. propose a comparison with “the recently recognized Zealandia paleocontinent (Mortimer et al., 2017),” but Zealandia is in fact mainly defined by its continental crustal basement, not by its cover (e.g., Adams, 2010).

Landing et al.’s Comment also considers that western Avalonia, based on lithostratigraphic similarities and the record of common Carboniferous strike-slip faults, should include the Caledonian Highlands and the Brookville and New River belts, two areas that more commonly are assigned to Ganderia (e.g., van Staal et al., 2021a, 2021b). However, this discussion was not taken from our commented paper, and hence we will not enter here into a discussion about the Avalonia/Ganderia contact.

Rift basins are theoretically produced by orthogonal extension, parallel to the main axis. However, nature is capricious and this is rarely the case. Oblique rifting commonly accounts for changes in fault polarities and offset depocenters displaying some features characteristic of another end-member model: strike-slip or oblique-slip basins. Until now, neither transcurrent nor inverse faults have been described in the Cambrian of western Avalonia. In some cases, syndepositional extensional faults are neither preserved nor sealed by younger strata, but their activity can be deduced by the presence of their slope-related, wedge-shaped breccias and conglomerates. They have been nicely illustrated with isopach maps by Hutchinson (1962), linked (half-)grabens and onlapping geometries by Smith and Hiscott (1984, figures 4, 7, 10, and 12), alluvial fans in Tanoli and Pickerill (1990, figure 10), onlapping and sealing geometries in Fletcher (2006, figure 10), related to unconformity-bearing carbonates by Álvaro (2021) and, even based on geophysical data, by Miller (1983).

Landing et al. deny a rift setting for the Cambrian of Avalonia. They write “Álvaro et al.’s Avalonian depositional regime as a linear series of half grabens has no modern analogue.” On the contrary, recent syntheses about the topic, such as Sapin et al. (2021), illustrate a classification of rift margins with linear series of (half-)grabens. Their examples with “multiple rifting events” fit well our tentative reconstruction of (half-)grabens (or “depocenters” if this term is preferred) separated by normal faults responsible for the episodic record of wedge-shaped slope-related breccia deposits.

In contrast, Landing et al. prefer a strike-slip regime, for which they provide a single reference in support: Noda (2013). The authors selected only two of Noda’s arguments: (1) the “thickness of the terminal Ediacaran–Lower Ordovician succession of the Caledonian Highlands is dwarfed by comparison with thicknesses in modern plate boundary transform fault settings as the Dead Sea,” and (2) Noda’s model explains the Dead Sea basin as “coeval extensional and collisional igneous melt production—a feature not part of a rift/half graben model.” The first argument does not support a strike-slip regime for Avalonia and, after reading Noda (2013), we have found no references to “collisional igneous melt production” (these four words do not occur in the paper). Again, Landing et al.’s arguments appear unfounded.

According to Nilsen and Sylvester (1995), McClay et al. (2002), Smit et al. (2008), and Noda (2013), among others, common characteristics of strike-slip basins include: (1) an elongate geometry of basins and depocenters, (2) dominance of axial infilling, (3) depocenter migration opposite to the direction of axial sediment transport, (4) very thick strata relative to the burial depth, (5) high sedimentation rate, (6) compositional changes that reflect horizontal movement of the provenance, and (7) rapid subsidence in the initial stage of basin formation. Rift basins commonly share other features, such as asymmetry of both sediment thickness and facies pattern, abrupt lateral and vertical facies changes and unconformities (close to main faults), and abundant synsedimentary slumping and deformation.

Therefore, strike-slip basins are generally elongate, narrow, and display very high patterns of subsidence and sedimentation rate, with a total thickness of ~14 km since late Miocene times for the Ridge Basin, along the San Andreas Fault, 14 km since Miocene times for the Dead Sea Basin, or 10–17 km since Eocene times for the Yinggehai Basin (see Noda, 2013). In contrast, the Cambrian infill of the Avalonia Basin lasted ~53.4 m.y. and reached a total thickness, without taking into account stratigraphic gaps, of ~0.6 km in the Caledonian Highlands and up to 2 km in the Avalon Zone of Newfoundland.

In addition, no sedimentological mismatches have been found close to reported slope-related wedge-shaped breccias and conglomerates (e.g., basal part of Handren Hill and Mystery Lake members in the Caledonian Highlands, Holyrood granite/Brigus Formation contact on Conception Bay, Random/Brigus formations contact at Little Dantzig Cove Brook on the Burin Peninsula, Random/Bonavista contact at Sunnyside; Hutchinson, 1962; Álvaro, 2021; Álvaro et al., 2023) reflecting horizontal (transcurrent) movements: the reworked clasts were derived from underlying (so eroded) strata after uplift and denudation of exposed shoulders. No superposition of synsedimentary normal and inverse faults have been described until now in the area.

The final argument is related to the preservation of a supposed “ribbon continent roughly along depositional strike, allowing recognition of a common lithostratigraphic framework and depositional sequence boundaries from Rhode Island to Belgium.” Obviously, all the references to support such an idea are “Landing et al.” However, the “ribbon-like” arrangement of tectonostratigraphic units in Avalonia, as in any massif core subsequently affected by younger orogenies, is dramatically affected by younger deformations (e.g., Waldron et al., 2015). Neither the Caledonian Highlands nor the Avalon Zone of Newfoundland preserve an original arrangement, as compared by Landing et al. with the Holocene Ridge and Dead Sea basins.

The arguments to support a strike-slip regime for Avalonia are, therefore, exclusively based on a supposed diachronous migration of “depocenters” bounded by “major faults with minor extensional faults.” The authors deny the existence of a fault scarp located on the NW edge of the Caledonian Highlands “as the fault scarp would be the NW limit of a succession of terminal Ediacaran–Cambrian formations, members and sequence boundaries,” so they consider that any “fault scarp” separating distinct Cambrian facies associations must mark the contact with the Avalonian craton, which represents another misconception. In summary, the arguments offered by Landing et al. neither support a strike-slip basin nor contradict a rift basin.

In our previous paper we described and illustrated a series of stratigraphic and sedimentological data that can be observed in the reported outcrops. We demonstrated that the stratigraphic information published by Landing and MacGabhann (2010) and related to the presence of a supposed glaciogenic level, 40 cm thick, is misleading: their supposed glaciogenic structures are neither glaciogenic nor found in a level, 40 cm thick, but were taken from strata more than 100 m above. We offered several sedimentological interpretations, based on our described and illustrated data, and proposed a paleogeographic model for the Cambrian of the Caledonian Highlands and, in figure 9C, for western Avalonia. Our lithostratigraphic framework followed the rules of the International Commission of Stratigraphy and the Geological Survey of New Brunswick. Our new illustrated microfossils offered different chronostratigraphic correlations with Newfoundland Avalon, challenging the forced lithostratigraphic assignations and synonymies established by Landing et al. without taking into account lithology. Any geodynamic model to interpret the Cambrian of Avalonia should fit with a geodynamic scenario linking a Neoproterozoic arc-related basin and the Ordovician drift of Avalonia toward Baltic-Laurentian positions. The presence of “depocenters” bounded by short-term uplifted shoulders, limiting wedge-shaped slope-related breccias and linked to carbonate karstification (Hutchinson, 1962; Smith and Hiscott, 1984; Fletcher, 2006) and coeval hydrothermal ore bodies of polymetallic sulphides, sulphates, and oxides encased in the unconformity-bearing carbonates described in the Avalon Zone of Newfoundland (Álvaro, 2021) and the Caledonian Highlands of New Brunswick (Tanoli and Pickerill, 1990; Álvaro et al., 2023), are in need of holistic interpretation. In our opinion, the simplest model that fits all the above-reported information is a rift-related succession of (half-)grabens affected by extension-dominant faults that led to local shoulder uplifts and lateral deepening of hanging-walls. Such a model also fits well with the tholeiitic-alkaline magmatism recorded locally in western Avalonia. Pure strike-slip interpretations would support elongate, narrow depocenters displaying very high patterns of accommodation space and sedimentation rates, at the scale of tens of kilometers, which are not supported by data. Of course, we are open to other interpretations, but they must be based on data, not on dogmatic assumptions and circular reasoning.

The authors appreciate the constructive revisions by two anonymous reviewers and technical editing by Wenjiao Xiao.