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

Paleogeographic syntheses require a thorough and convincing understanding through time of terrane sizes, numbers and tectonic regimes, their relationship to and interactions with other terranes and continents, latitudinal/climatic and, hopefully, longitudinal distribution to avoid being speculative. Adequately defined terranes contribute to reconstructing ancient plate tectonics and orogeny and the evolution of the Earth-life system. These concerns are particularly important to Avalonia, the largest terrane of the Appalachian-Caledonian orogen. Avalonia is best defined by its terminal Ediacaran–Ordovician sedimentary rock–dominated cover sequence (e.g., Rast et al., 1976) unconformable on a largely Mesoproterozoic–early Neoproterozoic basement (e.g., Murphy et al., 2019; Fig. 1). Lateral and vertical changes through time of the Avalonian cover sequence (e.g., thicknesses and rates of accumulation, dominant lithofacies, depocenter migration, development of local volcanism and intrusives) record the epeirogenic history of Avalonia (e.g., Landing et al., 2022, 2023).

Our concerns with Álvaro et al.’s (2023) synthesis of this diagnostic cover sequence include: (1) use of a rift/half graben model in a small part of Avalonia (Caledonian Highlands, SW New Brunswick, Canada) although a strike-slip regime is supported by regional study of North American–European Avalonia; (2) limiting Avalonia to a small New Brunswick area (Álvaro et al., 2023, figures 9A and 9B) that actually extended far SW and NE of Saint John and thus well beyond the Caledonian Highlands; and (3) placing a major rift fault with a high fault scarp in a uniform sedimentary blanket that extends 20 km NW from the Caledonian Highlands to include the Brookville and New River belts. We also note (4) errors of 10 m.y. or greater in litho-, bio-, and chronostratigraphic correlations that compromise an understanding of basin evolution; (5) a division of Avalonia into tiny “terranes” despite a regionally extensive cover sequence with thin units (<10 m) and depositional sequence boundaries that show Avalonia was a ribbon microcontinent; and (6) errors in reporting minor to major depositional features.

Álvaro et al. (2023) claim that, e.g., Landing (1996a) did “not take into account the wedge-like stratal patterns characteristic of half-graben and graben structures” in their interpretation of the Caledonian Highlands as showing deposition in a rift regime. However, broader studies along Avalonia do not document “wedge-shaped” units (Landing et al., 2022, 2023; Fig. 1). Rather, many stratigraphic units are thin (<10 m, often ~1 m) lithosomes. These include the Fosters Point Formation which extends over 120 km across SE Newfoundland. Indeed, Fosters Point persists into Cape Breton (Landing, 1991; Landing and Geyer, 2023) along with the Bonavista Group and Brigus Formation (not shown in the Mira area of Álvaro et al., 2023, figures 2 and 9C), SW New Brunswick (Cradle Brook, Fig. 1, CrB), eastern Massachusetts, USA (“Nahant Limestone”), and is termed the Home Farm Limestone in the English Midlands (e.g., Landing, 1996a; Landing et al., 2022).

The Fossil Brook Member (≤8 m) of the Chamberlain’s Brook Formation has a similar distribution and “non-wedge” geometry, with lower limestone and upper green mudstone and basal and upper unconformities (e.g., Landing et al., 2023; Fig. 1). This limestone-mudstone unit is the lower “Forest Hills Formation” of Álvaro et al. (2023; abandoned by Landing, 1996a; Landing and Westrop, 1996; Landing et al., 2023), and is far younger than Álvaro et al. (2023) indicate (discussed below). The thin Fossil Brook Member extends from Rhode Island (USA), along SW New Brunswick from Beaver Harbour to Hanford Brook, reappears NE of the poorly exposed Nova Scotia sections to blanket SE Newfoundland, and is recognized as the cap of the Purley Shales in Warwickshire in the English Midlands (e.g., Landing et al., 2023; Fig. 1).

A rift basin model is inappropriate given the stratigraphic relationships of the cover sequence in central New Brunswick and elsewhere in Avalonia (e.g., Landing et al., 2022, 2023). The alternative is a strike-slip regime for trans-Avalonian cover sequence deposition (e.g., Landing and Benus, 1988; see Landing et al., 2022, sections 16.1 and 16.2 for review). This strike-slip regime (e.g., Noda, 2013) developed close to the proposed Avalonian transform fault (Atf; Landing et al., 2022) and shows step-wise, SE depocenter migration. This type of migration does not characterize rift settings and featured sequential opening of syndepositional basins to the SE beginning in the latest Ediacaran. The easterly movement of depocenters was on NNE-trending major faults with minor, E-trending, splayed extensional faults shown by sediment-filled fissures in the Proterozoic basement and in early cemented Cambrian sandstone and limestone (e.g., Landing and Benus, 1988; Landing, 1996a).

The timing of depocenter migration in SE Newfoundland and the Welsh Borderlands–English Midlands follows from all available stratigraphic data and the succession of trans-Avalonian depositional sequences (Ads; Landing, 1996a; Rees et al., 2014; Landing et al., 2022). This movement in SE Newfoundland (Fig. 1) started with the onset and thickest accumulation of subaerial red beds—massive shelf sandstone (Rencontre-Random formations, Ads 1 and 2, terminal Ediacaran–middle Terreneuvian) on the NW margin of Avalonia (i.e., marginal platform of the Burin Peninsula and in SW New Brunswick including the Caledonian Highlands).

This initial marginal platform deposition was followed by a middle–late Terreneuvian SE depocenter shift with the greatest thicknesses of the Bonavista Group (Ads 2) in Bonavista and west Trinity bays of SE Newfoundland. Finally, another easterly shift is shown by the thickest Series 2 (Brigus Formation, Ads 4a and 4b) and basal Miaolingian (Easter Cove Member, Ads 5) in St. Mary’s and east Trinity bays (Fig. 1). The second and third depocenter shifts generally are marked by marine transgression of the Random Formation (and synonymous local units) that had earlier onlapped to the east of the marginal platform and across the Avalonian basement of the inner platform (e.g., eastern Massachusetts, SE Newfoundland, English Midlands). In the latter areas, younger units (Ads 3, 4a, and 6) can form the lowest cover sequence (Fig. 1).

Limiting Avalonia in SE New Brunswick to the small area of the Caledonian Highlands is problematical to Álvaro et al.’s (2023) reconstruction of depositional facies. Simply stated, a terminal Ediacaran–Cambrian cover sequence succession precisely identical in the stacking of formation- and member-level units, depositional sequences, and coeval with the Caledonian Highlands has long been known to the NW of the Caledonian Highlands in the ~30-km-wide New River and Brookville belts (e.g., Hayes and Howell, 1937). This cover sequence succession, with a volcanic edifice that was the source of ashes in the Hanford Brook Formation in Saint John, extends ~55 km farther SW along the “New River terrane” to Beaver Harbour (Landing et al., 2008; Fig. 1).

A cover sequence, as that of the recently recognized Zealandia paleocontinent (Mortimer et al., 2017), records a detailed geological history and is the key means for defining a terrane by its depositional architecture (e.g., Howell and Howell, 1995; Landing et al., 2022, sections 4.1, 4.2). In contrast to Álvaro et al. (2023, figure 9A), an areally much more widespread cover sequence means that Avalonia in SW New Brunswick must include the Caledonian Highlands along with the Brookville and New River belts. These latter two belts have been assigned to a “Ganderia” zone (e.g., Fyffe et al., 2012; Barr et al., 2014a, 2014b) but are best regarded as the continuation of the Avalonian marginal platform to the NW of the Caledonian Highlands (e.g., review in Landing et al., 2022; Fig. 1).

The Caledonian, Brookville, Silurian-dominated Kingston belt, and New River areas were defined by Carboniferous strike-slip faults that fractured a broad Avalonian marginal platform and should not be regarded as early Paleozoic terranes (Landing et al., 2022). One reason for distinguishing these “terranes” was isotopic examination of terminal Ediacaran–Early Cambrian igneous rocks that contrasted the New River (extensional) and Brookville (subduction/compressional) “terranes” (e.g., Fyffe et al., 2012). However, coeval extensional and compressive igneous rocks occur along transforms, such as the North Anatolian fault, and would be expected along the proposed Avalonian transform fault that controlled epeirogenic activity and produced volcanic edifices on the NW edge of the marginal platform (e.g., Landing et al., 2022, sections 4.1, 4.2, 5.3, 7.3). Similarly, purportedly distinct detrital zircon ages and whole rock stable isotope signatures of the New River and Brookville belts (e.g., Barr et al., 2014a, 2014b) are likely inherited features from the underlying blocks of the Avalonian basement collage and do not provide a basis for referring the Avalonian cover sequences exposed in inliers along the Appalachians to separate terranes (Keppie, 1985; Landing et al., 2022; Landing and Geyer, 2023).

The known lateral extent of the Avalonian Cambrian cover sequence in SW New Brunswick requires a depositional model that extends well beyond the limit of the modern Caledonian Highlands (i.e., Álvaro et al., 2023, figure 9B). Thus, almost all of coastal SW New Brunswick, including the Caledonian, Brookville, and New River belts, is referable to the Avalonian marginal platform. This large area is geologically and depositionally comparable to the marginal platform of the Burin Peninsula (Canada) and South Wales (UK) (Landing et al., 2023, figures 3 and 5; Fig. 1).

In terms of an appropriate tectonic regime, a syndepositional “rift fault” and an imposing fault scarp cannot be located on the NW edge of the Caledonian Highlands (see Álvaro et al., 2023, figure 9B) as the fault scarp would be the NW limit of a succession of terminal Ediacaran–Cambrian formations, members, and sequence boundaries. However and as noted above, this cover sequence succession extends unbroken across the Saint John region (Caledonian Highlands) and the Brookville and Little River belts to the NW (e.g., Hayes and Howell, 1937; Tanoli and Pickerill, 1988; Landing, 1996a; Landing et al., 2022, and references therein; Fig. 1).

The SE limit of this marginal platform is lacking in Álvaro et al.’s (2023, figures 1B and 9B) reconstruction of the Caledonian Highlands basin. The marginal platform includes outcrops on the Fundy Coast Parkway and on the coast NNE of St. Martins ~20 km east of Hanford Brook. This coastal marginal platform succession includes the Ratcliffe Brook Group of Landing (1996a; i.e., Rencontre Formation [which is actually most of the Quaco Road Member as mapped by Park et al., 2017; see Landing et al., 2022]). Higher units include the Chapel Island Formation with Quaco Road and Mystery Lake members, the Random (“Glen Falls”) Formation, and the Hanford Brook Formation (i.e., Park et al., 2017).

Faults mark the SE limit of the marginal platform of the Caledonian Highlands and isolate an Avalonian inner platform remnant on the coast at Cradle Brook (Fig. 1, CrB). This SE margin of the Caledonian Highlands should have been part of the Álvaro et al. (2023, figure 9B) synthesis. Indeed, the same NW-SE transition from the marginal to inner platform occurs in SE New Brunswick as elsewhere in North American and British Avalonia and serves to emphasize the unity of the Avalonian terrane (e.g., Landing et al., 2022). At Cradle Brook, a thin Random Formation is unconformable on Ediacaran pillow basalts with ~0.75 m of pebbly remané sediment at the contact. The Random and overlying Bonavista Group (likely with the fossiliferous Fosters Point Formation), and Brigus Formation form an inner platform succession at Cradle Brook (Landing, 1996b; Landing et al., 2022, section 10.6; Fig. 1).

Reconstructing the depositional history of the Caledonian Highlands must include accurate bio- and chronostratigraphic correlations in the context of the eleven trans-Avalonian depositional sequences (Ads) in North American, British, and Belgian Avalonia (Landing, 1996a; Landing and Westrop, 1998a, 1998b; Landing et al., 2013, 2022, 2023; Rees et al., 2014; Fig. 1). The Ads are key to reconstructing and timing a syndepositional epeirogenic history that largely defined the depositional architecture of Avalonia (e.g., strata thicknesses; depocenter location; onlap, offlap, and unconformity formation; depth of erosion; hiatus duration between depositional sequences; occurrence of volcanic ashes and development of volcanic edifices). Eustatic changes certainly took place through deposition of the Avalonian terminal Ediacaran–Ordovician cover sequence, but such coordinated developments at the beginning and end of Avalonian depositional sequences as volcanic activity, local transtensional faulting with uplifts and subsidence, and depositional basin migration have suggested epeirogeny and strike-slip faulting as dominant controls on deposition (Landing et al., 2022).

An understanding of the depositional history of the Caledonian Highlands is compromised because a number of global Cambrian series and stages are miscorrelated by ≥10 m.y. by Álvaro et al. (2023, e.g., figure 2). Thus, Series 2, which essentially brackets Earth’s oldest trilobites (e.g., Peng et al., 2012), and its lower Stage 3, must not be shown to include the sub-trilobitic Sunnaginia imbricata and Camenella baltica small shelly fossil (SSF) zones of Landing et al. (1989).

Accurate correlation of these two SSF zones is key to reconstructing Avalonian (and Caledonian Highlands) deposition. These zones and the Bonavista Group (Ads 3) are thickest in Placentia and west Trinity bays, SE Newfoundland, following eastern depocenter migration in the Avalonian strike-slip regime (Ads 3; Fig. 1). These zones are ~10 m.y. older and comprise the upper Terreneuvian Series 1 and upper Stage 2 contra Álvaro et al. (2023, figure 2).

Only the top of the C. baltica Zone, which is the upper Fosters Point Formation (“Foster Point” [sic] in Álvaro et al., 2023) and uppermost Ads 3, ranges into lowest Series 2 (Fig. 1). This correlation is based on microfossils and a strong negative δ¹³C excursion used to indicate correlation into strata with the lowest Moroccan and Siberian trilobites at ca. 519 Ma (Landing and Kouchinsky, 2016; Landing et al., 2021). The Fosters Point Formation must be “lowered” in Álvaro et al.’s (2023) figure 2 by ~10 m.y.

The Brigus Formation comprises two successive unconformity-bound depositional sequences, Ads 4a and 4b (Fig. 1). The lower (Ads 4a) has Callavia/C. broeggeri Zone trilobites (e.g., Westrop and Landing, 2011). By comparison, Álvaro et al. (2023, figure 2) show the Brigus Formation comprised of only one depositional sequence. This difference in reported stratigraphy is important as Ads 4a shows another eastern migration of strike-slip basin depocenter in Series 2, Stage 3 (e.g., Geyer, 2019). Unlike in Álvaro et al. (2023, figure 2), the upper Brigus is a second depositional sequence (Ads 4b) that is limited to Trinity and east Conception bays. Ads 4b, in turn, was truncated following epeirogenic uplift and is unconformably overlain by purple, red, to greenish mudstone (Easter Cove Member, Ads 5) of the lowest Chamberlain’s Brook Formation (Landing and Westrop, 1996). Ads 5 is an important, lenticular, roughly N-S trending unit in SE Newfoundland that illustrates the timing and consequences of transform-fault driven epeirogenic activity in the earliest Middle Cambrian. Ads 5 shows uplift and erosion of Ads 4b, subsidence and accumulation of Ads 5, and erosion of Ads 5. Thus, Ads5 (Easter Cove Member) should be illustrated under the Braintree Member (Ads 6), but is absent in Álvaro et al. (2023, figure 2).

An important epeirogenically controlled feature in the Caledonian Highlands, as well as elsewhere in SW New Brunswick and across Avalonia, is a major unconformity under the higher Middle Cambrian Fossil Brook Member. The Fossil Brook (Ads 7; Fig. 1) is the lower part of the “Forest Hills Formation” of Álvaro et al. (2023). This unconformity is absent in Álvaro et al. (2023, figures 2 and 9B) although it records major uplift, erosion, and subsidence that cut out ~45 m of the upper Hanford Brook Formation (Ads 4b) in the Caledonian Highlands (i.e., Landing and Westrop, 1996; Landing et al., 2023).

As the Ads 4b–7 unconformity is absent in Álvaro et al. (2023, figures 2 and 9B), the Hanford Brook Formation and Fossil Brook Member are shown as laterally gradational and temporally equivalent. Indeed, the strata overlying the unconformity in Álvaro et al. (2023, figures 2 and 9B) are shown as lowest Middle Cambrian in the Caledonian Highlands. This is incorrect; the Fossil Brook Member overlying the unconformity is so young that it actually cannot be part of Álvaro et al.’s (2023, figure 2) Avalonian correlation scheme as it has Eccaparadoxides eteminicus Zone trilobites (e.g., Kim et al., 2002; Fletcher, 2006). Their middle Middle Cambrian age means the lower “Forest Hills” is much younger than the lowest Chamberlain’s Brook in Álvaro et al.’s (2023) figure 2.

This sub-Ads 7 unconformity is not just a key to understanding the Caledonian Highlands’ depositional history. Indeed, this unconformity also shows unity of the Avalonian terrane as it is recognized from Rhode Island to the English Midlands (Landing, 1996a; Landing et al., 2023). The thin, widespread Fossil Brook (Ads 7) unconformably overlies strata as low as Ads 4A (lower Brigus Formation) and as high as Ads 6 (Braintree Member) in SE Newfoundland. A key regional development is that mudstones of Ads 7 form the highest greenish unit immediately under the trans-Avalonian green-black boundary from Rhode Island to Belgium (Landing, 1996a; Landing et al., 2023; Fig. 1).

Use of the “Forest Hills Formation” of Tanoli and Pickerill (1988) is unjustified. It is separated easily into a lower Fossil Brook Member of the Chamberlains Brook Formation, with lower thin shell hash limestones and higher greenish mudstone and higher black mudstone. The unconformably overlying black mudstone is best referred to the Manuels River Formation in the Caledonian Highlands and elsewhere in SW New Brunswick (Landing and Westrop, 1996), and across the rest of North American Avalonia (e.g., Landing et al., 2023). Coeval black mudstone masked by local names occurs in British and Belgian Avalonia (Landing and Westrop, 1998b; Landing et al., 2023).

There is no “Miaolingian sealing of inherited paleoreliefs” in Avalonia (Álvaro et al., 2023, figure 9B). The Avalonian Miaolingian consists of five depositional sequences (Ads 5–lower 9) each with basal unconformities and did not form a permanent blanket that would have led to a “sealing” (Landing, 1996a; Landing et al., 2023). The first blanketing (“sealing”?) of the entire Avalonian marginal and inner platforms was by Ads 4a and 4b (i.e., Brigus Formation mudstone widespread in North American Avalonia, coeval Hanford Brook sandstone in the Caledonian Highlands, and Purley Shales in the English Midlands; Fig. 1).

Mis-correlation of the upper Hanford Brook Formation with its Protolenus- and higher Berabichia-bearing trilobite assemblages (Landing and Westrop, 1996; Westrop and Landing, 2000) by Álvaro et al. (2023) blurs the depositional history of the Caledonian Highlands. These two assemblages have long been regarded as reliably upper Lower Cambrian (e.g., Westrop and Landing, 2000; Geyer, 2005, 2019). Álvaro et al. (2023, figure 2) note Palacios et al.’s (2017) recovery of purported Miaolingian acritarchs and a ca. 508 Ma U-Pb date from the Protolenus-bearing interval (Landing et al., 1998; recalculated by Schmitz, 2012) as Middle Cambrian. However, the Hanford Brook trilobites reliably counter the “younger” acritarch correlation into the Middle Cambrian. In addition, U-Pb dates indicate a ca. 506 Ma or younger age on the Miaolingian base is more appropriate (i.e., Sundberg et al., 2020). Thus, a ca. 508 Ma date on Protolenus-bearing strata puts all coeval depositional events in the Caledonian Highlands into the pre-Miaolingian, as this interval correlates with Ovatoryctocara granulata–bearing strata in Moroccan West Gondwana and on the Siberian Platform (e.g., Geyer, 2019). We have reevaluated a ca. 509 Ma date on the Miaolingian base in Avalonia (Williams et al., 2013) as likely due to re-worked zircons. Our lower Miaolingian dates are closer to the proposed 506.5 Ma or younger age (E. Landing and M.D. Schmitz, 2023, personal comm.).

The acritarch succession is used by Álvaro et al. (2023) to argue that key trans-Avalonian depositional units, as the middle Terreneuvian, massive Random Formation–type sandstones (upper Ads 2; e.g., Landing et al., 2013; Fig. 1), are diachronous. Thus, Random-type sandstones purportedly appear later in the Caledonian Highlands (Álvaro et al., 2023, as in their figure 2 “Glen Falls Sandstone”) than elsewhere in Avalonia (compare Landing et al., 2013). The problems with Lower Cambrian acritarch correlation and its use to reconstruct the depositional history of the Caledonian Highlands is detailed elsewhere (Landing et al., 2013, 2022, sections 17.4, 17.5) and is only reviewed here: A Skiagia ornata-Fimbriaglomerella membranacea Zone (S-F Zone) and overlying Heliosphaeridium dissimilare-Skiagia ciliosa Zone (H-S Zone) are described from the Chapel Island Formation in the Caledonian Highlands (i.e., Ratcliffe Brook Formation under the “Glen Falls Formation” in Álvaro et al., 2023, figure 2; see Palacios et al., 2011; Fig. 1). In Baltica, these acritarch zones are tied, respectively, to the lowest trilobite zones: the successive Schmidtiellus mickwitzi and Holmia kjerulfi zones (Moczydłowska, 1991) with trilobites that are actually younger than the oldest Moroccan and Siberian trilobites at ca. 520 Ma (Landing et al., 2021).

The “acritarch problem” in Avalonian depositional history is that recovery of S-F and H-S zone acritarchs seems to make the Ads 1–2 succession much younger in the Caledonian Highlands. The alternative view of this interval (Fig. 1) follows Landing et al. (2013) and includes (1) the terminal Ediacaran–middle Fortunian Stage with basal subaerial and marginal marine red beds, (2) overlying marine shelf with Ads 1–2 boundary bracketed by lowest Watsonella crosbyi Zone microfaunas, and (3) upper massive quartzite. How these units maintain a uniform succession often through >1.5 km in each Avalonian marginal platform region (Fig. 1) but differ greatly in age along depositional strike (i.e., Palacios et al., 2011) is difficult to understand if the acritarch correlations are temporally valid. Resolution of this seeming diachroneity of epeirogeny and related siliciclastic deposition along Avalonia lies in the documented diachroneity of Lower Cambrian acritarch zones (e.g., Moczydłowska and Vidal, 1988; Vidal et al., 1995). This diachroneity of Early Cambrian acritarch zones allows a resolution of the apparent diachroneity of epeirogenic activity and related siliciclastic deposition in the Caledonian Highlands.

Moczydłowska and Vidal (1988) and Vidal et al. (1995) showed that H-S Zone taxa as those reported by Palacios et al. (2011) under the Random (“Glen Falls”) sandstone in the Caledonian Highlands in Saint John occur in the sub-trilobitic, Tommotian Dokidocyathus regularis Zone of the Siberian Platform. This horizon is well below their Baltic occurrence with H. kjerulfi Zone trilobites (e.g., Moczydłowska, 1991). Palacios et al. (2017) also recovered S-F zonal taxa near the top of the Random Formation in SE Newfoundland, which also emphasizes the “acritarch problem.” In SE Newfoundland, the oldest strata (Ads 3, Petley Formation) unconformable on the Random Formation are far older (basal Sunnaginia imbricata Zone; Landing and Benus, 1988; Landing et al., 1989). The S. imbricata Zone is basal Tommotian, if not older, and certainly not an S. mickwitzi Zone equivalent. Further correlation problems are suggested by largely unresolved acritarch morphological plasticity (e.g., Moczydłowska, 2010; Wallet et al., 2022).

The Tommotian has a ca. 525 Ma base (Landing et al., 2013, 2021). This age means that the two acritarch zones are as much or more than 5 m.y. older in Siberia and Avalonia (i.e., Caledonian Highlands and SE Newfoundland) than the same two acritarch zones in the Baltic. Actually, dates on the origins of S-F and H-S taxa should be regarded as unknown as multi-species assemblages diagnostic of the zones appear abruptly, perhaps at non-sequences, on the East European platform of Baltica (e.g., Moczydłowska, 1991, figures 5, 6, and 8).

The ca. 530 Ma ash 7.6 m below the Random Formation (“Glen Falls”) at Somerset Street in Saint John (530.7 ± 0.9 Ma by Isachsen et al. [1994], recalculated to 530.02 ± 1.07 Ma by Schmitz [2012]) is important in reconstructing the deposition of the Caledonian Highlands. This ash can be regarded as showing a locally great age on the H-S Zone in Avalonia (Landing et al., 2013). In discussing the ash-bearing Somerset Street section, Landing and Westrop (1998a, figure 21) noted a bedding plane-parallel fault at the Chapel Island–Random (“Ratcliffe Brook”–“Glen Falls”) contact. The 8.6 m of strata cut out by the fault yield H-S Zone acritarchs where the section is traced SW (Palacios et al., 2011). Álvaro et al. (2023, figures 2 and 3) place a previously unreported and unrecognized (Landing and Westrop, 1998b) fault below the H-S Zone assemblage, which separates these acritarchs from a “too old” U-Pb ash date. They reassign the strata below this major fault, with bedding plane parallel and wrench faults that seem to have limited throw, to the older Quaco Road Member of Landing (1996a), although the wave-dominated sandstone with gray shale of the Quaco Road does not comprise the lower Somerset Street section. This reassignment of the sub-Random strata to the Quaco Road “accommodates” sparse acritarchs low in the Somerset Street section and allows a very low Cambrian correlation consistent with but based on an unillustrated Pterospermella velata Moczydłowska.

A higher correlation of sub-Random strata is suggested by skeletal fragments from the Mystery Lake Member tentatively referred to as trilobitic and perhaps showing a Series 2 assignment (Álvaro et al., 2023, figures 4L–4O). However, the lowest occurrence of trilobites should not be used as a marker for Series 2 (contra Peng et al., 2012), as trilobites, likely initially poorly skeletonized, probably ranged low in Series 1 (Landing et al., 2013). Bayesian analysis suggests a cryptic history with origin of trilobite major clades as far back as 530 Ma (Paterson et al., 2019), which is the same age as the Somerset Street ash. One of two ashes (NB 19-397) from the Mystery Lake Member (Barr et al., 2023) overlaps in age with the oldest trilobites in Morocco (Landing et al., 2021), but Pb-loss in this fractured succession with common hydrothermal veins may be a factor in the date.

Álvaro et al. (2023; also Johnson et al., 2019) question where Landing and MacGabhann’s (2010) glacial diamictite is located. This bed is important as it shows a high temperate/polar Avalonian (and Caledonian Highlands) location far from coeval tropical successions in West Africa (southern Morocco) with carbonate platforms, local evaporites, and archaeocyath reefs (e.g., Landing, 1996a; Landing et al., 2013, 2022). A detailed Hanford Brook East section and location of the glacial diamictite above trough cross-bedded sandstone on the Ads 1–2 boundary (Fig. 1) are in Landing and Westrop (1998a, figure 22) and Landing and MacGabhann (2010, figures 4–6C).

The glacial diamictite shows abundant dropstones (Figs. 2A and 2B) in the same outcrop figured by Álvaro et al. (2023, supplemental material 3, figure S5C2). It is 35 cm below the tip of the yellow arrow in Landing and MacGabhann (2010, figure 5B). With a lapsus, a sandstone with convolute bedding but no dropstones from a different horizon was mistakenly illustrated as a third picture of the diamictite (Landing and MacGabhann, 2010, figure 5C). Arguments that Landing and MacGabhann (2010, see figures 6A and 6B) do not describe a glacial diamictite (Álvaro et al., 2023) are countered by oversized clasts in a bed with non-erosive lower contacts (i.e., not a debris flow), sediment drapes on clasts, and no evidence of lateral movement of the bed. Absence of “incisions” that indicate a non-glacial origin (Álvaro et al., 2023) is puzzling. This non-standard term may refer to glacial striae that are noted by Landing and MacGabhann (2010) as rare in glacial clast studies and not observable on clasts in this indurated lithology. The argument that ball-and-pillow structures are absent (Álvaro et al., 2023) is countered by slabs (Landing and MacGabhann, 2010, figures 6A and 6B) showing discrete masses of sand separated by muddy matrix within and at the base of the bed. These are classic ball-and-pillows (e.g., Kuenen, 1965).

Álvaro et al.’s (2023, figure 9A) Avalonia reconstruction has about eight rift/half graben blocks (their “terranes”/“tectonostratigraphic units”) that move independently but have similar, coeval stratigraphic successions from Massachusetts to SE Newfoundland. Each “terrane” generally corresponds to one of the Appalachian Avalonian inliers defined by later orogenic faulting. Missing from this reconstruction is the Avalonian Northern Antigonish Highlands inlier in mainland Nova Scotia (e.g., Landing, 1996a, with references).

These inliers are not separate “terranes.” They do not accord with standard definitions of tectonostratigraphic/suspect/exotic terranes as fault-bounded geologic units in an orogen with distinctive geological histories that differ markedly from adjacent terranes and are not explained by facies changes (e.g., Howell and Howell, 1995). Earlier attempts to reconstruct a sort of “multi-Avalonia” as a number of terranes have been countered by the documentation of numerous lithologically identical formation- and member-level units that are of the same age and have coeval upper and lower contacts (i.e., eroded depositional sequence boundaries) between Avalonian tectonic inliers in the Appalachian-Caledonian orogen. The lithologic similarities allowing reconstruction of Avalonia as a unified terrane are numerous and include, among others, such features as a 0.4–1.0 m Coleoloides (“tube worm”) mud mound at the top of the sub-trilobitic Lower Cambrian (eastern Massachusetts, Northern Antigonish Highlands, Cape Breton, SE Newfoundland, English Midlands); abundant bentonites and other extrusives in the lower part of black Middle Cambrian mudstone (SE Newfoundland, North Wales); and a massive, middle Terreneuvian wave and tidal dominated mature quartz arenite in all Avalonian inliers from northern Rhode Island to Belgium (e.g., Landing et al., 2022, 2023).

Álvaro et al. (2023, figure 9C) even fragment some of the inliers into smaller “terranes”/“tectonostratigraphic units” with no relation to their original paleogeographic setting along Avalonia. Thus, the Burin Peninsula and Trinity and eastern Conception bay regions are shown as separate “terranes” and half grabens. A more appropriate reconstruction is to make them contiguous, not separated by terrane-defining faults in the Cambrian as they have quite similar cover successions (Fig. 1) with no evidence of a marginal rift fault. Indeed, these three areas are “stitched together” by the Random, Brigus, and younger cover sequence formations and form the best exposed marginal-inner platform region on the unified Avalonia microcontinent and terrane.

Álvaro et al.’s (2023, figure 9A) reconstruction shows Avalonia as a linear series of half graben blocks on an unexplained NW-SE transect. These “terranes,” which should be regarded as tectonic inliers, should be rotated ~90° into their roughly NNE alignment along the Avalonian terrane in the Appalachians. They should also be oriented roughly parallel to the proposed Avalonian transform fault (Atf) and illustrated without rift fault margins. The complete (and larger) Caledonian Hills area with the New River and Brookville “terranes” and the Bourinot Hills of Cape Breton Island would also be part of a strike-slip reconstruction of Avalonian North America (e.g., Landing et al., 2022, 2023).

Álvaro et al.’s (2023, figure 9C) Avalonian depositional regime as a linear series of half grabens has no modern analogue. However, deposition in a strike-slip regime with migrating depocenters has modern analogues and has similarities to the Yinggehai (or Yinggehai-Song Hong Basin in the South China Sea; e.g., Noda, 2013). As reviewed elsewhere, the Avalonian North American–British Isles–Belgium successions and Meguma zone developed in a strike-slip, not rift, regime defined by the Atf (Landing et al., 2022). The thickness of the terminal Ediacaran–Lower Ordovician succession of the Caledonian Highlands (~800 m; e.g., Tanoli and Pickerill, 1988) is dwarfed by comparison with thicknesses in modern plate boundary transform fault settings as the Dead Sea (~14 km) and Yinggehai (~17 km) basins (e.g., Noda, 2013, and references therein). The Atf was responsible for coeval extensional and collisional igneous melt production—a feature not part of a rift/half graben model (i.e., Landing et al., 2022).

A strike-slip regime maintains the relative position of the modern Avalonian inliers back into the Cambrian along a ribbon continent. The significance of the occurrence of the Avalonian tectonic inliers roughly along a NE-trend in the Appalachians (in modern coordinates) is that a traverse from inlier to inlier is roughly along depositional strike, and this frequently allows recognition of not only similar but coeval lithostratigraphic units and depositional sequence boundaries from Rhode Island to Belgium (Landing et al., 2022, 2023; Landing and Geyer, 2023; Fig. 1). A strike-slip regime integrates depositional sequences, facies and thicknesses, and minor coeval collisional and extensional igneous rocks as the result of Atf activity.