Comment and Reply Early Paleozoic rifting and reactivation of a passive-margin rift: Insights from detrital zircon provenance signatures of the Potsdam Group, Ottawa graben: Comment

Ed Landing1,†, Osman Salad Hersi2, Lisa Amati1, Stephen R. Westrop3, and David A. Franzi4 1New York State Museum, 222 Madison Avenue, Albany, New York 12230, USA 2Department of Geology, University of Regina, Regina, Saskatchewan S4S 0A2, Canada 3Oklahoma Museum of Natural History and School of Geology and Geophysics, University of Oklahoma, Norman, Oklahoma 73091, USA 4Center for Earth and Environmental Sciences, State University of New York at Plattsburgh, Plattsburgh, New York 12901, USA

This comment outlines fundamental prob- relief, much Quaternary cover, and many syn- ford and Arnott’s (2010) Covey Hill (Ausa­ble) lems in Lowe et al.’s synthesis (2018) of the and post-depositional faults. As suggested, in Formation and is a different stratigraphic con- early Paleozoic stratigraphic architecture of part, above, problems in Lowe et al.’s (2018, also cept as they assigned the Chippewa Bay and the eastern Ottawa-Bonnechere aulacogen that 2017) OBa synthesis include: (1) a stratigraphic Edwardsville members of Sanford and Arnott preclude an accurate analysis of the region’s nomenclature that redefines, without comment, (2010) to their Keeseville Formation. Another geological and depositional history and even an a number of earlier defined units (e.g., their nomenclatural problem in Lowe et al. (2018, adequate stratigraphic provenance of the ­detrital “Potsdam” and “Hannawa Falls”); (2) exten- also 2017) is restriction and upgrading of San- zircons that they analyze. These problems in- sion of the “Potsdam Group” up into the Ordo- ford and Arnott’s (2010) Hannawa Falls Mem- clude: inaccurate lithostratigraphic and contra- vician; (3) correlations (e.g., their “Rivière aux ber (to 199 m of red fluvial siliciclastics and up- dictory biostratigraphic correlations that have Outardes Member”) that subvert the allostrati- per dune sand) within the Covey Hill (Ausable) also led to extension of the Potsdam “Group” graphic synthesis; (4) not relating the succession Formation to 0–25 m of aeolian sandstone of into the Lower Ordovician; a proposed allo­stra­ to eustatic signatures and sequence stratigraphy a “Hannawa Falls Formation” that overlies the tigraphy that contradicts itself when tested with elsewhere in east Laurentia; (5) biostratigraphic Ausable. NACSN (2005) recommendations on existing litho- and biostratigraphic data; redefi- errors in correlation; (6) misrepresentation of “redefinition” mean the Hannawa Falls of San- nitions of earlier defined lithostratigraphic units stratigraphic contacts and depositional facies ford and Arnott (2010) should have a different that lead to confusion on close reading; and er- (e.g., Altona and Theresa formations). name when “restricted” to the dune sandstone. rors in interpreted depositional environments The older rocks in the eastern OBa are termed Furthermore, the selection of different horizons and lithostratigraphic contacts. “Potsdam Formation” in the type area of north- for the Covey Hill (Ausable)–Keeseville contact Rodinia break-up and the early Paleozoic his- ern New York, and have a feldspar-rich, nonfos- leads to distinction of a “Potsdam Group” sensu tory of NE Laurentia are recorded by structural siliferous lower interval and an upper, generally Sanford and Arnott (2010), sensu Lowe et al. evidence, volcanics, and siliciclastic rocks asso- white-colored quartz arenite unit (respectively, (2017, 2018), and sensu Clark (1966). Such ciated with an ~700-km-long graben system that Ausable and Keesville members; e.g., Fisher, awkward terms are simplified below by use of extends from the Sudbury, Ontario, region into 1968; Landing et al., 2009; Fig. 1). Clark (1966) the traditional “Potsdam Formation” (Fig. 1). the Appalachian orogen. This down-dropped elevated the Potsdam to group status, likely as he The generally unfossiliferous, lowest silici- Ottawa-Bonnechere aulacogen (OBa), a major wanted to subdivide the sandstones members, clastics in the eastern OBa (SE Ontario–north- cratonic graben terminating in an orogen, is one but did not then have the ability to name formal ern New York–SW Quebec) are regarded in of the oldest recognized aulocogens (Shatsky, submembers, as proposed by the North Ameri- many reports to record Ediacaran subsidence 1946, 1955; Burke, 1977; Landing et al., 2009). can Commission on Stratigraphic Nomenclature (e.g., Sanford and Arnott, 2010, their fig. 5; It may be the failed arm of a triple junction, sug- (NACSN, below; Brett et al., 2018). “Potsdam review in Landing et al., 2009). However, the gested by evidence of a mantle plume in north- Group” is not a uniform concept. Clark’s (1966) oldest sedimentary rocks of the eastern OBa ern Vermont (Fig. 1, Tibbet Hill volcanics), or a “Potsdam Group” included upper sandstones are surprisingly young and show abrupt late rift segment offset by transform faults (review and dolostones now brought to the Lower Ordo­ Early Cambrian marine onlap with subsidence in Webster and Landing, 2016). The age(s) and vician Theresa Formation, and Sanford and of the “mouth” of the OBa south and SSW of vertical and lateral continuity of sedimentary Arnott (2010) and Lowe et al. (2017, 2018) Montreal, with coeval faulting and down-drop rocks are key to determining sediment and de- expanded the “Group” by including the litho- of the northern Vermont shelf ~80 km ESE to trital zircon provenance and paleogeography of logically dissimilar Altona Formation (Landing form the anoxic Franklin Basin (Landing, 2007, the OBa. Lowe et al. (2018, also 2017) combine et al., 2009) as a basal member, while defining 2012; Landing et al., 2007, 2009; Fig. 1). This outcrop with drill core data in an area with low nomenclaturally different “Potsdam” subdivi- rejuvenation of a passive margin (eastern OBa sions. Thus, the Ausable Formation sensu Lowe and on-strike Franklin Basin) persisted into †ed.landing@​nysed​.gov et al. (2018) is up to 400 m thicker than San- the middle Cambrian with deposition of higher

GSA Bulletin; March/April 2019; v. 131; no. 3/4; p. 695–698; https://doi.org/10.1130/B35104.1; 1 figure; published online 25 January 2019.

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Figure 1. Composite eastern OBa and Frank- of the Altona in its type area (Landing et al., lin Basin successions (modified from Land- 2009, their fig. 4). Almost all “Altona” localities ing et al., 2009, fig. 9). Covey Hill Ausable)(­ that yielded Proterozoic detrital zircons (Lowe of Sanford and Arnott (2010) to 400 m et al., 2018, their table 1, samples AS-2–5) are thicker than Ausable of Lowe et al. (2018); “braided fluvial.” This is not an Altona facies intra-Keeseville unconformity (Lowe et al., (Landing et al., 2009) and is better referred to 2018) not reported in equivalent Cairnside- as an “undifferentiated Ausable Formation.” Keeseville-Nepean Formation (Sanford and Reevaluation of Lowe et al.’s (2018) “Altona” Arnott, 2010). RaO is purported Rivière outcrops in Quebec and Ontario may be needed. aux Outardes Member of Keeseville For- The Altona Formation is an important unit mation of Lowe et al. (2017, 2018), but is in the OBa. It is relatively thick in its type area a siliciclastic-dolostone interval correlated (84 m) and the only Cambrian OBa unit that is herein with Tribes Hill Formation and iden- bracketed biostratigraphically. Upper lower and tical Saint-Clotilde Member. Figure divides middle Middle Cambrian trilobites from the Al- Cambrian into three global subseries (e.g., tona (Landing et al., 2009) indicate it brackets Landing et al., 2018, fig. 1 caption and ref- an ~7 m.y. interval (i.e., late Early and middle erences therein): Lower—Terreneuvian Middle Cambrian dates in Landing et al., 1998, Series + Series 2; Middle—Mialoingian 2015) that may exceed any other OBa Cambrian Series; Upper—Paibaian Series. Abbrevia- unit in duration. tions: C.—Cambrian; carb.—carbonate; The Altona is not a “tidal flat” facies (i.e., Lowe Dap.—Dapingian; E.—Ehmaniella; Fm.— et al., 2018, fig. 3; also 2017). Trough cross-bed- Formation; Gp.—Group; Kees.—Kees- ded sandstone likely deposited under trade wind eville; l./low./Low.—lower/lower/Lower; control dominates in lower member 1, and hum- ls.—limestone; m—middle; Mbr.—Mem- mocky cross-stratified (HCS) sandstone occurs ber; O.—Olenellus; P.-P.—Prosaukia- in upper member 2 (Landing et al., 2009; Lowe Ptychaspis; phyll.—phyllite; RaO—“Rivière et al., 2017, their fig. 8A). As HCS has depths of aux Outardes”; ss.—sandstone; u.—upper; formation of 13–50 m (e.g., Dumas and ­Arnott, U.—Ulrichodina quadraplicatus, V.—Vari- 2006), assignment of the Altona to a tidal flat abiloconus bassleri. facies is incorrect. Thick dolostones with large burrows in the upper Altona do not reflect inter- tidal, evaporitic facies (Lowe et al., 2017, p. 7) but record the hydrothermal dolomitization ­marine deposits­ of the Altona Formation and with minor sulphide mineralization common in their upward replacement by Ausable Formation eastern New York (Landing et al., 2007, 2009). fluviatile arkoses (Landing et al., 2009). Thinner dolostones with planar laminae (e.g., Sanford and Arnott (2010, p. 26) substituted Lowe et al., 2017, their figs. 8B and 8C) likely “Jericho Member” for Landing et al.’s (2009) show downslope(?) transport of carbonate and Altona Formation and noted the latter “were quartz silt, and the dolomite–organic-rich lami- presumably unaware of the term “Jericho Mem- nae figured by Lowe et al. (2017, figs. 8B, 8C) ber” previously proposed for the same strata” in are not intertidal “microbial mats” but horizontal a 2004 technical report for the New York State stylolites. Similarly, normally graded feldspathic Energy Research and Development Authority. quartz arenites high in the Altona (Landing et al., However, stratigraphic units cannot be proposed 2009, their fig. 4; Lowe et al., 2017, p. 10, their in unpublished reports (NACSN, 2005). In any fig. 8D caption) do not record “sheetfloods near case, the Altona became the lowest member of the mouth of a nearby braided river” but are the Covey Hill (Ausable) in Sanford and Arnott likely subtidal shelf tempestites. (2010) and Ausable of Lowe et al. (2017, 2018) near Montreal ~60 km NNE of the type local- The Altona Formation reflects long-term formations. ity, Lowe et al. (2018, fig. 3, upper) also show (7 m.y.) marine deposition at the mouth of the Lowe et al. (2018, p. 1380, their fig. 3) state the Altona separated from middle Proterozoic OBa and represents a facies belt across which that the Altona “intertongues with fluviatile basement by several meters of undifferentiated little sand was transported into the Franklin Ba- arkose of the Ausable Formation” and incor- Ausable fluvial arkose. This is contradicted by sin during Parker Formation deposition (Fig. 1). rectly cited Landing et al. (2009) as holding this Sanford and Arnott (2010, p. 26) who assign the Lowe et al. (2018) speculate that the open shelf opinion. Lowe et al. (2018, fig. 3, lower) incor- lowest part of this core to the Altona (“Jericho (not intertidal) Altona intertongues with fluvia- rectly show the Altona above fluviatile arkose Member”). Furthermore, the top of the pur- tile sandstone of the undifferentiated Ausable of an undifferentiated Ausable Formation in its ported “Ausable Formation” under the Altona Formation. However, Altona deposition may type area. However, a borehole in its type area in the core (Lowe et al., 2017, their fig. 7A and have largely preceded possible middle Middle shows the Altona resting on middle Proterozoic table 3 caption) differs from the fluviatile, “red Cambrian intensification of OBa faulting, in- basement without intervening arkose (Land- to grey coarse-grained arkose” of the Ausable creased production of “Ausable arkoses,” and ing et al., 2007, 2009; Fig. 1). In the Quonto- and is most similar to the trough cross-bedded, abrupt upward replacement of the Altona shelf International St. Vincent de Paul No. 1 core whitish, feldspathic quartz arenite of member 1 by Ausable fluviatile sands (Fig. 1).

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Sanford and Arnott (2010) provide no reason bites Ptychaspis and Prosaukia (Fisher, 1968). (their allounit 2) as they only note that Salad to downgrade the Altona Formation (their “Jeri- These taxa must not be figured at the Cambrian– Hersi and Lavoie (2000b) “did not recognize the cho Member”) to a member and include it as Ordovician boundary (Lowe et al., 2018, fig. 3 cryptic unconformity between the Ausable and the base of their Covey Hill (Ausable) Forma- lower, assemblage b), but occur about in the Keeseville.” Actually, a “paleokarst” caps the tion. Lowe et al.’s (2017, p. 6) argument using middle upper Cambrian in sandstone-rich facies RaO in its type area (Salad Hersi and Lavoie, NACSN (2005) Article 25b to make the Altona a of the Upper Mississippi Valley. A ca. 500 Ma 2000b, fig. 5), which alternatively, might fit well member of their Ausable Formation is moot: no estimate on lower Keeseville trilobites and a as the unconformity at the top of the Covey Hill evidence indicates that the geochronologically ca. 494 date (hazarded herein) on the Upper (Ausable) of Sanford and Arnott (2010) and long-ranged Altona is intercalated with, rather Mississippi Valley Ptychaspis-Prosaukia Zone within the same strata later, confusingly, termed than largely older than, Ausable Formation flu- correlated into the Ellipsocephaloides-Idahoia “Keeseville Formation” by Lowe et al. (2018). vial arkoses (Landing et al., 2009; Fig. 1). Even Zone of south Laurentian carbonate facies (see Unfortunately, there is a second unconformity if part of the Altona is intercalated with the un- chronology Peng and Babcock, 2008) mean the above the type RaO and in the quartz arenites differentiated Ausable Formation of Lowe et al. oldest and youngest trilobites known from the of the “Cairnside Formation” (=Keeseville) in (2018), the lithic differences (marine shelf feld- Keeseville suggest a duration roughly com­para­ Quebec (Salad Hersi and Lavoie, 2000a). This spathic quartz arenite, red mudstone, and dolo­ ble to or even less than the Altona Formation. second unconformity is not accounted for in stone versus fluviatile, subaerial red and gray The belief that northerly and westerly diach­ Lowe et al.’s (2018, fig. 3) allostratigraphy and arkose) and the relatively tabular form of the Al- ronous onlap of the Potsdam continued into the lithostratigraphic synthesis and leads to a ques- tona comport with assignment to a formation—a Early Ordovician (e.g., Fisher, 1968) is not sup- tioning of their stratigraphic architecture of the lithostratigraphic grade that reflects a recom- ported by any evidence. This conclusion has been Potsdam Formation. mendation that formations should correspond “justified” with the reassignment, without dis- The correlation and best name for Lowe whenever possible to genetic units (NACSN, cussion by Sanford and Arnott (2010), of litho­ et al.’s (2018, fig. 3, upper) “RaO” and its sig- 2005, Articles 23e, 24 introduction, 24c). logically problematical sandstones with upper nificance as a key unit of their “allounit 2” Stratigraphic continuity of the Potsdam For- Tremadocian conodonts from the Theresa For- are problematical. Sanford and Arnott (2010) mation is broken by several regionally extensive mation (Dix et al., 2004) to the Nepean Forma- termed this interval the Edwardsville and Chip- hiatuses (Fig. 1). The durations of these breaks tion and correlation of these sandstones with the pewa Falls members of the Covey Hill (Ausa­ are difficult to bracket because of the absence purportedly coeval Keeseville. However, an un- ble) Formation, and G.S. Nowlan (in Lowe of fossils in the Ausable Formation sensu Lowe broken Cambrian–Ordovician succession in the et al., 2017, their appendix 1) recovered cono­ et al. (2018) (but without the Altona Formation as OBa is unlikely. With exception of the Great Ba- dont elements from the “Chippewa Falls Mem- a basal “member”) and their rarity in the quartz sin and Southern Oklahoma aulacogen, continu- ber” at the Rockland outcrop in Ontario. The arenite-dominated Keeseville Formation sensu ous boundary interval sequences are unknown lithostratigraphic assignment of these conodont- Lowe et al. (2017, 2018, fig. 3). As discussed be- on the Laurentian shelf, where an unconformity bearing strata is, frankly, obscure, as the Rock- low, available fossils restrict the Potsdam Forma- separates upper Cambrian and lower, not lower- land locality has three formational assignments tion to the Cambrian and preclude its description most, Tremadocian rocks (e.g., Landing, 1988). in the earlier literature. The “Chippewa Falls as “Cambrian–Ordovician” (e.g., Sanford and The best evidence that the upper Potsdam Member” strata of G.S. Nowlan’s report (i.e., Arnott, 2010; Lowe et al., 2017, 2018). Formation is not Lower Ordovician and that the Covey Hill [Ausable] Formation of Sanford and The oldest fossils known in the Keeseville stratigraphic architecture of the OBa preserves a Arnott, 2010) are also termed the RaO Member Member (Fig. 1) are Laurentian Crepicephalus eustatic signature and is more complex than por- of the Keeseville “Formation” in the same re- Zone trilobites found near the Quebec–New trayed is presented by Lowe et al. (2018, fig. 3, port (Lowe et al., 2017). In addition, Sanford York border (Fisher, 1968) and farther south upper, assemblage e). They figure a “Cambrian– and Arnott (2010, p. 37) described the Rockland near Fort Ann, New York (Landing et al., 2007, Ordovician boundary” fauna from the top of al- rocks as Nepean (i.e., Keeseville) Formation. stop 1.4). Not noted by Lowe et al. (2018, lounit 2 and top of their “Rivière aux Outardes However, the dolostone (unknown elsewhere fig. 3, lower), these upper middle Cambrian Member” (RaO) of the “middle Keeseville.” in the Nepean)-quartz sandstone association at trilobites suggest the upper Altona Forma- This report is very problematical. The RaO was Rockland is comparable to facies traditionally tion–lower Keeseville interval was ~3 m.y. named (Clark, 1966) from SW Quebec based referred to the Lower Ordovician Theresa For- long (i.e., ca. 503 Ma date on the upper Al- on sandstones that are finer-grained than the mation (e.g., Salad Hersi et al., 2002). tona, noted above, and ca. 500 Ma date in the overlying uppermost Potsdam and have swale- Nomenclatural problems aside, the age-diag­ Crepi­cephalus Zone fide Peng and Babcock, like structures. He never reported the RaO as “a nostic conodont Variabiloconus bassleri was 2008). Thus, the Ausable Formation (without locally calcareous marine unit” fide Lowe et al. found in “RaO” samples at Rockland; this sup- the Altona), the Ausable–Keeseville hiatus, and (2017, p. 16), although Salad Hersi and Lavoie plies a fourth stratigraphic interpretation that is the lower Keese­ville (Lowe et al., 2018, fig. 3, (2000b) found a dolomitic sandstone-rich inter- preferred herein and mandates reevaluation of lower) bracket less time than the Altona. Lowe val of the RaO north of Montreal. the OBa stratigraphic architecture of Lowe et al. et al. (2018, fig. 3, lower) show a long hiatus Lowe et al. (2018, fig. 3, upper) show their (2018, fig. 3). This fossil occurrence is placed below the Keese­ville with almost none of the “RaO” extending from Quebec along the north- at the Cambrian–Ordovician boundary and top member extending into the upper middle Cam- ern OBa outcrop belt into Ontario. Their as- of allounit 2 by Lowe et al. (2018, fig. 3, up- brian. However, the widespread occurrence of signment of their “RaO” to the lower part of per, assemblage e). However, V. bassleri is a Crepicephalus Zone trilobites in eastern New the quartz arenite-dominated Keeseville is un- diagnostic upper lower Tremadocian form and York suggests that much more of the Keeseville warranted as the RaO is feldspathic in its type suggests an age of ca. 483 ± 1 Ma (Landing in the SE OBa is upper middle Cambrian. area and best referred to the Ausable (e.g., Salad et al., 1997), which is ~5 m.y. younger than the The youngest fossils that can be confidently Hersi and Lavoie, 2000a). It is unclear how top of the Cambrian and ~9 m.y. younger than related to the Keeseville Member are the trilo- Lowe et al. (2017, p. 16) recognize the “RaO” the “guesstimate” on the youngest confidently

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known Keeseville trilobites. More importantly, all “Theresa” reports in the southeasternmost (latest early–middle Cambrian) marine onlap of cra- tonic New York and Quebec: Geological Magazine, V. bassleri is characteristic of the lowest Ordo- OBa have grouped the upper Cambrian Galway v. 146, no. 4, p. 552–566, https://​doi​.org​/10​.1017​ vician depositional sequence that occurs across and a Lower Ordovician Theresa with a cryptic /S0016756809006013​. Laurentia (Fig. 1, “RaO”). unconformity between them into a “Theresa”­ Landing, E., Westrop, S.R., Adrain, J., and Kröger, B., 2012, Early Ordovician (Tremadocian) eustasy and biotas on Coeval, but carbonate-dominated, upper (e.g., Lowe et al., 2018, fig. 3, upper). The The- a tropical passive margin––Tribes Hill–Rochdale For- lower Tremadocian rocks occur in the southern resa s.s. is a second Ordovician depositional mations in eastern Laurentia (New York and adjacent Vermont): Geological Magazine, v. 149, no. 1, p. 93– OBa in New York (Fisher’s, 1968, “Spellman sequence above the lowest Ordovician Tribes 123, https://​doi​.org​/10​.1017​/S0016756811000598​. Formation,” and Tribes Hill Formation of Land- Hill/“RaO” unit in Lowe et al. (2017, 2018) that Landing, E., Geyer, G., Buchwald, R., and Bowring, ing et al., 2012) and SW Quebec (identical Saint- records eustatic, likely not epeirogenic, control S.A., 2015, Geochronology of the Cambrian: a precise Middle Cambrian U-Pb date from the Ger- Clotilde Member in Clark, 1966). However, in OBa deposition. man margin of West Gondwana: Geological Maga- these Lower, not lowermost, Ordovician Tribes zine, v. 152, no. 1, p. 28–40, https://​doi​.org​/10​.1017​ Hill–Saint–Clotilde carbonates are not figured REFERENCES CITED /S0016756814000119​. Landing, E., Antcliffe, J., Geyer, G., Kouchinsky, A., An- nor are their relationships shown to lower Paleo- Brett, C.E., Pratt, B.R., and Landing, E., 2018, North Ameri- dreas, A., and Bowser, S.S., 2018, Early evolution of zoic allounits in the OBa by Lowe et al. (2018, can Commission on Stratigraphic Nomenclature Note colonial animals (Ediacaran Evolutionary Revolu- fig. 3, lower). These carbonates correlate with 68—Application for addition of submembers to the tion–Cambrian Evolutionary Radiation–Great Ordovi- North American Stratigraphic Code: A case for for- cian Diversification Interval): Earth-Science Reviews, the “RaO” locality with V. bassleri. Thus, this malizing lithostratigraphic units of intermediate rank: v. 178, no. 1, p. 105–135, https://​doi​.org​/10​.1016​/j​ “RaO” locality is much younger than allounit 2/ Stratigraphy, v. 15, no. 2, p. 103–108, https://​doi​.org​ .esrrev​.2018​.01​.013 /10​.29041​/strat​.15​.2​.103​-108​. Lowe, D.G., Arnott, R.W.C., Nowlan, G.S., and McCracken, allounit 3 of the Potsdam Formation in the OBa, Burke, K., 1977, Aulacogens and continental breakup: An- A.D., 2017, Lithostratigraphic and allostratigraphic and cannot be fundamental to defining allounit 2. nual Review of Earth and Planetary Sciences, v. 5, framework of the Cambrian–Ordovician Potsdam The conodont-bearing “RaO” locality seems to p. 371–396, https://​doi​.org​/10​.1146​/annurev​.ea​.05​ Group and correlations across early Paleozoic southern .050177​.002103​. Laurentia: Canadian Journal of Earth Sciences, v. 54, record a siliciclastic-rich equivalent of the Tribes Clark, T.H., 1966. Châteauguay area: Châteauguay, Hinting- p. 550–585, https://​doi​.org​/10​.1139​/cjes​-2016​-0151​. Hill depositional sequence, the lowest part of ton, Beauharnois, Napierville and St. Jean counties: Lowe, D.G., Arnott, R.W.C., Chiarenzelli, J.R., and Rain- which has siliciclastic mudstone and thin sand Québec Department of Natural Resources, Geological bird, R.H., 2018, Early Paleozoic rifting and reacti- Report 122. vation of a passive-margin rift: Insights from detrital in New York (Fig. 1; e.g., Landing et al., 2012). Dix, G.R., Salad Hersi, O., and Nowlan, G.S., 2004, The zircon provenance signatures of the Potsdam Group, As detailed by Landing (2012), the Pots- Potsdam–Beekmantown Group unconformity, Nepean Ottawa graben: Geological Society of America Bul- Formation type section (Ottawa, Ontario): a cryptic letin, v. 130, no. 7/8, p. 1377–1396, https://​doi​.org​/10​ dam is succeeded by two formations of the sequence boundary, not a conformable transition: Ca- .1130​/B31749​.1​. Beekmantown Group composed of carbonates nadian Journal of Earth Sciences, v. 41, no. 8, p. 897– North American Commission on Stratigraphic Nomencla- (limestone/hydrothermal dolostone) with mi- 902, https://​doi​.org​/10​.1139​/e04​-040​. ture, 2005, North American Stratigraphic Code: The Dumas, S., and Arnott, R.W.C., 2006, Origin of hummocky American Association of Petroleum Geologists Bul- nor quartz arenites in eastern New York. Both and swaley cross stratification—The controlling influ- letin, v. 89, no. 8, p. 1547–1591. of these similar formations have been mistak- ence of unidirectional current strength and aggradation Peng, S., and Babcock, L., 2008, Cambrian Period, in Ogg, enly referred to the “Theresa Formation” of the rate: Geology, v. 34, no. 12, p. 1073–1076, https://​doi​ J.G., Ogg, G., and Gradstein, F.M., eds., The Concise .org​/10​.1130​/G22930A​.1​. Geologic Time Scale: Cambridge University Press, eastern OBa in a number of reports. The The- Fisher, D.W., 1968, Geology of the Plattsburgh and Rouses p. 37–46. resa in its type region of southern Ontario and Point, New York–Vermont quadrangles: New York Salad Hersi, O., and Lavoie, D., 2000a, Pre-Cairnside For- into SW Quebec is a Lower Ordovician unit. State Museum, Map and Chart Series 10, 51 p. mation carbonate-rich sandstone: evidence for a Car- Landing, E., 1988, Cambrian–Ordovician boundary in North bonate platform in southwestern Quebec?: Geological It has conodonts (e.g., Ulrichodina quadrapli- America: Revised Tremadocian correlations, uncon- Survey of Canada, Current Research 2000-D3, 8 p. cata; Calaptoconus quadraplicatus in Lowe formities, and “glacioeustasy,” in Landing, E., ed., The Salad Hersi, O., and Lavoie, D., 2000b, Lithostratigraphic Canadian Paleontology and Biostratigraphy Seminar, revision of the Upper Cambrian Cairnside Formation, et al., 2017) no older than late Tremadocian, is Proceedings: New York State Museum Bulletin 462, upper Potsdam Group, southwestern Quebec: Geologi- thus younger than the Tribes Hill depositional p. 48–58. cal Survey of Canada, Current Research 2000-D4, 8 p. sequence, and unconformably locally overlies Landing, E., 2007, Ediacaran–Ordovician of east Lau- Salad Hersi, O., Lavoie, D., Hilowle Mohammed, A., and rentia—Geologic setting and controls on deposition Nowlan, G.S., 2002, Subaerial unconformity at Pots- quartz arenites of the Potsdam “Group” (Salad along the New York Promontory, in Landing, E., ed., dam–Beekmantown contact in the Quebec reentrant: Hersi et al., 2002; Dix et al., 2004). The The- Ediacaran–Ordovician of East Laurentia: S.W. Ford regional significance for the Laurentian continen- resa is regarded as a siliciclastic-rich equivalent Memorial Volume: New York State Museum Bulletin tal margin history: Bulletin of Canadian Petroleum 510, p. 5–24. ­Geology, v. 50, p. 419–440, https://​doi​.org​/10​.2113​/50​ of an upper Tremadocian depositional sequence Landing, E., 2012, Time-specific black mudstones and .3​.419​. known as the carbonate-dominated Rochdale global hyperwarming on the Cambrian–Ordovician Sanford, B.V., and Arnott, R.W.C., 2010. Stratigraphic and slope and shelf of the Laurentia palaeocontinent: structural framework of the Potsdam Group in in east- (=“Fort Ann”) Formation in eastern New York Palaeo­geography, Palaeoclimatology, Palaeoecology, ern Ontario, western Quebec and northern New York: (e.g., Landing et al., 2012; Fig. 1). v. 367–368, p. 256–272, https://​doi​.org​/10​.1016​/j​ Geological Survey of Canada Bulletin 5907, 84 p. A similar dolostone and quartz arenite unit .palaeo​.2011​.09​.005​. Shatsky, N.S., 1946, The great Donets Basin and the Wichita Landing, E., Bowring, S.A., Fortey, R.A., and Davidek, K., system: comparative tectonics of an ancient platform: is the late Cambrian Galway Formation, which 1997, U-Pb zircon date from Avalonian Cape Breton SSSR Akademie Nauk Izvede’stvo, Geologiscii Serii, conformably overlies and is laterally equivalent Island and geochronologic calibration of the Early v. 1, no. 1, p. 5–62. to the upper Potsdam in eastern New York (e.g., Ordo­vician: Canadian Journal of Earth Sciences, v. 34, Shatsky, N.S., 1955, On the origin of the Pachelma trough: no. 5, p. 724–730, https://​doi​.org​/10​.1139​/e17​-059​. Byulletin Moskova Obschestra Lyuditeley Pribody: Landing, 2012), The Galway has been mapped Landing, E., Davidek, K., Westrop, S.R., Geyer, G., and Otdelenie Geologii, v. 5, no. 1, p. 5–26 [in Russian]. as “Theresa” on the OBa’s south flank (e.g., Heldmaier, W., 1998, Duration of the Early Cambrian: Webster, M., and Landing, E., 2016, Geologic context, bio- Fisher, 1968). Lowe et al.’s (2018, fig. 3, lower) U-Pb ages of volcanic ashes from Avalon and Gond- stratigraphic significance, and systematic revision of wana: Canadian Journal of Earth Sciences, v. 35, no. 4, late early Cambrian olenelloid trilobites from the Parker illustration of a sandstone-dolostone interval p. 329–338, https://​doi​.org​/10​.1139​/e97​-107​. and Monkton formations, northwestern Vermont: Aus- conformable with the Potsdam is likely correct, Landing, E., Franzi, D.A., Hagadorn, J.W., Westrop, S.R., tralasian Palaeontological Memoirs, v. 49, p. 193–240. Kröger, B., and Dawson, J., 2007, Cambrian of east but this overlying interval is upper Cambrian Laurentia: field workshop in eastern New York and Science Editor: Rob Strachan Galway and not Lower Ordovician Theresa western Vermont, in Landing, E., ed., Ediacaran–Ordo- Associate Editor: John Waldron vician of east Laurentia: S.W. Ford memorial volume: Formation. The “Theresa” is not a diachronous, Manuscript Received 6 September 2018 New York State Museum Bulletin 510, p. 25–80. Manuscript Accepted 28 December 2018 westerly younging “Cambrian–Ordovician” unit Landing, E., Amati, L., and Franzi, D.A., 2009, Epeiro- (Lowe et al., 2018, fig. 3, lower). Indeed, almost genic transgression near a triple junction: the oldest Printed in the USA

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