Tribes Hill–Rochdale formations in east Laurentia: proxies for Early Ordovician (Tremadocian) eustasy on a tropical passive margin (New York and west Vermont)

Abstract

Slow subsidence and tectonic quiescence along the New York Promontory margin of Laurentia mean that the carbonate-dominated Tribes Hill and overlying Rochdale formations serve as proxies for the magnitude and timing of Tremadocian eustatic changes. Both formations are unconformity-bound, deepening-shoaling, depositional sequences that double in thickness from the craton into the parautochthonous, western Appalachian Mountains. A consistent, 'layer cake' succession of member-level units of the formations persists through this region. The Tribes Hill Formation (late early Tremadocian, late Skullrockian, late Fauna B-Rossodus manitouensis Chron) unconformably overlies the terminal Cambrian Little Falls Formation as the lowest Ordovician unit on the New York Promontory. It was deposited during the strong early Tremadocian, or Stonehenge, transgression that inundated Laurentia, brought dysoxic/anoxic (d/a) slope water onto the shelf and led to deposition of the Schaghticoke d/a interval (black mudstone and 'ribbon limestone') on the Laurentian continental slope. The uniform lithofacies succession of the Tribes Hill includes a lower sand-rich member; a middle, dark grey to black mudstone that records d/a in eastern exposures; and an upper, shoaling-up carbonate highstand facies. A widespread (12 000+ km 2 ) thrombolitic interval in the highstand carbonate suggests the New York Promontory was rimmed by thrombolites during deposition of the Tribes Hill. Offlap and erosion of the Tribes Hill was followed by the relatively feeble sea-level rise of the Rochdale transgression (new) in Laurentia, and deposition of the Rochdale Formation. The Rochdale transgression, correlated with the Kierograptus Drowning Interval in Baltica, marks a eustatic rise. The Rochdale Formation represents a short Early Ordovician interval (early late Tremadocian, middle-late Stairsian, Macerodus dianae Chron). It correlates with a depositional sequence that forms the middle Boat Harbour Formation in west Newfoundland and with the Rte 299 d/a interval on the east Laurentian slope. The Rochdale has a lower carbonate with abundant quartz silt (Comstock Member, new) and an upper, thrombolitic (Hawk Member, new) highstand facies. Tribes Hill and Rochdale faunas are mollusc-rich, generally trilobite-poor, and have low diversity, Laurentian faunal province conodonts. Ulrichodina rutnika Landing n. sp. is rare in Rochdale conodont assemblages. Trilobites are also low in diversity, but locally form coquinas in the middle Tribes Hill. The poorly preserved Rochdale trilobites include the bathyurid Randaynia, at least two hystricurid species and Leiostegium.

Lower Ordovician trilobites from the Septembersø formation, North-East Greenland

“…Chapmanopyge pygidia have elevated pleural lobes, and pleural and interpleural furrows, if developed, generally do not extend to the pygidial margin. Loch (2007) assigned Randaynia Boyce, 1989 to the subfamily Bathyurellinae; Landing et al (2012) agreed that it was a bathyurid, but did not assign it to a subfamily. We follow Fortey & Bruton (2013), who assigned it to the Symphysurinidae (see below).…”

“…Whittington (1953), Fortey (1979) and Boyce (1989) and Fortey & Bruton (2013) assigned Bathyurellus?affinis Poulsen, 1937to Uromystrum Whittington, 1953 on the basis of the wide, concave-upward pygidial border. However, Loch (2007) reassigned it to Randaynia, and Landing et al (2012) accepted this assignment. This is rejected.…”

Rejoinder: Four-series concept of the Cambrian has a long history

“…In most subsequent work (e.g., Landing 2007;Landing et al 2009Landing et al , 2010Landing et al , 2012Landing & MacGabhann 2010;Landing 2011Landing , 2012 he has fairly consistently referred to a three-part subdivision of the Cambrian into Lower, Middle, and Upper parts, or a three/four-part subdivision with the Lower Cambrian embracing the Placentian and Branchian intervals. This approach contradicts the proposal for subdividing the Cambrian into four series and ten stages (Peng 2004) approved by Voting Members of the ISCS in 2005(Babcock et al 2005, Peng & Babcock 2005.…”

“…The Etcheminian Series yields trace fossils and small shelly fossils (SSFs), and corresponds roughly to the Terreneuvian Series of current global usage. If we follow Landing (2012) in recognizing that the Avalon sequence includes the Placentian (Landing et al 1989) and Branchian (Landing 1992) series, plus the traditional Middle and Upper Cambrian, then Matthew's Etcheminian Series added to the then-contemporary Lower, Middle, and Upper Cambrian meant that he likewise envisioned the lowermost Paleozoic (= Cambrian) as comprising four series.…”

“…Among the topics reviewed was the four-series, ten-stage model of the Cambrian voted into practice by the Subcommission (Peng & Babcock 2005, Babcock et al 2005. Landing (2012) pointed out that in our review (Peng & Babcock 2011) we neglected to cite his suggestions (Landing 1998a(Landing , 1998b that the Cambrian could be subdivided into four parts, and notes that the origin of the four-part concept evolved from studies in Avalonian sequences. In our review paper, we acknowledged that a four-fold subdivision of the Cambrian System had been discussed by various workers and, in the interest of brevity, exemplified this by "(e.g., Palmer 1998)".…”