A5: SEDIMENTOLOGY AND STRATIGRAPHY OF THE CAMBRIAN-ORDOVICIAN POTSDAM GROUP (ALTONA, AUSABLE AND KEESEVILLE FORMATIONS), NORTHEASTERN NY
Dave Lowe
Department of Earth Sciences,
University of Ottawa
Ottawa, ON, Canada
Ryan Brink, Charlotte Mehrtens
Department of Geology
University of Vermont
Burlington 05405
INTRODUCTION
Geological Setting
The Cambrian to Lower Ordovician Potsdam Group is the lowermost unit of the Paleozoic platform succession overlying rocks of the 1 – 1.5 Ga Grenville orogen within the Ottawa Embayment and Quebec Basin, two semi-connected fault-bounded basins that form an inboard salient of the St. Lawrence platform succession in eastern Ontario, northern New York State and western Quebec, over an area of approximately 26,000 km2 (Figure 1). Potsdam strata was deposited from the Late Early Cambrian until the Early Ordovician at subequatorial latitudes between 10° – 30° south approximately 200 – 400 km inboard of the south-facing Laurentian margin, during the late postrift and subsequent passive margin phase associated with the rifting and breakup of Rodinia (Torsvik et al. 1996; McCausland et al. 2007, 2011; Lavoie, 2008; Allen et al., 2009; Figure 2). It is mainly a siliciclastic unit that, in general, exhibits an upwards progression from terrestrial to marine deposits (Sanford and Arnott, 2010; with some exceptions, including the Altona Formation and Riviere Aux Outardes Member, see below). This terrestrial to marine progression is shared by stratal equivalents of the Potsdam Group on the Laurentian margin and in other North American cratonic basins that collectively form at the base of the Sauk Megasequence, a transgressive Megasequence that records eustatic rise across much of Early Paleozoic Laurentia preceding the Taconic Orogeny (Sloss, 1963; Lavoie, 2008).
There is no consensus from the existing Potsdam literature to suggest whether the Ottawa Embayment and Quebec Basin were pre-existing basins that were passively filled during the Early Paleozoic or were active rift or tectonically-reactivated basins. Also poorly understood is the relationship between these basins to structures within the rift margin to the east. However, features of the Potsdam including its highly irregular isopach distributions, presence of internal unconformities, variable provenance, localized soft-sediment structural deformation, in-situ brecciation, and debris flows adjacent to faults all suggest that tectonism played a role in Potsdam sedimentation, probably by the reactivation of inherited zones of tectonic weakness and pre-existing faults that developed in the Proterozoic (e.g. Wiesnet and Clark, 1966; Wolf and Dalrymple, 1984; Landing et al., 2009; Sanford and Arnott, 2010). In fact, the Ottawa Embayment and Quebec Basin lie in an area of prolonged rifting referred to as the Ottawa-Bonnechere Graben, a probable aulocogen that became active in the Neoproterozoic and was reactivated episodically until the Mesozoic and even until more recent times (Kay, 1942; Kumarapeli and Saull 1966; Kamo et al. 1995; Roden-Tice et al. 1999; Aylesworth et al. 2000; Malka et al. 2000; Bleeker et al. 2011); though the exact timing of episodic tectonic events is poorly constrained. Also poorly understood from existing literature is the approximate configuration of the basins and adjacent highlands during the Cambrian-Ordovician and the original extent of Cambrian-Ordovician strata due to post-Ordovician faulting, Mesozoic hotspot migration and unroofing of the Adirondack Dome (Kay, 1942; Fisher, 1968; Isachsen, 1975; Crough 1981; Roden-Tice et al. 1999).
The Potsdam Group is made up of four formations: the Altona, Ausable, Hannawa Falls and Keeseville Formations (Figure 3), which are described in more detail below. This trip will focus on the Potsdam strata in from the southern Quebec Basin and westward along the axis of the Oka-Beauharnois Arch that separates the Quebec Basin and Ottawa Embayment. In this area the Altona, Ausable and Keeseville Formations are present and exposed (Figure 4).

A5: SEDIMENTOLOGY AND STRATIGRAPHY OF THE CAMBRIAN-ORDOVICIAN POTSDAM GROUP (ALTONA, AUSABLE AND KEESEVILLE FORMATIONS), NORTHEASTERN NY

STOP 1. Murtagh Hill, NY: The basal Altona Formation
Walk up the dirt road until power lines are visible ahead and then veer into the woods on the left. BE CAREFUL OF SYRUP PIPING AND SUPPORTING GUIDE WIRES!! Proceed ∼25’ to the small stream and walk upstream to Stop 1. NO HAMMER OUTCROP (there is plenty of float available to sample).
This outcrop (Figure 10) exposes the stratigraphically lowest horizons of the Altona Formation in the region. The 3.5 meter thick exposure of coarse-grained, poorly sorted arkosic sandstones of Lithofacies 6 can be observed in close proximity to Precambrian basement rocks. Beds have non-planar (erosional?) bases and may exhibit graded bedding. Planar cross bedding can be present.
Based on grain size, graded bedding and the stratigraphic position of this lithofacies above basement and below arenaceous dolostones it is interpreted as recording sheetflood deposition into the marginal marine setting.

STOP 2A. NO HAMMER!
The first stop exposes Lithofacies 1 of the Altona Formation, between 22 to 24 meters in the Altona composite section (Figure 11). Although covered in moss and lichen, patient viewing will reveal a variety of ripple morphologies that are very characteristic of much of the Altona sandstones. Lithofacies 1 is a very fine to medium grained, moderately well-sorted sandstone containing a variety of sedimentary structures including upward bundling ripple cross laminations, combined flow ripple cross laminations, and swaley cross stratification (Figure 9). Along with these structures, planar cross stratification and dwelling burrows have been identified. Planar cross stratification is up to 1 m thick, pinches out laterally over 10 m, and is generally topped by 2D or 3D weakly asymmetrical ripple cross lamination. Swaley cross stratification up to 2 m thick passes into upward bundling cosets of ripple cross laminations < 0.5 m thick. Individual ripples consist of both form concordant and form discordant morphologies with wavelengths of up to 10 cm and amplitudes < 8 cm. Sandstones are dolomitized, reacting weakly to dilute HCl.
Lithofacies 1 is interpreted to represent sediment that accumulated on a wave-dominated upper to middle shoreface that was periodically inundated by storms. This interpretation is based on the combination of sedimentary structures including upward bundling ripples, swaley cross stratification, combined flow ripple cross lamination and planar cross stratification.
Walk upriver and cross the small access stream before ascending up to the field (while it is possible to walk in the shallow channel and view the stratigraphy, for efficiency with a large group we will ascend up to the field, walk northward and descend to the river at various points. WARNING: old, difficult to see barbed wire is present in the woods. Be careful!).

STOP 2B. Mudstone
This portion of the Altona stratigraphy, between 62-65 meters in the Altona composite section (Figure 12), exposes Lithofacies 4, a heterolithic unit dominated by mudstone but containing sandstone, siltstone, and dolostone. Note the interbedded nature of the sandstones and mudstones as well as the sedimentary structures found within the sandstones. Mudstones are red and grey in color with mm scale laminations. Trace fossils are present but rare. Sandstones are poorly sorted, fine to coarse-grained, and contain planar cross beds and ripple cross lamination. Slightly thicker and finer-grained sandstone beds contain structures resembling hummocky cross stratification (HCS). Planar cross beds have erosional bases, are occasionally graded, and change thickness laterally over ∼50 meters from 4 to 30 cm. Ripple cross laminations are present with cosets less than 10 cm thick and are either form concordant or discordant. Siltstones contain 1 cm thick ripple cross lamination and locally there are 1–2 cm thick lenses or nodules of dolostone.
Lithofacies 4 is interpreted to record deposition of offshore muds near the transition from the lower shoreface to offshore zones. The intercalated beds of sand are interpreted as tempestite deposits; episodically high energy deposition in an overall lower energy, further offshore setting. Transitions between lithologies are abrupt; there are no mud-rich dolostones, sandstones or muddy siltstones that would be expected on a graded shelf. This suggests the possibility that the deposition of mud represents a salinity controlled discharge-related event from hypopycnal flow.

STOP 2C. Dolostone bench
A prominent dolostone bench forms a small waterfall between 67-69 meters in the Altona composite section (Figure 13). Dolostone beds may contain disseminated sand-sized clastic material but the majority of beds are carbonate. Thin section analysis has not revealed fossils, sedimentary structures or fabrics that can be used to interpret paleoenvironments as there is evidence of multiple dolomitization events. The carbonate material may be shallow water (photic zone) algal in origin or it may be a chemical precipitate (“whitings”). The shoreline setting suggested for the clastic lithofacies of the Altona suggests that the near shore clastics may have passed offshore into more clear water carbonate ramp deposits. During sea level highstands, clastic material would be trapped in terrigenous fluvial systems which allowed carbonate deposition to commence. The interbedding of sandstone/mudstone and dolostone would record discharge events rather than sea level fluctuations. Dolostone beds containing clastic material frequently exhibit graded bedding of the sand, which may represent punctuated mixing during storm events. Dolostones containing fine sand disseminated throughout the carbonate may record eolian input of clastics.

STOP 2D. Poorly exposed uppermost Altona
This portion of the Altona stratigraphy, 81-84 meters in the Altona composite section, is poorly exposed but includes Lithofacies 5, a poorly sorted, feldspathic fine to medium-grained sandstone (Figure 14). 50 cm thick beds appear to be graded with faint ripple cross laminations. Due to its graded nature, stratigraphic position interbedded with dolostones, and stratigraphic position (overlain by Lithofacies 6), this lithofacies is interpreted to have been deposited under conditions of waning flow velocities associated with discharge events in the adjacent terrestrial fluvial system (Ausable Fm.). This lithofacies is interpreted to represent “event beds”: siliciclastic sediment washed onto the carbonate ramp following a runoff event (storm). This lithofacies may represent the nearshore component of the event beds represented by the graded planar cross bedded sandstones of Lithofacies 4.
Ascend up to the fields and walk south back to the vehicles, which need to turn around to retrace the route back to the Military Turnpike.

STOP 3. Great Chazy River, North Branch Near Ellenburg: The Ausable Formation
Park at the angler’s parking along Cashman Road. Walk south almost to the intersection of Cashman Road and Rt 190 (Star Road) and work your way down to the river. The section is continuous on the east side of the river from beneath the bridge southwards for some distance upriver. The section at this stop is along a river and waterproof footwear may be needed, depending on the current water level. Watch your step, as rocks are strewn across the riverbed and may be slippery.
This stop showcases the lithofacies and architectures of braided fluvial strata of the Middle Cambrian Ausable Formation, stratigraphically beneath the Keeseville but above the Altona. We are now on the foot wall of the east-dipping normal Havelock fault upon which lower potions of the Potsdam succession are exposed on the axis of the Oka-Beauharnois Arch (Figures 1 and 4), but nevertheless the Altona –Ausable contact is probably several hundred meters in the subsurface at this location. Strata here records deposition of pebbly arkose by braided rivers in a humid climate setting. The braided rivers flowed roughly SE and had nearby hinterlands 70 – 150 km to the NW. These rivers drained into a ∼N-S trending sub-basin that covered ca. ∼10,000 - 15,000 km2 in this region, resulting in the deposition of ∼300 – 400 m of braided fluvial arkose that occurs from Montreal south to Plattsburgh (Figure 6a).
Most of the section consists of ∼5 – 15 cm thick cross-stratified sets of moderately sorted upper medium to very coarse-grained pebbly sandstone formed by 2D or 3D dunes in shallow high energy steady unidirectional currents; although thicker (∼15 – 45 cm) planar-tabular sets are interpreted to have been deposited by simple downstream-accreting solitary bars (i.e. unit bars, see Bridge, 2003; Reesink and Bridge, 2011) (Figure 15). Architectural analysis reveals that dunes and unit bars coalesced to form larger-scale accretional elements (bounded by the red lines in Figure 15) where in which sets and cosets bounded by accretion surfaces dip at low angles in the downstream direction. These accretional elements are interpreted as the deposits of low relief compound braid bars in shallow braided rivers, ubiquitous components of modern and ancient braided fluvial environments. Other features include 4 – 10 cm thick pebble conglomerates that record energetic bar-top deposition and ∼ 1.5 – 6 cm thick argillaceous fine-grained sandstone beds and associated local rip-up clasts. In thin section the latter beds are shown to contain preferentially degraded feldspars, swollen biotite and eluvial clay and silt (Figure 16) recording preferential weathering (mostly by hydrolysis) and illuviation, and are therefore interpreted as overbank floodplain deposits.
Relatively thick (∼1.3 – 2.5 m) repetitive successions usually capped by thin overbank deposits are recognized here and elsewhere (bounded by heavy black lines in Figure 15) and interpreted as channel belt successions. At this outcrop the at least one entire channel belt succession is exposed, over- and underlain by parts of others.

STOP 4A. High Falls viewpoint
This very thick section exposes a sharp but conformable contact between two distinct parts of the Upper Cambrian quartz arenitic Keeseville Formation (Figure 17). The lower part was deposited in a coarse-grained sandy braided fluvial setting and exhibits large-scale cross-stratification formed by large dunes and unit bars with ∼westward paleoflow. Although in the past this lower unit has been correlated to the Ausable Formation due to similarity in sedimentary lithofacies, its detrital composition, detrital zircon ages and its westward paleoflow instead suggest that this is a part of the Keeseville Formation. The upper part of the section exhibits a planar-tabular character. Based on lithofacies analysis it is interpreted to have been deposited mostly by unconfined terminal ephemeral sheet floods in an arid or semi-arid terrestrial environment (i.e. ephemeral fluvial). Each sheet flood deposit was largely deflated and reworked by the wind during the long recurrence intervals between flooding, which resulting in a generally tabular architecture with abundant windblown deflation lags. In places, particularly near the top of the section, upstream,-migrating scour-filling features are exposed and interpreted as supercritical chutes-and-pools or cyclic steps. These features, particularly cyclic steps, are better exposed and thus described at stop 6B.
The sharp contact separating the braided and ephemeral fluvial strata records a shift in climate of from humid to semi-arid. This climate shift is correlated to a global Late Cambrian climate fluctuation at around ∼494 Ma recorded by numerous isotopic and lithofacies indicators and attributed to orbital forcing (Cherns et al., 2013).
Return to the parking lot, where we will proceed back to Jerdon Road and make our way to the section above the High Falls that exposed the ephemeral fluvial unit.

STOP 4B. Above High Falls
After leaving the High Falls parking lot, turn left (south) on to Jerdon Road. Then turn left (east) on to Pulp Mills Road, cross the bridge and turn right (south) on to La Plant Road. Park along La Plant Road and walk back to Pulp Mill Road and turn left (west). Cross the bridge and then make a right and walk north along a beaten path towards the Chateguay River. The outcrop is continuous along both sides of the river. From this (the east) side the finer detail of the ephemeral fluvial deposits can be seen, while spectacular large-scale features, including cyclic steps, can be seen by looking across to the other side of the river.
Most of the section along the west side of the river consists of planar-stratified medium to very coarse grained sandstone (Figure 18). Many if not most planar laminations are inversely graded and record deposition by the migration of wind ripples. Many planar layers are capped by coarse- to very coarse-grained deflation lags. Bedding surfaces are either featureless or expose low relief regular or irregular features suggesting adhesion and/or microbial processes. Sand Grains, although coarse, are rounded with common pits and bulbous edges, suggesting aeolian processes. Eluvial silt and clay is common along grain margins, indicating illuviation of fine-grained windblown material. Scours and low angle cross-stratified sets, probably formed by antidunes under energetic sheet floods, are common (as in Figure 18). Altogether this association records episodic deposition by terminal sheet floods (i.e. terminal splays) followed by long periods of aeolian deflation and reworking in an ephemeral fluvial environment. Most of these features can be observed close-up while walking north along the exposed sections. Farther north, pink colored apatite-cemented layers occur in lower parts of the section. These preserve high primary intergranular volumes and prohibit silica overgrowths present elsewhere (Figure 19) and thus are early intergranular cements. The origin of the phosphate is unknown, but was most likely derived from phosphate-bearing brines from underlying basement rocks.
As we walk back along the outcrop to Pulp Mill road, a large scour-filling feature can be seen on the other side of the river. This feature (outlined in Figure 20) is interpreted as a cyclic step deposit. Cyclic steps form under high energy and shallow supercritical flows in which the formation of hydraulic jumps permits the buildup and upstream-migration of cross-stratified sandstone (see Figure 20). This scour records the upstream-migration of the erosional trough of a cyclic step bedform. On the upstream side of the trough the flow accelerated causing erosion, while further downflow the flow expanded under a stable hydraulic jump resulting in stoss-side deposition (e.g. Cartigny et al. 2014). The sets are bounded by erosion surfaces that record natural fluctuations in the quasi-stable hydraulic jump during the same flood event. The final position of the cyclic step trough and its subsequent fill under subcritical flow conditions is recorded by the symmetrical scour immediately upstream of cyclic step sets, which contains downstream-migrating dune cross-stratified sandstone (Figure 20). Thus this features records high energy sheet flood conditions, perhaps during an atypically voluminous high energy flood or a flood closer to the source of overland drainage (such as a nearby isolated storm cell or front). The erosion and filling of cyclic step troughs (the scour) are crucial to the preservation of these features from subsequent aeolian reworking/deflation.
Following some discussion/observation, views of the east side can be made from the Pulp Mill Bridge and by a small beaten path on the other side of the bridge. But USE CAUTION the bank is VERY STEEP and quite high on this side. DO NOT GET TOO CLOSE TO THE EDGE! From here we can examine the large-scale stratal architecture of these deposits that previously we examined close-up. Most of the upper section is made up of the planar-stratified aeolian-reworked terminal splay deposits we examined close-up, while some parts below are interpreted as scour-filling antidune cross-strata or cyclic step sets (Figure 21).
Ascend the path back to Pulp Mill Rd and return to the vehicles parked on La Plant Road. We must turn the vehicles around and proceed back to the T junction with Pulp Mill Road.

STOP 5. Rainbow Quarry: The Keeseville Formation
Park at or just beyond the quarry entrance. The best section is exposed in the southwest corner of the Quarry. Here relatively thick (∼4-6 m) sets and cosets of cross-stratified medium grained sandstone are present and are interpreted to have been formed by the migration of large aeolian dunes in a generally dry aeolian erg setting (Figure 22). Close examination of the cross beds reveals inverse graded grain flow deposits in foreset strata and inversely-graded wind ripple laminations in the toe sets. At this location and at a similar quarry ∼1.6 km southwest of here the aeolian dune sets are separated by generally planar stratified sandstone underlain by horizontal surfaces erosion that most likely record dry to damp interdune erosion and deposition of interdune sand flats (see Hagadorn et al. 2008 for more details). Aeolian erg strata here are interpreted to be coeval to the ephemeral fluvial strata at the top of the previous stop (Chateauguay Chasm) and most likely accumulated by the deflation and downwind transport of the ephemeral fluvial splay deposits at the Chateauguay Chasm. This correlation is supported by similarities in detrital zircon ages.

STOP 6. Lasalle Dam, Altona NY: The Keeseville Formation
Park at the Feinberg Park and walk south along Devils Den Road across the bridge. The outcrop is along both sides of the river, and the section is continuous upriver to the abandoned McGregor Powerhouse. Following some discussion on the south side of the river, the north side can be accessed directly from Feinberg Park.
This section exposes a part of the Upper Cambrian Keeseville Formation which underlies the surrounding area in Altona north of the Flat Rock State forest (Figure 4). Here the Keeseville consists of a sheetflood-dominated ephemeral fluvial facies association that was deposited on a Late Cambrian coastal plain inboard of the paleo-shoreline located at or somewhere south of the Ausable Chasm section, ca. 40 km to the south, which was mostly a coastal sabkha throughout the Late Cambrian. A good modern approximation for such a setting might be the coastal plain of the Skeleton Coast of Northern Namibia in which ephemeral rivers discharge directly into the Atlantic forming ephemeral fluvial “deltas”.
Most of this section consists of beds of slightly inclined (≤10°) or horizontal planar-stratified medium grained quartz arenite usually truncated by low angle or horizontal surfaces (Figure 23). Internally these beds consist of finely-interlaminated shallow water (depth-limited orbital ripples, upper plane bed, rare current ripples) and aeolian (wind ripple and adhesion) lithofacies recording alternating wet and dry conditions. Coarse-grained deflation lags are common and desiccation cracks are also present but rare. These beds are interpreted terminal splay lobes in an ephemeral fluvial setting, and were built up over time by the accretion of episodic flood deposits. Flood deposits were largely reworked by the wind between episodic floods, resulting in common aeolian and adhesion features. Features such as oscillation ripple marks, parting lineations and adhesion ripples and warts immediately catch the eye on the bedding planes of these terminal splay lobes but belie the aeolian influence on deposition. Keep an eye out for less obvious features, such as inversely-graded laminations or low relief step-like features on bedding planes attributed to wind ripple stratification, coarse-grained deflation lags, or featureless planar laminations and surfaces most probably recoding the adhesion of windblown sand onto damp surfaces (i.e. adhesion stratification: Hunter, 1980; Kocurek and Fielder, 1982). Infiltrated matrix is also present in these strata and can be seen in thin section, and record the infiltration of clays into the vadose zone by illuviation, probably during rainstorms.
Upper medium to coarse-grained antidune cross-stratification is also present at this section and record high energy, probably more proximal, waning shallow sheet flood conditions. Antidune strata are recognized by low angle symmetrical trough-cross stratified bottom sets and convex-up low angle symmetrical formsets with wavelengths in the order of ∼25 cm – 1 m (Figures 23 and 24). It has a very similar structure to HCS, something noted from numerous fluvial deposits (Cotter and Graham, 1991; Rust and Gibling, 1990; Fielding, 2006) and also from experimentally produced antidune cross-stratification (Alexander et al., 2001; Cartigny et al., 2014). However, several features including the small thickness and wavelengths of sets and formsets, its 2D form on bedding planes, its relatively coarse grain size and lack of association with bioturbated lower shoreface deposits rules out classic HCS.
Also present near the base of the section are erosionally-based and crudely channelized trough cross-stratified sandstone formed by the migration of subaqueous dunes. These strata record the filling of an erosional distributary channel (like an arroyo or wadi) that had previously funneled energetic flood waters to terminal splays, probably during the late stages of lower energy floods leading to the abandonment of this particular distributary channel.
Isolated Proticnites trackways formed by arthropods have been observed at this location, but are rare.