FASHIONABLY LATE:
Slide 1: What I will discuss are events that took place about 510 m.y. ago, and
took place surprisingly quickly, on the margin of Laurentia, or “ancestral North
America.” An aulocogen, red oval in title, is a major linear zone of subsidence or
foundering on the margin and within continents.
Slide 2: Aulocogens are related to the rifting, or pull-apart of continents. They
form Y-shapes with the broken edge of a continent. An aulocogen is defined by a
major fault system that leads to foundering and minor igneous activity not only on
the rifted margin of a continent, but can extend several thousand km across a
continent..
The Ottawa–Bonnechere aulocogen is a major down-dropped structure. It is
filled with Cambrian- and Ordovician-age sedimentary rocks in the St. Lawrence
River lowlands of northern New York and adjacent Quebec and Ontario. These
almost flat-lying sedimentary rocks face the folded and faulted Appalachian
Mountains. Regionally, the fault system of the Ottawa aulocogen extends 1,500 km
way to the WNW almost to Lake Huron—this is a giant geological structure. The
field area I’ll describe is at the red star in NE New York.
If you stand just south of the field area, almost at the limit of the Adirondack
Mountains—
Slide 3: …you dimly see Mont Real around which Montreal is build. This 100 km
distance across the St. Lawrence low-land spans the Ottawa aulocogen.
Slide 4: New York rocks range from basically unlithified Ice Age sediments that
cover almost all the state. Age of Dinosaurs rock, Triassic through Cretaceous, are
the red and beige in SE New York. The sedimentary rocks of most of the state are
flat lying in the Allegheny Plateau, and include much of the Paleozoic. The
basement of New York is everywhere 1.1 billion years old or more, and is domed
up to form the Adirondack Mountains. The field area of this talk, again a red star,
is in Cambrian-age rocks of the Ottawa aulocogen north of the Adirondacks.
I’ll give a conclusion early, the northern New York and adjacent Canadian
basement foundered in the Ottawa aulocogen in the late Early Cambrian—
basically, about 510 m.y. ago eastern Laurentia begins to crack up.
1
Slide 5: In 1915, almost 100 years ago, Alfred Wegener, a physicist and
meteorologist, proposed continental drift. Continental drift was the ancestor of
modern plate tectonics. 1915 was the wrong time for Germans to propose anything,
by a German, and continental drift languished for generations against the idea of a
fixist Earth.
Wegener observed that some continents, as Africa and South America, had
margins that could be fitted together like a puzzle, and this suggested break up of
an early supercontinent. Also, his late Paleozoic–early Mesozoic supercontinent
explained the intercontinental distributions of fossil organisms that could not cross
wide oceans—the Glossopteris flora, in green, or land and fresh- water reptiles that
also could not cross oceans, the beige, brown, and blue bands. Finally, Wegener
proposed that mountain belts were formed by the collisions of continents—as the
Himalayas. Pretty good.
Wegener’s Urkontinent was renamed Pangaea. Now, there is appreciation of
a supercontinent cycle, with supercontinent break-up and rafting away of smaller
continents leading to formation of a new supercontinent. Supercontinent break-up
is important—increased provincialism allowed by small isolated continents is
associated with evolutionary radiations. Pangaea’s break-up was followed by land
and marine developments as the appearance of dinosaurs and mammals.
An older supercontinent called Rodinia, largely constructed about a billion
years ago largely broke up after the early Ediacran—a break-up followed by the
origin and diversification of multicelled Ediacaran animals followed by the
Cambrian Radiation and origin of modern marine organisms.
Slide 6: This talk details local rifting on the northeast margin of Laurentia—one of
Rodinia’s daughter continents. The lower left picture shows an early stage in the
break-up of Rodinia, which left Laurentia and the New York region in tropical
latitudes in the Ediacaran through most of the Paleozoic.
Wegener noted the angular margins formed by the rifting of continents—as
the west South America and central west African margins. These mark the active
arms of so-called triple junctions in plate tectonics activity. A passive arm, as the
modern, fault bounded, volcanic, subsiding region of the African Chain of Lakes
area is the third arm of a triple junction. The corresponding active arms are marked
by the Red Sea and Gulf of Aden north of the Horn of Africa with Saudi Arabia
separating from Africa.
2
Slide 7: The eastern margin of Laurentia was known certainly since the 1960s to
consist of a series of saw-tooth-like re-entrants and promontories; these are ancient
active arms. The Ottawa aulocogen, a failed arm, was first called the Ottawa
graben and compared with the African Chain of Lakes by 1966.
Key developments since then have included documenting a 1,500 km-long,
WNW-trending fault province that continues the Ottawa aulocogen into western
Ontario.
In addition, fairly precise dates have been determined on rift related, mafic
igneous rocks that lie along the eastern margin of Laurentia and commonly form
the oldest rocks above Grenvillian, or billion year-old basement. These rocks
indicate late Cryogenian, as in North Carolina, to relatively late rifting in the
Tibbett Hill volcanics in northest Vermont. The Tibbett Hill volcanics at 554 m.y.
are not tooo different in age from the minor Callender and Mt. Rigaud volcanics
right in the Ottawa aulocogen.
Slide 8: The 554 m.y. Tibbet Hill volcanics are up to 600 m of massive pillow
basalts and intrusives. They rapidly thin to the south in Vermont where they form
blue-green interlayers, lower right photo, in rift siliciclastics.
Slide 9: The Tibbit Hill volcanics are thickest immediately along the strike of the
Ottawa aulocogen. Being about the same age as the Callander and Mt. Rigaud
mafic volcanics, these roughly simultaneous events indicate Ediacaran rifting of
east Laurentia and early formation of the Iapetus Ocean along the Quebec Reentrant and New York Promontory.
So faulting, volcanism, and formation of the Ottawa aulocogen are
Ediacaran events—but when did the aulocogen subside and accumulate sediments?
Slide 10: The oldest sedimentary rocks on the northeast Laurentian craton and in
the Ottawa aulocogen are commonly quartz arenite of the upper Potsdam
Formation. The red stars mark regions where a nonconformity exists between
billion year-old Grenvillian gneisses or intrusives and overlying quartz arenite.
Remember, the Laurentian craton and Ottawa aulocogen are west of the folded and
faulted Appalachian Mountains—which is the largely white area in the right half of
the picture.
Slide 11: The upper Potsdam Formation is commonly called Potsdam Sandstone,
but the unit includes sandstone, some dolstone, and mudstone. The upper Potsdam,
or Keeseville Member, is a pink or white hard siliceous quartz arenite. It forms
canyons and cliffs around the Adirondack Mountain and in the Ottawa aulocogen.
The red star in the stratigraphic column shows its position as the lowest Paleozoic
unit in eastern New York and around Ottawa in the aulocogen.
3
Slide 12: Age of the Keeseville? Around most of the Adirondacks and around
Ottawa, this quartzose sandstone nonconformably overlies billion year old gneiss,
and is the basal unit of a thin Cambrian shelf succession—that often less than 300
m thick with the Keesville up to 150 m, but mostly less. All sedimentary features
show shallow marine deposition, with common fossils, particularly burrows, and in
eastern NY, large stranded, sand-filled jellyfish common on several surfaces.
Complex burrows show the Keeseville is Cambrian, not Ediacaran, and younger
than formation of the aulocogen..
Slide 13: Again, trace fossils show a late Early Cambrian or younger age with
arthropod telson marks and foot prints of Prototaxites. The largest known
Cambrian trace fossil Climactichnites, maybe made by a mollusk, is common in
the Keeseville and known elsewhere in Laurentia in the Middle Cambrian.
Trilobites occur sparingly in the Keeseville, and show that it is terminal Middle
Cambrian to middle Late Cambrian in age. Again, the Keeseville, locally the oldest
sedimentary rock, is much younger than the Ottawa aulocogen’s time of formation.
Slide 14: The lower Potsdam consists of a different type of sandstone. The lower
Potsdam, marked by green stars, reaches a substantial 750 m thick in the mouth of
the aulocogen and commonly overlies billion year-old basement. Is this the oldest
aulocogen sedimentary fill and evidence of its initial subsidence?
Slide 15: The lower Potsdam is the Ausable Member. It nonconformably overlies
billion year-old gneiss in the aulocogen, lower left, may record some subaerial
deposition, but is poorly studied, and has a conformable contact with the overlying
Keeseville.
The Ausable Member is feldspathic. Thus, this mineralogically immature,
locally thick sandstone marks a foundering of the Ottawa aulocogen and just
possibly marks formation of the passive arm. Problem is that the Ausable has no
known fossils or known datable volcanics—so what is its age?
Slide 17: Speculations on the Ausable Member’s age have fallen everywhere along
the vertical green bar.
Its local 750 m thickness thickness has been associated with origin and
foundering of the Ottawa graben, perhaps in the late Precambrian. More recently,
middle Ediacaran ages of the Callander and Mt. Rigaud mafics further NW in the
aulocogen have been used to support an Ediacaran age for its base. So maybe the
rest of the Ausable just falls somewhere in the pre-Keeseville Cambrian.
4
Slide 18: Useful evidence on the age of the Ausable Member and the first
foundering and depositional fill of the Ottawa aulocogen is now known. This
evidence was found in a small outcrop area on the NE edge of the Adirondacks.
Old 1960’s reports noted an area of red soil in this area under the Potsdam
Formation outcrops—this is peculiar as most glacial soils here are brown or
blackish. In addition, red shales were noted in this area northwest of Plattsburgh,
New York.
Slide 19: This area has discontinuous outcrops of very peculiar, non-Potsdam-type
rock. One continuous stretch of outcrops along West Chazy River is on the
Atwood Farm in the red oval in the upper left picture. The section has thick
dolostone beds to a meter thick. A small waterfall in the lower right picture is over
a light colored, burrow churned dolostone. Between and above the massive
dolostone beds are red shales and argillaceous dolostones. This is not Potsdam, it
really looks like the Rome Formation in the southern Appalachians.
And in one of these shales, a layer with trilobites.
Slide 20: The trilobites from this 20 m-thick section shown on the left are a
monospecific assemblage of the shallow-water form Ehmaniella. Isolated heads
and heads with partial bodies occur, but preservation is poor. Which species of
Ehmaniella is present can’t be determined. The key thing is that Ehmaniella is
middle Middle Cambrian. For comparison, Ehmaniella specimens were swept
offshore in western Canada, and occur in the famous Burgess Shale.
In terms of the history of the Ottawa aulocogen, Ehmaniella occurs less than
10 m under, again under, the Ausable Member which occurs on the hill slopes just
above West Chazy River. The trilobites occur in a non-Potsdam lithofacies with
red to gray mudstones and dolostones that I called the Altona Formation.
Slide 21: The trilobites of the new Altona Formation give a general middle Middle
Cambrian age bracket for the base of the overlying Ausable Member. The
geochronology of the Middle Cambrian is imperfectly known, but a middle Middle
Cambrian age on the basal Ausable Formation is probably only several million
years older than the late Middle Cambrian age on the lower Keeseville.
Things happened quickly in the later Middle Cambrian at the mouth of the
Ottawa aulocogen—the bottom is fell out and provided accommodation space for
up to 750 m of immature Ausable sandstone, some of it possibly subaerial,
followed by a rapid transition to the mature, quartzose Keeseville Member.
But, all of this is much younger than the likely middle Ediacaran origin of
the Ottawa aulocogen.
5
Slide 22: Another section of the new Altona Formation is in the blue oval south of
Atwood Farm—see the upper left picture.
This Old Military Turnpike section is along a road build to defend the US
against Canada. OK….
The section starts on 1.1 billion year-old Grenville gneiss. The gneiss is cut
by post-Grenville orogeny basic dikes thought to be Ediacaran. The Ediacaran call
is based on very limited evidence. Note the hammer in the red oval on one of these
Rand Hill intrusives.
There are 50 m of gently west-dipping section in the ditch along Old
Military Turnpike—trust me! The section is dominated by reddish feldspathic
sandstones interbedded with red shales, Both the sandstones and shales have a low
diversity, Cambrian-aspect trace fossil assemblage.
The sandstones were deposited by south-flowing currents, under present
coordinates. Given a 90 degree clockwise rotation of Laurentia in the Cambrian,
these were west-flowing, perhaps driven by trade winds on the east margin of
Laurentia.
Slide 23: Yes, a poorly exposed section. Gneiss and quartz pebble conglomerates
occur about 9 m above the Grenville. More important is a fossil hash bed with
moldic trilobite fragments. One of these is a strongly convex, large eyed cheek of
the trilobite Olenellus. Trivially, this is the oldest trilobite found west of the
Appalachians in the northeast US, and records the oldest marine onlap. More
importantly, this fossil shows that deposition started in the late Early Cambrian
near the mouth of the Ottawa aulocogen.
Slide 24: Despite this limited fossil record, the early epeirogenic history of the
lower Ottawa aulocogen is finally revealed…somewhat.
A rapid succession of dated Cambrian events shows that there is no record of
sediment deposition following the apparent middle Ediacaran development of the
Ottawa aulocogen.
Only 50 million years later do late Early Cambrian sediments, about 512
million years ago, appear in the lower Ottawa aulocogen. The aulocogen founders
gradually through the middle Middle Cambrian, possibly until about 503 m. y. ago.
This perhaps 9 m.y. interval is the Altona Formation.
Then the bottom really fell out, gneissic highlands are elevated and eroded,
and up to 750 m of feldspathic Ausable Member accumulate in the aulocogen in
only several million years in the middle to late Middle Cambrian.
By the late Middle Cambrian, mature quartz arenite of the Keeseville
Formation appears and the aulocogen becomes essentially quiescent again. This
quiescence lasts perhaps 60 m.y. into the onset of the Late Ordovician Taconic
orogeny.
6
Slide 25: Several questions remain. A less significant one is
“how major of a stratigraphic unit,” or “how thick,” is the Altona Formation?
We only have two major sections. There is a third section probably corresponding
to the top of Old Military Turnpike section, but it is isolated somewhat to the south
and right on the Adirondack north slope.
The lower part of the Altona Fm is 50 m of current-deposited, feldspathic
sandstone- and red shale-dominated and rests on billion year old gneiss on Old
Military Turnpike. There is no direct way to relate this late Early Cambrian, in
part, section to the 25 m of red mudstones, massive dolostones, and upper wavedeposited sandstone that form the upper part of the Altona Formation, other than to
say the upper part of the formation is certainly younger.
The problems are that Pleistocene cover is extensive and densely-spaced
Ordovician and, maybe, Mesozoic normal faults limit continuous or long
exposures in these almost flat-lying Cambrian rocks.
Fortunately, a hydrologic research well is only 1.0 km west of the Atwood Farm
section. Based on cuttings, the well passed through about 40 m of feldspathic
Ausable Member. It then passed through about 90 m of reddish mudstone,
feldspathic sandstone, and dolostone of Altona Formation type before reaching
Precambrian diabase and meta-anorthosite. Magnetic susceptibility and gamma-ray
logging also show a distinct profile with positive gamma ray spikes likely
indicating mudstone-rich intervals of Altona-type not present in the Ausable.
Slide 26: So, a previously unrecognized, fairly major, 90 m-thick formation lies
under the Potsdam in the Ottawa aulocogen in northeastern New York. Its
deposition beginning in the late Early Cambrian records the oldest marine
inundation and subsidence of the Ottawa aulocogen. These events are about 50
m.y., or fashionably late, after the middle Ediacaran origin of the aulocogen.
But this epeirogenic history doesn’t stop here, the Altona Formation is abruptly
overlain by feldspathic sandstones of the Ausable Member. Up to 750 m of
Ausable Member is dumped into the allocogen in perhaps several million years
from the middle Middle to late Middle Cambrian. Then the aulocogen goes
quiescent and the Keeseville, a normal cratonic-like mature sandstone, covers the
lower Ottawa aulocogen and extends south across eastern New York, appears as
the Danby Formation on the Vermont shelf, and loses identity and disappears, as in
New Jersey, with distance from the Canadian shield.
7
Slide 27: What’s happening, I speculate, is a rejuvenation and a slow to
accelerating subsidence of the lower Ottawa aulocogen beginning in the late Early
Cambrian.
A similar epeirogenic history seems to be take place at the mouth of the aulocogen
in the now thrusted Appalachian Mountain succession of NW Vermont.
In this area, a stable late Early Cambrian carbonate shelf that extended N-S from
eastern Connecticut and Massachusetts and along the Green Mountain axis of
Vermont into Quebec suddenly down-dropped in NW Vermont. The shallow
Vermont carbonate shelf was replaced by dysoxic to anoxic mudstone deposition
of the Franklin Basin. Franklin Basin mud and debris flow deposition continued
through the Early Ordovician.
How did this happen? Apparently by down-dropping along ESE-directed faults
along the trend of the Lemoille River. These faults also parallel the trend of the
lower Ottawa aulocogen, run right through the intersection of the Ediacaran triple
junction, and through the greatest thickness of the middle Ediacaran Tibbett Hill
volcanics.
Local sections show how the shelf carbonates of the Dunham Formation become
sandy upwards, and then the shelf fractures to produce a submarine scarp down
which giant Dunham blocks slide into mudstones of the Parker Formation. The
timing of the faulting is latest Early Cambrian with upper Olenellus–bearing faunas
occurring in the Dunham and in the lower Parker.
8
Slide 28: I can’t give an explanation or an analog for this delayed foundering and
rejuvenated fault action of the lower Ottawa aulocogen and an area further
outboard.
This entire east Laurentian region had undergone a transition from a rifted margin
that faced a new Iapetus Ocean in the middle Ediacaran to Early Cambrian
interval. My opinion is that rift sedimentation extended even into the lower range
of Olenellus further south in Vermont, but nothing of rift sediments are preserved
in the aulocogen.
The NY–southern Quebec region, in fact an area extending along the Appalachian
foreland to Georgia and from NY to southern Labrador and western Newfoundland
became a long-term passive margin and a mixed quartz arenite and carbonate
platform until Middle and Late Ordovician collision with the Taconic island arc.
The exception to this long interval of epeirogenic quiescence along east Laurentia
was the abrupt foundering of the lower Ottawa aulocogen and Franklin Basin in the
latest Early Cambrian through middle Middle Cambrian.
The question mark in the figure means I don’t know what is happening. Something
peculiar in regional cooling history?, perhaps subsidence related to cooling and
density increase of the Tibbet Hill volcanics?, or stresses related to far-field effects
maybe related to the detachment of what would become the Argentinian
Precordillera from south-central Laurentia?
Slide 30: I’ll end this little report without further speculations, but by noting the
utility of the most traditional of geological techniques—to biostratigraphy and
fossil-based relative time correlation. I give my appreciation to the audience, to
Adam for his invitation, and to Clara for her patient efforts in getting this to you.
9