Brian Pratt | University of Saskatchewan (original) (raw)

Papers by Brian Pratt

Sedimentology, May 21, 2004

Silurian pinnacle reefs in the subsurface of the south‐western Ontario portion of the Michigan Ba... more Silurian pinnacle reefs in the subsurface of the south‐western Ontario portion of the Michigan Basin display a variety of laminated carbonates (laminites) within predominantly muddy reef‐capping facies in the upper part of the Guelph Formation and the overlying A‐1 Carbonate of the Salina Group. Laminites, which are limestone, dolomite or partially dolomitized limestones, have a range of morphologies, from simple planar to a variety of wavy and serrated forms. Individual laminae are composed mainly of micrite, microspar or replacive dolomite, and vary internally from isopachous and continuous over the diameter of the core to non‐isopachous and often discontinuous. Clotted and peloidal micrite, sometimes defining small knobs and chambers, is interpreted as being microbial in origin and occurs within all types of laminites. Fibrous cement locally comprises laminite clasts in breccias or coats clasts in breccias, and also occurs as spherulites in the interparticle spaces in breccias. Although similar laminites have been described from elsewhere in the Michigan Basin and interpreted as caliche, travertine and abiotic subtidal stromatolites, the laminites in south‐western Ontario are most realistically regarded as microbial. The causes for the variations in morphology and characteristics of the constituent laminae are uncertain, although fluctuations in local microenvironmental conditions would have been important, set against a backdrop of an increasingly restricted overall setting. Caliche or travertine origins for these laminites are unlikely in general, except perhaps locally at the subaerial exposure surface at the tops of pinnacle reefs.

EGU General Assembly Conference Abstracts, May 1, 2014

Earth-Science Reviews, Jun 1, 2021

Abstract Carbonate rocks of the lower Belt Supergroup (ca. 1.45 Ga) in west-central North America... more Abstract Carbonate rocks of the lower Belt Supergroup (ca. 1.45 Ga) in west-central North America include the Haig Brook, Tombstone Mountain, Waterton and Altyn formations which crop out in the eastern Rocky Mountains of northwestern Montana, southwestern Alberta and southeastern British Columbia. They record the development on the present-day northeastern side of the Mesoproterozoic Belt Basin of a carbonate platform early in the basin history while it was still relatively deep. The Waterton–Altyn succession documents a shallowing-upward, westward-prograding, broadly ramp-style platform composed of mixed carbonate–siliciclastic sediments. Five main facies types are recognized: Laminite, Ribbon, Grainstone, Oolite and Stromatolite. The first two were formed of lime muds deposited on the ramp and outer platform under low-energy conditions. The Grainstone facies consists of sand-sized peloids, aggregates and intraclasts plus admixed quartz and feldspar sand, microspar grains, radial ooids and silicified oolite and anhydrite. The Oolite facies is dominated by ooids with a concentric cortex. These are allochthonous coarse-grained particles interpreted to have been transported westward to a marginal belt and outer platform mainly by tsunami off-surge from the platform interior, coastal sand shoals and tidal-flat sabkhas, outcrops of which are not preserved. Absence of hummocky or swaley cross-stratification suggests that the platform was not affected by strong storms. Instead, flat-pebble conglomerates on the ramp are ascribed to episodic, tsunami-induced wave action which caused localized rupture and imbrication of flat pebbles. Scouring by tsunami off-surge produced intraclasts in the outer platform. There, this sediment lay undisturbed, but in the shallower marginal belt it was reworked by strong tidal currents which generated variably directed cross-lamination from dune and ripple migration and, locally, large sand bars with northwest-dipping clinoforms. Deformation features caused by synsedimentary earthquakes are common, with the various seismite types reflecting the facies-specific rheology of the sediment. Seismites in the lower Altyn Formation appear not to be linked to individual tsunami-lain dolograinstone beds in the outer platform, suggesting that these two sets of features were not generated by the same faults. The carbonate factory shut down when the platform was suffocated by siliciclastic mud sourced from the west, and tidal activity diminished as the whole basin shallowed. The interpretation of the carbonate platform presented herein is radically different from previous views that considered the Waterton and Altyn formations to be predominantly of shallow-subtidal and tidal-flat origin. These rocks are relevant to the appreciation of other carbonate platforms, especially in that tsunamis may be an under-appreciated agent of erosion and sediment transport offshore.

Rocky Mountain Section - 69th Annual Meeting - 2017, 2017

Rocky Mountain Section - 69th Annual Meeting - 2017, 2017

GSA Annual Meeting in Denver, Colorado, USA - 2016, 2016

Al ga l mats form a variety of s e diment-s tabilizin g cryptalgal struc tures in t he Boca J e w... more Al ga l mats form a variety of s e diment-s tabilizin g cryptalgal struc tures in t he Boca J e wf i s ll area of th e Lac, Ronai.r e . These st:r uct u rfls are rela ted to the blue-g r een a1gA..l communi l"S t.hnt: comrrisP the m.1r-s , desiccation during int e rtidal period s n.nd ••urrcnt act i.on. Vnr Lat. ion in these paramet e rs produces a zonation of ex t e rnal mor phol ogies of c ryptalgal structures. Sediment is agglutinated onto mucilag e s ecret e d by f ine filaments of blue-green algae in subtidal and intertidal oncolites. Sediment is trapped and enmeshed by blue-green algae in the rigid smooth mat. Subtidal colloform mats hold sediment loosely by entrapment and a gg lutination onto mucilagenous algal sheaths. The sediments of the tufted mat zone a re caught and bound by large sticky filaments. Lithification of oncolites and chunks of smooth mat occurs in the int e rtidal by growth of por e-destroying fibrous aragonite cement. Indurated crust pavements that are found in the study area appear to have been cemented in the intertidal and supratidal environment. Cementation here is principally cryptocrystalline aragonite. In the present regressive stage in the Lac, there has not been sufficient time for significant vertical accretion in the smooth and tufted mat zones.

Journal of Paleontology, May 1, 2002

Geologic Field Trips of the Canadian Rockies: 2017 Meeting of the GSA Rocky Mountain Section

The Ecology of the Cambrian Radiation, 2000

Pervasive horizons of various kinds of soft-sediment deformation structures were identified in th... more Pervasive horizons of various kinds of soft-sediment deformation structures were identified in the lacustrine sediments of the Eocene Green River Formation, USA. These features are present in a variety of sediments deposited in paludal (coal, sand) to profundal (oil shale) environments. Deformation is represented by brittle and plastic behavior, as well as sediment injection, and the deformed layers are confined by undeformed beds with similar thickness and lithology to the deformed ones. Based on the (1) tectonic setting of the subbasins; (2) the sedimentary environment and sedimentological characteristics of the successions in which the deformed layers occur; (3) their lateral extent; (4) their recurrence at different stratigraphic levels; and (5) their similarity to those described as seismically induced deformation structures in other areas and reproduced experimentally, we interpret these features as having developed as a result of increased pore pressure and vertical or horizontal stresses induced by seismic activity.

American Association of Petroleum Geologists eBooks, 1993

New Zealand Journal of Geology and Geophysics, Oct 1, 1988

Page 7. New Zealand Journal of Geology and Geophysics, 1988, Vol. 31: 397-404 0028-8306/88/3104-0... more Page 7. New Zealand Journal of Geology and Geophysics, 1988, Vol. 31: 397-404 0028-8306/88/3104-0397 $2.50/0© Crown copyright 1988 397 Early Paleozoic history of the central Transantarctic Mountains: evidence from ...

Canadian Journal of Earth Sciences, Aug 18, 2004

The Kennedy Channel and Ella Bay formations are the two oldest stratigraphic units exposed in the... more The Kennedy Channel and Ella Bay formations are the two oldest stratigraphic units exposed in the Franklinian margin sedimentary sequence in the Canadian Arctic Islands. An Early Cambrian age had previously been accepted by the occurrence of trilobites and small shelly fossils in the type section of the Kennedy Channel Formation. Reinvestigation of the area around the type section shows that several large strike-slip faults cut the succession and that the olenelloid trilobites are from an infaulted slice of a younger unit, the Lower Cambrian Kane Basin Formation. Thus, there is no unambiguous paleontological evidence for the age of either the Kennedy Channel or Ella Bay formations. However, the abundance of stromatolites, absence of trace fossils, and separation from overlying Lower Cambrian clastics by a regional angular unconformity indicate a probable late Neoproterozoic age for these two formations. The Ella Bay Formation likely correlates with the Portfjeld Formation in North Greenland, the Spiral Creek Formation in East Greenland, and the Risky Formation of the Mackenzie Mountains in northwestern Canada. The passive margin that existed in northern Laurentia during the early Paleozoic was, therefore, established in the late Neoproterozoic, and the onset of rifting must have preceded this, rather than occurring in the Early Cambrian as some authors have suggested.

Ichnos, Dec 28, 2018

A new ichnospecies of Protopaleodictyon Ksia˛_ zkiewicz, 1958, Pr. aitkeni isp. n., is named from... more A new ichnospecies of Protopaleodictyon Ksia˛_ zkiewicz, 1958, Pr. aitkeni isp. n., is named from material recovered from the mid-Cambrian Stephen-Eldon formation transition in Banff National Park, Alberta, Canada. Specimens occur in convex hyporelief on the sole of a dolomitic lime mudstone bed, and exhibit straight to gently curving strands with a 'zigzag' shape up to 45 cm in length. Strands are fairly regular, with branching angles ranging from 110 to 120. Branch segments forming the strand are approximately the same length and produce strands with open and occasionally closed hexagonal polygons arranged alternatively along the specimen's axis. Hexagons are 25-40 mm wide and string widths are 5-10 mm. The dimensions of Pr. aitkeni are large compared to other ichnospecies of the ichnogenus and graphoglyptids in general. The host interval is interpreted to have been deposited in a relatively shallow-water environment in the interior of a carbonate platform, contrasting with the deeper siliciclastic settings in which younger examples of the ichnogenus typically occur. This occurrence further supports the hypothesis that graphoglyptid ethology initially developed in shallow shelf environments before shifting into deeper facies over geologic time.

Sedimentology, May 21, 2004

Silurian pinnacle reefs in the subsurface of the south‐western Ontario portion of the Michigan Ba... more Silurian pinnacle reefs in the subsurface of the south‐western Ontario portion of the Michigan Basin display a variety of laminated carbonates (laminites) within predominantly muddy reef‐capping facies in the upper part of the Guelph Formation and the overlying A‐1 Carbonate of the Salina Group. Laminites, which are limestone, dolomite or partially dolomitized limestones, have a range of morphologies, from simple planar to a variety of wavy and serrated forms. Individual laminae are composed mainly of micrite, microspar or replacive dolomite, and vary internally from isopachous and continuous over the diameter of the core to non‐isopachous and often discontinuous. Clotted and peloidal micrite, sometimes defining small knobs and chambers, is interpreted as being microbial in origin and occurs within all types of laminites. Fibrous cement locally comprises laminite clasts in breccias or coats clasts in breccias, and also occurs as spherulites in the interparticle spaces in breccias. Although similar laminites have been described from elsewhere in the Michigan Basin and interpreted as caliche, travertine and abiotic subtidal stromatolites, the laminites in south‐western Ontario are most realistically regarded as microbial. The causes for the variations in morphology and characteristics of the constituent laminae are uncertain, although fluctuations in local microenvironmental conditions would have been important, set against a backdrop of an increasingly restricted overall setting. Caliche or travertine origins for these laminites are unlikely in general, except perhaps locally at the subaerial exposure surface at the tops of pinnacle reefs.

EGU General Assembly Conference Abstracts, May 1, 2014

Earth-Science Reviews, Jun 1, 2021

Abstract Carbonate rocks of the lower Belt Supergroup (ca. 1.45 Ga) in west-central North America... more Abstract Carbonate rocks of the lower Belt Supergroup (ca. 1.45 Ga) in west-central North America include the Haig Brook, Tombstone Mountain, Waterton and Altyn formations which crop out in the eastern Rocky Mountains of northwestern Montana, southwestern Alberta and southeastern British Columbia. They record the development on the present-day northeastern side of the Mesoproterozoic Belt Basin of a carbonate platform early in the basin history while it was still relatively deep. The Waterton–Altyn succession documents a shallowing-upward, westward-prograding, broadly ramp-style platform composed of mixed carbonate–siliciclastic sediments. Five main facies types are recognized: Laminite, Ribbon, Grainstone, Oolite and Stromatolite. The first two were formed of lime muds deposited on the ramp and outer platform under low-energy conditions. The Grainstone facies consists of sand-sized peloids, aggregates and intraclasts plus admixed quartz and feldspar sand, microspar grains, radial ooids and silicified oolite and anhydrite. The Oolite facies is dominated by ooids with a concentric cortex. These are allochthonous coarse-grained particles interpreted to have been transported westward to a marginal belt and outer platform mainly by tsunami off-surge from the platform interior, coastal sand shoals and tidal-flat sabkhas, outcrops of which are not preserved. Absence of hummocky or swaley cross-stratification suggests that the platform was not affected by strong storms. Instead, flat-pebble conglomerates on the ramp are ascribed to episodic, tsunami-induced wave action which caused localized rupture and imbrication of flat pebbles. Scouring by tsunami off-surge produced intraclasts in the outer platform. There, this sediment lay undisturbed, but in the shallower marginal belt it was reworked by strong tidal currents which generated variably directed cross-lamination from dune and ripple migration and, locally, large sand bars with northwest-dipping clinoforms. Deformation features caused by synsedimentary earthquakes are common, with the various seismite types reflecting the facies-specific rheology of the sediment. Seismites in the lower Altyn Formation appear not to be linked to individual tsunami-lain dolograinstone beds in the outer platform, suggesting that these two sets of features were not generated by the same faults. The carbonate factory shut down when the platform was suffocated by siliciclastic mud sourced from the west, and tidal activity diminished as the whole basin shallowed. The interpretation of the carbonate platform presented herein is radically different from previous views that considered the Waterton and Altyn formations to be predominantly of shallow-subtidal and tidal-flat origin. These rocks are relevant to the appreciation of other carbonate platforms, especially in that tsunamis may be an under-appreciated agent of erosion and sediment transport offshore.

Rocky Mountain Section - 69th Annual Meeting - 2017, 2017

Rocky Mountain Section - 69th Annual Meeting - 2017, 2017

GSA Annual Meeting in Denver, Colorado, USA - 2016, 2016

Al ga l mats form a variety of s e diment-s tabilizin g cryptalgal struc tures in t he Boca J e w... more Al ga l mats form a variety of s e diment-s tabilizin g cryptalgal struc tures in t he Boca J e wf i s ll area of th e Lac, Ronai.r e . These st:r uct u rfls are rela ted to the blue-g r een a1gA..l communi l"S t.hnt: comrrisP the m.1r-s , desiccation during int e rtidal period s n.nd ••urrcnt act i.on. Vnr Lat. ion in these paramet e rs produces a zonation of ex t e rnal mor phol ogies of c ryptalgal structures. Sediment is agglutinated onto mucilag e s ecret e d by f ine filaments of blue-green algae in subtidal and intertidal oncolites. Sediment is trapped and enmeshed by blue-green algae in the rigid smooth mat. Subtidal colloform mats hold sediment loosely by entrapment and a gg lutination onto mucilagenous algal sheaths. The sediments of the tufted mat zone a re caught and bound by large sticky filaments. Lithification of oncolites and chunks of smooth mat occurs in the int e rtidal by growth of por e-destroying fibrous aragonite cement. Indurated crust pavements that are found in the study area appear to have been cemented in the intertidal and supratidal environment. Cementation here is principally cryptocrystalline aragonite. In the present regressive stage in the Lac, there has not been sufficient time for significant vertical accretion in the smooth and tufted mat zones.

Journal of Paleontology, May 1, 2002

Geologic Field Trips of the Canadian Rockies: 2017 Meeting of the GSA Rocky Mountain Section

The Ecology of the Cambrian Radiation, 2000

Pervasive horizons of various kinds of soft-sediment deformation structures were identified in th... more Pervasive horizons of various kinds of soft-sediment deformation structures were identified in the lacustrine sediments of the Eocene Green River Formation, USA. These features are present in a variety of sediments deposited in paludal (coal, sand) to profundal (oil shale) environments. Deformation is represented by brittle and plastic behavior, as well as sediment injection, and the deformed layers are confined by undeformed beds with similar thickness and lithology to the deformed ones. Based on the (1) tectonic setting of the subbasins; (2) the sedimentary environment and sedimentological characteristics of the successions in which the deformed layers occur; (3) their lateral extent; (4) their recurrence at different stratigraphic levels; and (5) their similarity to those described as seismically induced deformation structures in other areas and reproduced experimentally, we interpret these features as having developed as a result of increased pore pressure and vertical or horizontal stresses induced by seismic activity.

American Association of Petroleum Geologists eBooks, 1993

New Zealand Journal of Geology and Geophysics, Oct 1, 1988

Page 7. New Zealand Journal of Geology and Geophysics, 1988, Vol. 31: 397-404 0028-8306/88/3104-0... more Page 7. New Zealand Journal of Geology and Geophysics, 1988, Vol. 31: 397-404 0028-8306/88/3104-0397 $2.50/0© Crown copyright 1988 397 Early Paleozoic history of the central Transantarctic Mountains: evidence from ...

Canadian Journal of Earth Sciences, Aug 18, 2004

The Kennedy Channel and Ella Bay formations are the two oldest stratigraphic units exposed in the... more The Kennedy Channel and Ella Bay formations are the two oldest stratigraphic units exposed in the Franklinian margin sedimentary sequence in the Canadian Arctic Islands. An Early Cambrian age had previously been accepted by the occurrence of trilobites and small shelly fossils in the type section of the Kennedy Channel Formation. Reinvestigation of the area around the type section shows that several large strike-slip faults cut the succession and that the olenelloid trilobites are from an infaulted slice of a younger unit, the Lower Cambrian Kane Basin Formation. Thus, there is no unambiguous paleontological evidence for the age of either the Kennedy Channel or Ella Bay formations. However, the abundance of stromatolites, absence of trace fossils, and separation from overlying Lower Cambrian clastics by a regional angular unconformity indicate a probable late Neoproterozoic age for these two formations. The Ella Bay Formation likely correlates with the Portfjeld Formation in North Greenland, the Spiral Creek Formation in East Greenland, and the Risky Formation of the Mackenzie Mountains in northwestern Canada. The passive margin that existed in northern Laurentia during the early Paleozoic was, therefore, established in the late Neoproterozoic, and the onset of rifting must have preceded this, rather than occurring in the Early Cambrian as some authors have suggested.

Ichnos, Dec 28, 2018

A new ichnospecies of Protopaleodictyon Ksia˛_ zkiewicz, 1958, Pr. aitkeni isp. n., is named from... more A new ichnospecies of Protopaleodictyon Ksia˛_ zkiewicz, 1958, Pr. aitkeni isp. n., is named from material recovered from the mid-Cambrian Stephen-Eldon formation transition in Banff National Park, Alberta, Canada. Specimens occur in convex hyporelief on the sole of a dolomitic lime mudstone bed, and exhibit straight to gently curving strands with a 'zigzag' shape up to 45 cm in length. Strands are fairly regular, with branching angles ranging from 110 to 120. Branch segments forming the strand are approximately the same length and produce strands with open and occasionally closed hexagonal polygons arranged alternatively along the specimen's axis. Hexagons are 25-40 mm wide and string widths are 5-10 mm. The dimensions of Pr. aitkeni are large compared to other ichnospecies of the ichnogenus and graphoglyptids in general. The host interval is interpreted to have been deposited in a relatively shallow-water environment in the interior of a carbonate platform, contrasting with the deeper siliciclastic settings in which younger examples of the ichnogenus typically occur. This occurrence further supports the hypothesis that graphoglyptid ethology initially developed in shallow shelf environments before shifting into deeper facies over geologic time.

A new Lagerstätte in the Rockslide Formation, Mackenzie Mountains of northwestern Canada, is a Bu... more A new Lagerstätte in the Rockslide Formation, Mackenzie Mountains of northwestern Canada, is a Burgess Shale-type (BST) deposit of Drumian age (Bolaspidella Biozone). A 1 m thick mudstone (composed of illite, quartz, clinochlore and dolomite) contains a low-diversity biota similar to that of the Wheeler and Marjum formations of Utah, and to some extent the Burgess Shale itself. The lithologically heterogeneous composition of the formation (shale, dolomite, lime mudstone, sandstone, mudstone) and absence of metamorphism offer an opportunity to analyze the depositional environment and taphonomy of this deeper water unit via organic/inorganic geochemistry (trace element, ∂13Corg, biomarker, XRD and synchrotron XRF). Similar to other BST Lagerstätten, the environment could not have been deposited under stable anoxic conditions, as the amount of organic matter is too low (TOC= 0.10–0.32%). Trace element ratios, especially V/Sc, indicate low oxygenation of the bottom waters during the deposition of the fossil-bearing interval but a higher oxygen content thereafter. The results differ from most other BST deposits, which suggest oxygenated bottom waters were maintained throughout sedimentation. In addition, organic matter in the sediments and ∂13Corg values (-22.3–26.9) suggest the presence of benthic microbial mats. Heavy elements are concentrated mostly in nm-µm-sized particles, but some can reach up to 50 µm in size. These are most likely due to bioaccumulation by microbes as they only occur in circular organic residues several centimeters in diameter and there is no indication of hydrothermal activity. These organic residues and the evidence for hyoliths feeding on them suggest an in situ preservation of the biota, perhaps from sudden and short anoxic events and quick burial under either microbial mats and the muddy sediment. This is also supported by the presence of only very rare trace fossils.