Philip Fralick

The transition to a sulphidic ocean ∼ 1.84 billion years ago

The Proterozoic aeon (2.5 to 0.54 billion years (Gyr) ago) marks the time between the largely anoxic world of the Archean (> 2.5 Gyr ago) and the dominantly oxic world of the Phanerozoic (< 0.54 Gyr ago). The course of ocean chemistry through the Proterozoic has traditionally been explained by progressive oxygenation of the deep ocean in response to an increase in atmospheric oxygen around 2.3 Gyr ago. This postulated rise in the oxygen content of the ocean is in turn thought to have led to the oxidation of dissolved iron, Fe(II), thus ending the deposition of banded iron formations (BIF) around 1.8 Gyr ago. An alternative interpretation suggests that the increasing atmospheric oxygen levels enhanced sulphide weathering on land and the flux of sulphate to the oceans. This increased rates of sulphate reduction, resulting in Fe(II) removal in the form of pyrite as the oceans became sulphidic. Here we investigate sediments from the ∼1.8-Gyr-old Animikie group, Canada, which were deposited during the final stages of the main global period of BIF deposition. This allows us to evaluate the two competing hypotheses for the termination of BIF deposition. We use iron–sulphur–carbon (Fe–S–C) systematics to demonstrate continued ocean anoxia after the final global deposition of BIF and show that a transition to sulphidic bottom waters was ultimately responsible for the termination of BIF deposition. Sulphidic conditions may have persisted until a second major rise in oxygen between 0.8 to 0.58 Gyr ago, possibly reducing global rates of primary production and arresting the pace of algal evolution.

Geochemical discrimination of clastic sedimentary rock sources

Abstract Factors controlling the geochemistry of a clastic sedimentary rock can include: (1) composition of source terrain, (2) chemical weathering, (3) hydraulic sorting, (4) diagenesis, (5) metamorphism, and (6) hydrothermal alteration. A linear solution inferring source terrain composition from geochemistry of the sediment is impossible in this multivariate system as several unknowns will commonly be present. The use of graphical analysis of element pairs circumvents the problem. Chemically immobile elements will maintain invariant ratios during rock mass change caused by either addition or depletion of mobile elements. This results in chemically immobile element scattergrams exhibiting linear trends along radians extending from the origin, if the major mineral phases of the immobile elements have behaved in a hydrodynamically similar manner. As chemically mobile element plots will produce a scatter of points, this relationship can be used to test chemical immobility and similarity in hydrodynamic sorting history. The constraint on analysis is that the source area must be compositionally uniform or the sediment well mixed prior to delivery to the basin. A second technique, using SiO2 plots, has also been developed to investigate element mobility and hydrodynamic behaviour of the mineral phases containing the elements. SiO2-immobile element plots result in a linear arrangement of points extending towards either 0% or 100% SiO2. The 0 intercept position occurs for elements with major mineral phases concentrated in sand; the 100 intercept for those concentrated in clays. Plotting chemically mobile elements produces different patterns, and this can be used to gain information on alteration and sorting history. Elemental ratios for chemically immobile elements with similar hydrodynamic behaviour will be the same as those for the weighted average composition of the source material. This provides a powerful tool for deducing source terrain from sediment geochemistry. Techniques outlined above were tested on Archaean metasandstones from Superior Province, Canada. Immobile element ratio diagrams for NbAlTi and ZrAlTi indicate that a calc-alkaline extrusive-intrusive suite lying to the north of the study area was the source, and not five other volcanic suites in the region. This conclusion agrees with previous clast lithology studies and accentuates the applicability of the geochemical techniques.

Sedimentology of archean greenstone belts: Signatures of tectonic evolution

Abstract Stratigraphic styles in Archean greenstone belts are compared to those of modern and Phanerozoic depositional basins in order to test the conformity of tectonic style through time. Six lithological associations in greenstone belts are recognized: (1) mafic-ultramafic volcanic, (2) calc-alkaline volcanic, (3) bimodal volcanic, (4) quartz arenite-iron-formation or carbonate, (5) conglomerate-wacke and (6) conglomerate-arenite. Examples of the associations are described from the Kaapvaal, Superior and Zimbabwe Provinces and the Pilbara Block. Each association differs only slightly between the four regions, thereby emphasizing a common tectonostratigraphic theme. Sedimentary rocks are a minor component of the mafic-ultramafic volcanic association. They were deposited in two types of volcanic-basin environments: one similar to barred lagoons and bays around oceanic volcanic islands, and the other similar to sediment-starved platforms adjacent to coalesced volcanoes in inter-arc, intra-arc and back-are basins. In contrast, sedimentary rocks in the calc-alkaline volcanic association comprise thick wedges of epiclastic and volcaniclastic detritus deposited in elongate sedimentary basins, and thick wedges of pyroclastic and volcaniclastic detritus deposited adjacent to volcanic centers. Sedimentary facies were identical to those of forearc-trench and marine volcanoplutonic-arc settings. Chemogenic lithofacies in both volcanic associations were deposited in shallow to deep-marine waters, although shallow-water profiles were selectively preserved in some terrains. Sedimentary rocks of the bimodal volcanic association are thick wedges of siliciclastic and volcaniclastic deposits interbedded with subaerial to submarine erupted flows of komatiite, basalt, and rhyolite. Depositional environments ranged from braid-plain and braid-delta to coastal marine and submarine fan. Starved-basin deposits include banded iron-formation and sulfidic black shale. The association is identical to that of cratonic extensional basins in arc-continent and intracontinental rifts. Sedimentary rocks of the quartz arenite-iron-formation or carbonate association onlap weathered basement or bimodal-volcanic association rocks. Depositional environments were either transitional from fluvial to siliciclastic and carbonate marginal-marine to offshore pelagic-hemipelagic, or from inshore intrabasinal-clastic and volcaniclastic to offshore pelagic-hemipelagic. Siliciclastic deposits are supermature. Two tectonic sites are represented: those with siliciclastic facies represent continent-adjacent syn- to post-rift stable shelves and those dominated by banded iron-formation, with volcaniclastic deposits, represent arc-adjacent post-rift stable shelves. Sedimentary rocks of the conglomerate-wacke association are represented by thick wedges of siliciclastic or volcaniclastic sandstone, with minor volcanic components. These were deposited in braid-plain, braid-delta and submarine-fan environments within elongate sedimentary basins. Coastal-plain and pelagic basin facies are also present. Siliciclastic detritus was derived from syndepositional, magmatic and metasedimentary thrust-belts. Two stratigraphic styles are preserved: one that records a flysch-like stage of deep- to marginal-marine environments, and the other that records a molasse-like stage of marginal-marine to fluvial environments. The association is identical to that of compressional-foreland basins of arc-continent collisional and compressional-arc tectonic affinities. The conglomerate-arenite association is preserved in elongate structural basins that transgress crustal components and all other supracrustal units. It is dominated by thick wedges of siliciclastic sandstone, although bimodal volcanic and volcaniclastic rocks can also be present. Depositional environments ranged from talus and alluvial fan, and braid-plain to either lacustrine or coastal marine. Local basin margins often show lateral offsets from source terrains and vertically skewed facies patterns. Longitudinal facies geometries are typical. The association is similar to that of strike-slip collisional graben in hinterland tectonic-escape and terrane-accretion orogens. The examples show that, despite all the ramifications of secular geophysical, geothermal and geochemical global change, the stratigraphic style of sedimentary rocks in Archean greenstone belts can be matched with modern tectonic analogues, thereby emphasizing the conformity of stratigraphic style through time. The inseparable link between stratigraphic and tectonic styles implies that the tectonic style of greenstone belts was not temporally unique. Major differences exist with regards to the chemical composition of certain magmatic and sedimentary products, but these are compensated for by their constant stratigraphic function in terms of depositional processes, depositional environments and tectonic sites.

Discovery of distal ejecta from the 1850 Ma Sudbury impact event

A 25-70-cm-thick, laterally correlative layer near the contact between the Paleoproterozoic sedimentary Gunflint Iron Formation and overlying Rove Formation and between the Biwabik Iron Formation and overlying Virginia Formation, western Lake Superior region, contains shocked quartz and feldspar grains found within accretionary lapilli, accreted grain clusters, and spherule masses, demonstrating that the layer contains hypervelocity impact ejecta. Zircon geochronologic data from tuffaceous horizons bracketing the layer reveal that it formed between ca. 1878 Ma and 1836 Ma. The Sudbury impact event, which occurred 650-875 km to the east at 1850 ′ 1 Ma, is therefore the likely ejecta source, making these the oldest ejecta linked to a specific impact. Shock features, particularly planar deformation features, are remarkably well preserved in localized zones within the ejecta, whereas in other zones, mineral replacement, primarily carbonate, has significantly altered or destroyed ejecta features.

Sedimentology of the lower huronian supergroup (early proterozoic), Elliot lake area, Ontario, Canada

Abstract The Early Proterozoic Huronian Supergroup is a slightly metamorphosed sedimentary succession locally > 10 km thick, exposed to the north of Lake Huron, Ontario, Canada. The lower part of the supergroup, comprising the Matinenda, McKim, Ramsay Lake and Pecors formations, was studied in the Quirke Syncline area, Elliot Lake, and regions immediately to the west and south. Thirteen lithofacies are present: trough and planar cross-stratified sandstone, quartz pebble conglomerate, clay-rich sandstone, lenticular-bedded sandstone-siltstone-mudstone, laminated sandstone-siltstone-mudstone, massive sandstone- massive pebbly sandstone, massive diamictite, stratified matrix-supported diamictite, stratified clast-supported diamictite, graded diamictite, and massive siltstone. The lithofacies may be grouped into associations which correspond approximately to the mapped formations. The crossbedded sandstones and conglomerates comprise a fluvial association (Matinenda Formation), which was formed mainly in shallow braided channels. Clay-rich sandstone and the lenticular and laminated lithofacies, constituting portions of the McKim and Pecors Formations, were deposited on the strand and offshore areas of braid deltas, interfingering laterally and vertically with the Matinenda fluvial deposits. Four glacigenic subassociations are present in the Ramsay Lake Formation. An ice-proximal subassociation (I) consists of massive pebbly sandstone and diamictite which progressively truncates the Matinenda and McKim Formations to the north. The massive sandstone may be an ice-override deposit formed by erosion and reworking of the underlying Matinenda Formation, or a sub-ice shelf rain-out accumulation of locally eroded glacial debris. Subassociations II and III consist mainly of subaqueous sediment gravity flow and ice rain-out deposits. Subassociation IV is interpreted as an ice-proximal fluvial outwash deposit containing resedimented blocks of moraine material. Stratigraphic and paleocurrent data indicate a south-dipping paleoslope that tilted southeast during the deposition of the fluvial and deltaic units. A trough of relatively deep water extended NW-SE across the region, transverse to the E-W divergent continental margin hinge line that is assumed to have existed to the south of the project area. Analogies are suggested to Mesozoic-Recent basins on the Atlantic margin of Canada, where transverse paleogeographic trends reflect either landward extension of active oceanic fracture zones, or reactivation of earlier structural lineaments.

Stability of the nitrogen cycle during development of sulfidic water in the redox-stratified late Paleoproterozoic Ocean

Nitrogen cycling has been evaluated across a depth transect in the late Paleoproterozoic Animikie Basin (North America), spanning the end of Earth’s final period of global iron precipitation, and a major transition to euxinic conditions in areas of high productivity. Sediments from near shore, where productivity was highest, have δ15N compositions up to ∼3‰ higher than at more distal sites. This suggests that as NH4+ mixed vertically upward into the oxic photic zone from the anoxic ocean interior, it was either assimilated by organisms or oxidized. Subsequent enhanced production of N2 by denitrification or anammox (anaerobic ammonium oxidation) led to the observed increase in δ15N close to shore. Any deficit in biologically available N was overcome by N2-fixing organisms, but the input of N with low δ15N from this process did not overwhelm the increase in δ15N from denitrification. Because there is no evidence for conditions of severe N stress arising from trace metal limitation (particularly Mo) of N fixation during the transition to euxinic conditions, losses of N were either very small (potentially because low O2 levels limited NH4+ oxidation), or alternative pathways that retained N were important. The fact that Mo appears to have remained bioavailable for N fixation, either suggests that the extent or severity of sulfidic water column conditions was not sufficient to quantitatively sequester Mo on a global scale, or that rivers directly delivered Mo to surface waters on the inner shelf. The effects of N2 fixation on δ15N increased to more distal parts of the shelf, consistent with models invoked for modern upwelling zones over broad continental margins.

Paleohydraulics of chute-and-pool structures in a Paleoproterozoic fluvial sandstone

Abstract Chute-and-pool structures consist of an inclined surface leading into a pool where a hydraulic jump causes deposition of backset laminae. Structures fitting this description are present at the base of a very fine-grained sandstone bed in the fluvial Matinenda Formation of northern Ontario. Reconstruction of the paleohydraulic system indicates that a depth of approximately 1 cm and a velocity of approximately 65 cm/s formed these bedforms during the initial stage of a flow with increasing depth and decreasing velocity. This type of deposit should be most common at the base of sandy beds deposited as emergent bars are overtopped during increasing discharge events.

Sediment redeposition in Archean iron formation; examples from the Beardmore-Geraldton greenstone belt, Ontario

ABSTRACT Archean iron formation (I.F.) typically occurs over 5-15-m intervals within predominantly arenitic sedimentary rocks in the Beardmore-Geraldton greenstone belt. The I.F. consists primarily of dark, laminated magnetite (chemical component) with common intercalations of siltstone/sandstone (clastic component) which range in thickness from sub-millimeters to tens of centimeters. Sedimentary structures in the latter beds, which include graded bedding parallel lamination, and load casts, indicate deposition of clastic strata, together with some magnetitic material, by turbidity currents. Within intervals of alternating magnetite, siltstone, and sandstone beds, the clastic component may coarsen and thicken upward over a scale of meters. Such sequences eventually pass vertically into a 100% clastic succession. These features suggest progradation of submarine-fan, clastic-sediment lobes (with a minor magnetite component) into an environment where essentially iron-rich sediment (the magnetite precursor) was otherwise being deposited as background chemical rainout. Variations in coarseness and thickness of clastic beds in different sections within the I.F. are interpreted as reflecting proximal to distal locations relative to channel-feed systems on the fan. On the outcrop scale, the background (clastic-free) chemical sediment appears as blackish, laminated magnetite. Thin sections, however, reveal that it actually consists of regular sub-millimeter alternations between magnetite and chert. These magnetite-chert microlaminations are analogous to those of the Hamersley Basin in Western Australia (Trendall and Blockley 1970) and similarly may reflect annual varvings.

Petrographic analysis and INAA geochemistry of prehistoric ceramics from Robinson Pueblo, New Mexico

Abstract The examination of thin sections is a successful method for discovering provenance groupings of archaeological ceramics if non-clay inclusions in pottery paste can be identified with petrographically distinctive geologic sources. When this is not the case, geochemical analysis must be employed to search for more subtle evidence of such groupings. This situation is likely to arise in studies, such as the present one, where the objective is to discern local or subregional patterns of pottery manufacture and exchange. Ninety potsherds from site LA46326 in New Mexico were subjected to both INAA and petrographic analysis. The results of this conjoint approach are assessed in relation to geochemical theory and are analysed graphically and statistically.

Does the Paleoproterozoic Animikie Basin record the sulfidic ocean transition?: COMMENT

[Pufahl et al. (2010)][1] question the interpretation of data in [Poulton et al. (2004)][2], showing a transition from ferruginous to euxinic conditions on the continental shelf of a major ocean basin at ca. 1840 Ma. In contrast with [Poulton et al. (2004)][2] and also [Johnston et al. (2006)][3],