Paul E. Schenk

Sequence stratigraphy and proveance on Gondwana's margin: The Meguma Zone (Cambrian to Devonian) of Nova Scotia, Canada

The Meguma Zone is the second largest terrane in the Canadian Appalachians. Three thick sandstones (Cambrian, Upper Ordovician, and Lower Devonian) alternate with two thick shales (Lower Ordovician and Silurian). The succession is a marginal assemblage shoaling upward from deep-sea fan complexes to coastal facies. In terms of sequence stratigraphy, the succession consists of a basal type 1 sequence (the Meguma Supergroup) and three overlying type 2 sequences (collectively, the Annapolis Supergroup). Interpretations of sedimentary environments and stratigraphic relations agree with those of classical systems tracts. The Meguma Zone is a sedimentary sink for an enormous amount of well-sorted fine-grained sand and silt. This quantity and its westward dispersal indicate a Gondwanan derivation. Stratigraphic units of the West African craton mimic those of the Meguma Zone in lithology, provenance, dispersal, succession, and age. During Neoproterozoic time, continental ice sheets, rivers, and wind moved sediment southeastward down a cratonic paleoslope from what is now Morocco through Mauritania and Mali. Remnants of widespread sand sheets extend across southern Mali. This sand reservoir became the source rock for the Meguma Zone. In the Early Cambrian the paleoslope reversed, perhaps due to birth of Iapetus. These same agents eroded the Malian sand sheets episodically during relative sea-level lows in Cambrian, Late Ordovician, and Early Devonian times. Remnants of the resulting northwestward-moving cratonic sands and silts occur today as mesas and buttes in northern Mali, Mauritania, southern Morocco, and Algeria. The ultimate destination of this detritus was the continental margin of North Gondwana. This sediment now forms the Meguma Zone and other terranes of southern Europe, northern Africa, and the Middle East.

Chemosynthesis; an alternate hypothesis for Carboniferous biotas in bryozoan/microbial mounds, Newfoundland, Canada

Fossil tubes, stressed high-abundance low-diversity faunas, and abundant, low-temperature, hydrothermal mineralisation occur in Lower Carboniferous bryosoan/microbial carbonate mounds in southwestern Newfoundland, Canada. These occurrences, set in a rift-valley setting that was tectonically active during the Carboniferous, were previously regarded as marginal marine deposits laid down in subtidal, schisohaline environments adjacent to a rocky shoreline. We suggest that they may, instead, have formed by chemosynthetic processes at low temperature marine vents at a depth of 100 m, or greater

Events and sea-level changes on Gondwana's margin: The Meguma Zone (Cambrian to Devonian) of Nova Scotia, Canada

The Meguma Zone consists of thick, siliciclastic and volcaniclastic strata, and granitic plutons that underlie southern Nova Scotia and offshore areas. The strata were deposited from Late Cambrian through Early Devonian time on the continental margin of Gondwana. Grain size of the major stratigraphic units reflects the changing elevations of base level in the source region. Twelve major events in the zone can be related to changes in relative sea level and combined into four, second-order cycles. Each cycle consists of a fining-, then coarsening-upward sequence that is capped by subaerial volcaniclastic material. These subaerial events are assumed to represent significant hiati, and so are bounding unconformities. Two unconformities are at system boundaries; two, at sequence boundaries. Third-order cycles suggest idiosyncrasies of the source land. Ultimate cause of the Meguma cycles may be mantle activity that affected continental base level, relative sea level, and direction of continental drift. The Meguma curve is similar to that of northwest Africa.

Depositional environment of the Gowganda Formation (Precambrian) at the south end of Lake Tamigami, Ontario

ABSTRACT The Gowganda Formation is world-famous as evidence for Precambrian glaciation. As many features of tillites are similar to those of turbidites, the time seemed ripe for a re-examination of the evidence for its origin. The formation was studied in detail as it is exposed across the southern half of Lake Timagami, Ontario. Two main rock-types, conglomeratic graywacke and argillite-quartzite, alternate three times throughout the 10,000 ft of Gowganda exposed in the area. Conglomerate characterized by granite clasts occurs sporadically along the basal contact. Present attitudes of the formation in the area are initial. The mountainous pre-Gowganda topography was not controlled by basement structure. Most of the evidence points to a glacial origin. A pre-Gowganda glaciated surface of considerable extent is reported. The basal conglomerate is fluvial--possibly outwash. The conglomeratic graywacke is tillite deposited by glacial ice advancing from the northeast quadrant. The argillite-quartzite is an interglacial, flysch-like facies probably deposited in a cool, shallow-water environment with considerable ice-rafting. The periglacial lake or sea was characterized by frequent small-scale turbidity currents triggered by slumping of the unconsolidated sediment on the mountainous pre-Gowganda topography. Paleocurrent features in the argillite-quartzite correspond to the pre-Gowganda topography.

Facies and Phases of the Altamont Limestone and Megacyclothem (Pennsylvanian), Iowa to Oklahoma

The Altamont Limestone, Marmaton Group of the Desmoinesian Series, is a thin, widespread stratigraphic unit which crops out for more than 460miles along the eastern flank of the Western Interior basin. The formation consists of two carbonate members (older Amoret and younger Worland Limestones) and an intervening shale (Lake Neosho Shale)—the last member characterized by a median black phosphorite. The formation is the marine portion of the Altamont megacyclothem. The depositional environment has been interpreted after detailed field work, insoluble-residue analysis, shale disaggregation and sieving, and carbonate petrography of thin-sections and acetate peels. Phosphatic nodules were studied in thin sections and slabs and by qualitative chemical tests. Microscopic point-counts of 23 categories of fauna and flora in limestones and shales yielded 7 biofacies. Tectonic features affecting sedimentation were the Cherokee, Tri-State, and East-Central negative elements, and the Bourbon, West-Central, Lincoln, and Redfield positive elements. Greater turbulence on the south flanks of arches suggests a prevailing south wind. Composition, habit, association, and occurrence of phosphatic nodules conform to present concepts of phosphatic deposition. Maximum marine transgression occurred during phosphate precipitation. Carbonates are shallow-water fades; shales are deep-water facies. Maximum sea-level fluctuation in the area was between 50 and 200 m. A single transgressive-regressive cycle of the strandline produced the Altamont megacyclothem. Water depth was the controlling factor in vertical and lateral distribution of lithosomes and biosomes. Tectonic features modify only the carbonates. Eustatic change rather than diastrophism appears best to explain the megacyclothem.

The Macumber Formation of the Maritime Provinces, Canada--A Mississippian Analogue to Recent Strand-Line Carbonates of the Persian Gulf

ABSTRACT The Macumber Formation is the most distinctive and widespread carbonate of the Windsor Group (Mississippian). The formation is divisible typically into two distinct lithosomes. Lithosome A, the lower unit, is a slabby, thickly laminated, cross-stratified, pelletted, oncolitic, sparsely fossiliferous limestone. Lithosome B, the upper unit, is a fissile, thinly laminated, vuggy, dolomitic, unfossiliferous, mud-cracked, brecciated, algal stromatolite. Discontinuous, thick, boulder-sized calcirudite near the top of the lithosome has been called the Pembroke Formation. The carbonate of the Macumber and Pembroke Formations is overlain by red, gypsiferous, terrigenous lutite, marl, or locally, coarse, thick, red fanglomerate. Both Lithosome A and B are almost identical to strand-line carbonates of the Persian Gulf, Bahaman Islands, Florida Bay, and Australia. Lithosome A was deposited in the shallow subtidal to intertidal zone; Lithosome B in the high intertidal to low subtidal zone. The Macumber Formation and overlying lithosomes of the basal Windsor Group record sudden marine transgression followed by slow regression. Underlying red terrigenous detritus is overlain by subtidal then intertidal carbonate. Overlying high intertidal and low supratidal carbonates were fragmented by desiccation and scour, the clasts heaped into channel-fill breccia in all dimensions. This breccia, in part the Pembroke Formation, is a natural consequence of the Macumber environment. Penecontemporaneous dolomite, gypsum, and/or ed terrigenous detritus record passage of the hypersaline, high-supratidal environment over the depositional site. The lateral progression of bio- and lithosomes is interpreted from both vertical and longitudinal profiles. The seaward succession of lithotopes is from red, terrigenous, usually fine-grained alluvial fans to supratidal, dolomitizing salt flats with saline lakes and lagoons, through strandline carbonates with channels and algal mats to shallow, restricted lagoons, and finally toward more turbulent, more normal marine conditions. Rock units of the basal Windsor Group record environment, but not time except along depositional strike.

Loch Macumber (early Carboniferous) of Atlantic Canada

Following the Acadian Orogeny, Atlantic Canada accommodated several, large, relatively deep lakes within a wrench-fault basin complex called the Maritime Basin. Late Devonian and Tournaisian lakes were hydrologically open, shallow to deep, mainly fresh water bodies. Middle Visean lakes, here collectively called Loch Macumber, were closed, deep, and meromictic. Their deposits comprise the first and thickest of five sequences in the Maritime Basin. Salinity in the loch increased with time from restricted marine or penesaline, to saline. Basin-centre facies consist of a thin, but extensive, sheet of black, peloidal laminated lime mudstones and an overlying thick evaporite complex. The carbonate sheet grades laterally into both laminated to thinly bedded marlstones, siliciclastic sandstones, and microbial, biocementstone mounds. Laminae consist of alternating carbonate and either silty carbonaceous shale or siliciclastic clay and silt. The mudstone and marlstone are locally interbedded with siliciclastic and carbonate turbidites, resedimented (?deep water) breccias, and olistostromes.Seasonal changes in anoxia and/or carbonate production produced rhythmic laminae of carbonate and carbonaceous shale. Carbonate grains consist of silt-sized microbial clots and rare arthropod carapaces and brachiopod shells. The mounds originated as tufa precipitated around subaqueous hydrothermal springs that supported chemosynthetic communities. Resedimentation processes including incipient brecciation, sliding, slumping, debris flows, and turbidity currents were common. The mounds trapped hydrocarbons from the surrounding laminite and sulphides from underlying hydrothermal vents. Increasing salinity with time resulted in sulphate and chloride precipitation that filled the basins and ended the life of Loch Macumber. After the deposition of thick evaporites the topography became less accentuated, the seas less saline, and the faunas more normal marine.

Deep-basin/deep-water carbonate-evaporite deposition of a saline giant: Loch Macumber (Visean), Atlantic Canada

The Carboniferous saline giant of Atlantic Canada is approximately 500 m thick and extends over an area of 250×103 km2. It consits of three lithologies: 1. a relatively thin (usually less than 5 m) extensive carbonate sheet containing buildups, 2. an overlying thick (up to 400 m) evaporite complex of sulphates and chlorides, and 3. wedge-shaped units of conglomerate, breccia, and sandstone (up to 200 m). The latter interfinger with the preceding lithologies but locally may underlie the carbonate sheet. The basal carbonate occurs mainly as fine, laminar couplets of alternating peloidal grainstones and bituminous films; these pass laterally into peloidal marlstones and mounds. Laminites and marlstones have incipient brecciation, recumbent folds, siliciclastic and carbonate turbidites, rubble, and olistostromes. Fossils are rare. They are restricted in space to the basal decimeter and in diversity to conodonts and crustaceans. ANereites ichnofauna occurs at one locality. Mounds consist mainly of turbid fans of fascicular-optic calcite and botryoids of laminated calcite. Their fauna shows low diversity but high density of some species; tube worms and microbial growths are noteworthy. The overlying evaporites are mainly anhydrite with lateral and vertical transition into halite and potash. Siliciclastic sediments thicken toward boundary faults. Both the base and top of the giant are unconformities, the former abrupt, the latter karstic.The basal carbonate accumulated subaqueously in deep, physically and chemically stratified water. Initially, the desert floor of the complex rift basin was below ambient sea level. After catastrophic submergence, basal waters were at first dysaerobic and restricted marine, but changed to anoxic and pene- to hypersaline as the water column became stratified. Its sedimentary record corresponds to a giant, meromictic, saline lake — Loch Macumber. The laminite records either seasonal changes in rainfall and temperature, or episodic storms punctuating normal chemical sedimentation. Sulphate-reducing bacteria fed on terrestrial organic matter preserved in anoxic bottom water, and precipitated peloidal calcite. Concentrations of bacteria over hot springs may also have supported chemosynthetic organic communities and precipitated petroliferous and sulphide-rich mounds. Siliciclastic rocks increase toward basin margins. Locally steep depositional surfaces caused mass movements of sediment down basin margins. A long term increase in salinity resulted in precipitation of initially corroded and subsequently euhedral crystals of gypsum. Thick sulphates and chlorides followed. Sedimentary rock of Loch Macumber records topographic filling of a deep rift basin. Isostatic analysis suggests that both depth of the subaerial desert basin and initial water depth after submergence were considerably less than the preserved thickness of the saline giant. Its record corresponds closely to that expected from Schmaltz’s model of deep-basin/deep-water carbonate-evaporite deposition.

Carbonate mud flows and other indicators of excess pore-fluid pressure in the Macumber Formation (Lower Carboniferous, Viséan) at Ingonish, Nova Scotia, Canada

The Ingonish area of northern Nova Scotia offers the most extensive exposure of the Macumber Formation, the basal carbonate unit of the Lower Carboniferous (Middle Viséan) Windsor Group of Nova Scotia and New Brunswick. The area also shows tectonic-free sedimentary characteristics of this unit. The formation consists of two principal lithologic units, a basal, black, earthy, carbonaceous limestone normally about 0.5 m thick and a thinly stratified, peloidal dolomudstone up to 17.5 m thick. At Burke Head, the dolomudstone is cut by at least 25 massive tufa mounds, elongated in cross-section and intersecting their surrounding strata by rising northeastward at angles of 15 to 25 degrees. At east Ingonish Island the basal unit is anomalous in being both dolomitized and thicker (5 m); there, both units are cut by at least 20 smaller massive tufa mounds. The mounds at both localities formed over deep-water hydrothermal vents, i.e., marine hot springs; the fossils of abundant chemosynthetic tubeworms surround the lower mounds on east Ingonish Island. The Macumber Formation overlies Horton Group braided-stream facies sandstones, and presumably underlies nearby, thick gypsum deposits of the Windsor Group.Features in the formation indicating excess pore-fluid pressure include voluminous vugs and widespread subaqueous sediment flows. Miniature pockmarks, sedimentary volcanoes, and shafts show the release of small quantities of pore fluids. Larger quantities of pore fluids exited through tufa mounds and geyser-like shafts. The pressure generated mudflows; eruption of pore fluids brecciated some of their flanks.The general environment of rift tectonics was conductive to active hydrology. The seasonal semi-arid climate caused changes in pore-fluid pressures, due to episodic aqueous and sedimentary loading of basin floors. Igneous activity, either at the same time or before deposition of the Windsor Group, created significant geothermal gradients to drive hydrothermal circulation and increase pore-fluid pressures.Textures and structures of the Macumber Formation in the Ingonish area are due to syndepositional processes, driven by initial, high pore-fluid pressures.