Martin R. Gibling

Late Carboniferous cyclothems and alluvial paleovalleys in the Sydney Basin, Nova Scotia

Cyclothems are documented for the first time in alluvial to coastal plain strata (West-phalian D to Stephanian) of the Sydney Basin, Nova Scotia. Well-developed cyclothems commence with an extensive carbonaceous lime-stone or shale with a restricted marine biota that indicates maximal flooding. Succeeding gray detrital bay and lake fills with distributary channel deposits are overlain by stacked channel deposits (paleovalley fills) that mark an abrupt basinward facies shift and a profound change in alluvial style. Channel fills and red vertisols were deposited on an alluvial plain above the filled valleys, and the paleovalley is equated with thick vertisols in adjacent, inter-fluve areas. The cyclothems are capped by gray coastal deposits with economic coals. Agglutinated foraminifera are present in seat earths beneath the coals, which formed as a result of a brief stillstand during transgression. Red mudstones pass laterally into gray detrital bay fills in regions of greater subsidence associated with faulted basement blocks. Poorly developed cyclothems are bounded by extensive but split coals, and autogenic effects partially obscure allogenic patterns. Mean cyclothem duration approximates 200 ka.

Calcrete and coal in late Carboniferous cyclothems of Nova Scotia, Canada: Climate and sea-level changes linked

Modern tropical peats require continuity of precipitation for accumulation, whereas calcretes and calcic vertisols require strongly seasonal conditions. The 20-30-m-thick cyclothems of the Sydney basin, Nova Scotia, Canada, show a systematic alternation of coals and other hydromorphic paleosols with calcretes and calcic vertisols. This implies strong variation in seasonality during the duration of a cyclothem, estimated at 200 ka. In at least one cyclothem, calcic paleosols formed on an interfluve adjacent to a paleovalley cut through marine strata, suggesting that a more seasonal and probably drier climate prevailed during sea-level lowstand. The calcic paleosols are estimated to have formed during periods of 10[sup 4] yr, possibly indicative of partial control by obliquity and precessional cycles. 27 refs., 5 figs., 1 tab.

Calcretes at sequence boundaries in upper carboniferous cyclothems of the Sydney Basin, Atlantic Canada

Abstract Nodular and associated groundwater calcretes in coal-bearing cyclothems of the Sydney Basin mark the boundary between underlying marginal-marine and overlying alluvial deposits. They are inferred to represent lowstand surfaces (sequence boundaries). The host sediments include both sand/siltstones and palustrine limestones. Microfabrics of calcretes with detrital hosts show evidence of replacement, displacement and shrinkage (alpha fabrics), suggesting that the calcretes formed under relatively arid conditions. In contrast, microfabrics of nodular calcretes with limestone hosts exhibit predominantly beta fabrics, with root-induced brecciation and subordinate shrinkage fabrics. The calcretes and host limestones are inferred to have formed in topographic lows. Thick alluvial sediments with red calcic vertisols overlie the calcretes, and formed within the transgressive systems tract under conditions of abundant sediment supply. Coals and associated hydromorphic palaeosols lie near the transgressive maximum and in the highstand systems tract. The presence of calcareous palaeosols on lowstand surfaces suggests that the lowstands were times of relative climatic aridity.

Paleozoic vegetation and the Siluro-Devonian rise of fluvial lateral accretion sets

A long-standing hypothesis links the increased prominence of meandering rivers in the middle Paleozoic to the colonization of terrestrial environments by vegetation. This hypothesis is tested using a data set of Cambrian to Devonian fluvial literature and field examination of key stratigraphic units. According to some researchers, Cambrian to mid-Silurian river systems were braided in planform, with a sharp increase in the abundance of meandering rivers during the Silurian–Devonian. Although meandering systems were largely identified on the basis of thick mudstones and organized channel deposits, the data set record of lateral accretion sets appears to be a robust proxy for the abundance of meandering river point bars. Lateral accretion is first recorded from Pridolian–Lochkovian strata, but is noted in nearly 40% of fluvial case studies by the Famennian. This trend matches the known record of rooted vegetation, suggesting that vegetation progressively stabilized river banks and promoted single-thread channels. However, the presence of Precambrian and extraterrestrial meandering systems indicates that vegetation is not essential for meandering, and the lack of evidence for Cambrian to Silurian (Ludlow) point bars is surprising. If originally present, they may largely have been destroyed by extreme floods, chute cut-offs in coarse-grained meandering systems, and eolian activity.

Facies model for fluvial systems in the seasonal tropics and subtropics

Facies models that summarize the deposits of fluvial systems are well established for humid climate settings and for desert environments, but the deposits of rivers in the sub-humid and semiarid seasonal tropics have been largely ignored. Our observations and data from modern streams and recent deposits in northeastern Australia show how such rivers, with extremely variable discharge, have distinctive deposit characteristics that are substantially different from conventional fluvial facies models. These properties include (1) erosionally based channel-fill lithosomes that exhibit complex lateral facies changes, with (2) abundant, pedogenically modified mud partings, (3) complex internal architecture that may lack the macroform elements typical of other fluvial sediment bodies, (4) an abundance of sedimentary structures formed under high flow stage, and (5) an abundance of in situ trees that colonize channel floors and are adapted to inundation by fast-flowing water. We illustrate examples of this fluvial style from the rock record, and set out a new facies model. The recognition of such a distinctive fluvial character, and of changes in character through vertical successions, will aid paleoclimate and reservoir analysis.

Agglutinated foraminifera and thecamoebians from the Late Carboniferous Sydney coalfield, Nova Scotia: paleoecology, paleoenvironments and paleogeographical implications

Abstract Agglutinated foraminifera and arcellaceans (“thecamoebians”) were examined from Carboniferous (late Westphalian-Stephanian) cyclothems in the Sydney Basin of Nova Scotia. Four distinctive assemblages are documented from the coal-bearing strata, and indicate deposition took place on an extensive coastal platform. The eastern part of the basin is dominated by Ammobaculites-Ammotium assemblages that indicate estuarine-low marsh paleoenvironments. Relationships between test size, grain size within the test, and grain size of the substrate are used to differentiate between upper mudflat and lower mudflat sub-facies within estuarine facies. Mixed assemblages dominated by Trochammina, Ammotium and Ammobaculites indicate vegetated substrates, analogous to modern lower marsh environments, and flanked the estuarine system in the eastern and western areas of the basin. Rich Trochammina and thecamoebian assemblages are restricted to the western area of the basin, indicating deposition within the upper part of the tidal range. Trochammina assemblages, typically found underlying coal seams, may indicate vegetated substrates equivalent to modern high marsh facies. Thecamoebian assemblages, dominated by an agglutinated difflugid type, indicate freshwater paleoenvironments. Encystment of the thecamoebian populations suggests paleoenvironments were short lived.

Craton-derived alluvium as a major sediment source in the Himalayan Foreland Basin of India

Within the Himalayan Foreland Basin, the axial Yamuna River with Himalayan headwaters lies along the northern margin of the Indian Craton, giving the impression that cratonic rivers have contributed little to the basin compared with Himalayan drainages. However, the Betwa, Chambal, and other rivers, which drain northward into the Yamuna, are vigorous monsoonal rivers with large catchments. Stratigraphic and petrographic evidence shows that sediment derived largely from these rivers extends north of the axial Yamuna River. Red feldspathic sand and gravel underlie much of the southern foreland basin at shallow depth (>25 m), where its topmost strata are dated at ca. 119 ka ago, and extend at deeper levels (>500 m) to about one-third of the distance across the foreland basin. Petrographic analysis confirms a match with modern Betwa River sands, which derive their feldspar from granitic gneisses of the Bundelkhand Complex. Along the Yamuna Valley the red alluvium is overlain by gray alluvium dated at 82–35 ka ago, which also yields a cratonic signature, with large amounts of smectite derived from the Deccan Traps. Cratonic contributions are evident in alluvium as young as 9 ka ago in a section 25 km north of the Yamuna. This gray cratonic sediment was probably deposited in part by the Chambal River, which transports high-grade metamorphic minerals from the Banded Gneiss Complex of the Aravalli belt. Cratonic sediment appears to interfinger with Himalayan detritus farther north below the Ganga-Yamuna Interfluve. With its headwaters in the tectonically unstable Indus-Ganga watershed area, the Yamuna River may have occupied its present course late in the Quaternary, and if so, cratonic rivers may have provided the basin9s axial drainage for prolonged periods. The penetration of Himalayan sediment to the distal foreland basin may reflect avulsion of orogenic rivers along the craton margin, in addition to dynamic transverse drainage systems from the Himalaya that pushed the axial drainage to the basin9s feather edge. The wide spread of cratonic sediment would have been enhanced by slow subsidence in the distal foreland basin and focusing of rivers into a basin reentrant.

The geometry of drag zones adjacent to salt diapirs

Drag zones are highly strained regions developed adjacent to the flanks of salt diapirs, and are produced when the sedimentary overburden is folded or rotated into steeply dipping attitudes sub-parallel to the diapiric walls. This case study focuses on a diapiric province on Cape Breton Island, Nova Scotia, where five Visean-age salt diapirs penetrate Upper Carboniferous (Namurian–Stephanian) conglomerates, sandstones, shales and coal seams. 2D marine seismic coverage extends over several diapirs thus allowing the drag zones studied onshore to be located relative to the adjacent diapir. The width of diapiric drag zones within the case studies varies from 70 m up to 500 m, with narrow drag zones reflecting the low mean competence of shales and siltstones which may have been shallowly buried and poorly lithified at the onset of diapirism. Broader drag zones are dissected by two sets of extensional fractures together with major faults, suggesting that the overburden was semi-lithified and displayed greater flexural rigidity. Overburden displaying high competency contrasts results in strain localization and partitioning, with pervasive granulation seams and minor faults developed in sandstones and bedding-parallel shears in coal and shale horizons. Drag zones are segmented by steeply outward-dipping faults associated with decametric, asymmetric drag profiles. These faults facilitate the upward movement of material and effectively expand the diapiric process into adjacent overburden via a newly termed process of lateral diapiric accretion.

An early Pennsylvanian waterhole deposit and its fossil biota in a dryland alluvial plain setting, Joggins, Nova Scotia

The terrestrial ecology of Pennsylvanian tropical wetlands is understood in detail, but coeval dryland ecosystems remain highly enigmatic. To fill this gap in our knowledge, a Pennsylvanian (Langsettian) continental red-bed succession was studied at the classic Joggins locality, Nova Scotia. These units represent the deposits of seasonally dry, alluvial plains traversed by anastomosed drainage networks. One channel complex informally known as the ‘Hebert beds’ (the focus of this study) contains an unusual fossil assemblage and is interpreted as an alluvial waterhole deposit that formed following drought-induced cessation of channel flow. Adpressed and charred fossil plant remains indicate that the alluvial plain surrounding the waterhole was covered by fire-prone cordaite vegetation, with hydrophilic lycopsids and sphenopsids restricted to waterlogged riparian niches. Gigantic unionoid freshwater bivalves, locally in life position, and occurring in large numbers in the waterhole, were probably infaunal suspension feeders during periods of fluvial activity, but aestivated in channel bottom muds when flow ceased. Abundant terrestrial gastropods found clustered around fossil plant detritus may have been deposit feeders scavenging dry portions of channel floors. Common partially articulated remains of small to medium-sized tetrapods possibly represent animals drawn to the waterhole during drought when surface water was scarce elsewhere. In terms of both sedimentology and biology, the Hebert beds alluvial complex bears a very close similarity to the seasonal drainages and waterholes of present-day central and northern Australia. This unique deposit sheds significant new light on the nature of Pennsylvanian dryland tropical ecology.

Braidplain Evolution in the Pennsylvanian South Bar Formation, Sydney Basin, Nova Scotia, Canada

ABSTRACT The South Bar Formation is a 1 km-thick alluvial succession in the Pennsylvanian Morien Group of the Sydney Basin, Nova Scotia that fines upward from conglomerate to pebbly sandstone to sandstone facies assemblages. The conglomerate assemblage comprises mainly massive to horizontally stratified conglomerate and planar cross-stratified sandstone, attributed to deposition in proximal braided rivers confined between bedrock ridges. The sandstone assemblage shows uniformity of facies and paleocurrents across the basin, and is interpreted as a distal braidplain deposit; the pebbly sandstone assemblage is transitional between the other two. Both sandstone assemblages are dominated by trough cross-stratified sandstone formed by dune migration in channel tracts about 4 m deep. This succession differs markedly from Platte River and Icelandic sandy braidplain deposits, which are mainly planar cross-stratified sands formed by broad, shallow flow. The South Bar sandstones are more like South Saskatchewan River deposits, but differ in 1) greater abundance of trough sets and 2) rarity of planar sets and other components of shallow water channel fill. These features are ascribed to 1) inheritance of relatively deep channels from confined upstream reaches, 2) maintenance of deep flow in active tracts by local confinement between temporary banks, and 3) rapid avulsive abandonment of active tracts, which inhibited development of channel-fill sequences. Evidence for local temporary confinement is preserved as in situ and slumped channel-margin mudrock units. Lateral and vertical transitions from the upper South Bar Formation to the mud-rich Waddens Cove and Sydney Mines Formations are ascribed to changes in channel pattern from braided to meandering. The former is attributed to decreased slope, the latter to eustatic rise in sea level.