Ron Boyd

Highstand transport of coastal sand to the deep ocean: A case study from Fraser Island, southeast Australia

Deep-water sands form a new frontier for marine geology and petroleum exploration, but how does sand reach the deep sea? Existing geological models predict that deep-water sands are mainly supplied from rivers during times of low sea level, or by incision of canyons into the shelf to tap river or longshore-transport sand sources. Here, we demonstrate that at high sea level, southeast Australian deep-water sands are delivered by a wave-driven coastal transport system, interacting with estuarine ebb tidal flows, that transports sand over the shelf edge at a change in margin orientation. Discovery of this new process results from an investigation that combines multibeam acoustic, microfaunal, zircon and luminescence dating, oceanographic, Landsat, remotely operated vehicle, and sediment property methods. Our longshore transport–driven model is capable of forecasting new locations for deep-water sand deposits in a predictive paleoclimatic and paleotectonic setting.

On balanced and unbalanced accommodation/peat accumulation ratios in the Cretaceous coals from Gates formation, Western Canada, and their sequence-stratigraphic significance

Coal composition was investigated by means of photometric and maceral analyses on closely spaced lithotype-based strip samples over the full thickness of several paralic coal seams from the Cretaceous Gates Formation of the Western Canadian Sedimentary Basin. The aim of this investigation was to test various methods of identifying accommodation trends in coal and use them to refine sequence-stratigraphic interpretation of continental sediments. Conventional sequence stratigraphy derives its subdivisions and significant surfaces from the records left by relative sea-level oscillations. These records either do not project into the continental realm, or are difficult to recognise in clastic non-marine sediments. Paralic coal seams have been selected to study this problem, because they are not entirely removed from marine influence and, compared with most inorganic deposits, coal has stored a greater wealth of information that can be analysed at a higher level of resolution. The study has led to the identification of five new surfaces with chronostratigraphic potential in the sequence-stratigraphic analysis of non-marine sediments. Two of these surfaces, called paludification surface (PaS) and terrestrialisation surface (TeS), occur at the bases of the investigated coal seams, while two other surfaces, referred to as non-marine flooding surface (NFS) and give-up transgressive surface (GUTS), form the tops of the coal. The fifth and probably most important new surface, called the accommodation-reversal surface (ARS), is independent of any particular facies and may either coincide with some of the other surfaces or occur separately. The proportion of detrital minerals has been used as the chief discriminator between different mire types and accommodation trends. Other useful indicators of mire type and peat dispersal have been the proportions of sporinite and inertodetrinite, as well as some derived maceral and/or mineral ratios, e.g., the groundwater influence index and the tissue preservation index. Isometamorphic variations of telovitrinite reflectance and fluorescence, as well as their coefficients of variation were also found to contribute to the identification of cyclic shifts between balanced and unbalanced accommodation/peat accumulation ratios. Some of these cycles, which are backed up by clastic stratigraphy, appear to correspond to the development of shallowing-upward and deepening-upward parasequences. Superimposed high-frequency, low-amplitude perturbations in the coal cycles relate to smaller-scale accommodation cycles of sub-parasequence level, not always recognised in non-marine strata. These sub-parasequence coal cycles do not always continue the shallowing-upward trend typical of conventional parasequences. Several coals were found to contain stacks of small-scale cycles with upward increasing accommodation signatures either in their lower or upper halves, or over the whole seam section.

Wisconsinan glacial and sea-level history of Maritime Canada and the adjacent continental shelf: A correlation of land and sea events

The Wisconsinan glacial history of Maritime Canada records the interaction of land-based glaciers with the sea, which dissects the region along the deep embayments of the Bay of Fundy and the Laurentian Channel. Ice centers, collectively termed the Appalachian ice complex, shifted in a clockwise manner across the varied bedrock terranes of the region, producing cross-striated bedrock outcrops and compositionally distinct till sheets on land. Offshore glacial sediments can be correlated to their terrestrial counterparts on the basis of provenance, thus establishing crucial land-sea links in the glacial record. Five glacier flow events have been recognized. During the Caledonia phase in early to middle Wisconsinan time, ice from eastern Appalachian upland sources crossed Nova Scotia and extended to the continental shelf edge, where a calving margin was established. Slumping at this margin produced proximal wedge-shaped bodies of diamicton that interfinger with glaciomarine sediments. The Caledonia phase glacier retreated during middle Wisconsinan time to the inner Scotian Shelf. During the Escuminac phase in late Wisconsinan time (22–19 ka), an ice center formed over the Magdalen Shelf (Escuminac ice center) and transported large quantities of local red-bed material southward to the outer shelf-slope margin. This glacier retreated about 18 ka to a tidewater margin at Sable Island Bank and Banquereau. Sea level during this time was falling as a result of isostatic recovery from earlier Escuminac phase maximum ice loads. Just after 18 ka, ice wasted back across the midshelf basins to the tidewater margin of the Scotian Shelf end moraine complex and was stabilized by falling sea levels. Escuminac ice was reorganized into the Scotian ice divide (Scotian phase) over mainland Nova Scotia and the continental shelf south of Cape Breton Island. Between 15 and 13 ka another period of glacier retreat ensued, and the margin settled close to the present-day land areas. Between 13 and 12.5 ka an ice margin was established from local terrestrial ice centers in northern Nova Scotia (Chignecto phase) into the Bay of Fundy and just beyond the present-day Atlantic coast. Responding to increasing climatic warming, these smaller glaciers dissipated and their margins retreated landward. A sea-level lowstand of −65 m on the inner shelf occurred ca. 11.7 ka. Isolated terrestrial remnants of Chignecto phase glaciers were reactivated as a result of cooling during the Collins Pond phase (Younger Dryas) ca. 10.8 ka. Enhanced ice rafting, evident in the North Atlantic deep oceanic record, appears to correlate with periods of glacier retreat in the Appalachian ice complex. During melting and sea-level rise, both the quantity and thickness of icebergs increased due to grounding-line retreat within the Magdalen Shelf, thereby increasing the net flux of hematite-stained quartz to the North Atlantic.

Depositional Styles in a Low Accommodation Foreland Basin Setting: An Example from the Basal Quartz (Lower Cretaceous), Southern Alberta

ABSTRACT The Lower Cretaceous Basal Quartz (BQ) of Southern Alberta (Townships 1-40, Ranges 1 W5-5W5) can be informally divided into seven mappable units (A Sandstone, composed of the Regional A, Carmangay, Mesa IV, Valley and Terrace; Horsefly, BAT, Ellerslie). The study area is considered to be in an accommodation-limited setting due to the presence of multiple, closely spaced unconformities, and the general absence of marine deposits. Multiple levels of cyclicity exist in the BQ. There are two cycles of increasing-upward mineralogical and textural maturity, the first associated with the A Sandstone and the second associated with the Horsefly-BAT-Ellerslie succession. There are multiple southward marine transgressive events, both on the unit scale and on the cycle scale, indicative of a backstepping stacking geometry. The high resolution subdivision of the BQ allows for the recognition of changing BQ paleodrainage through time. There is both a progressive spatial and stratigraphic change in incised valley organization, from thin and wide valley forms in the south and at the base of the cycles, to thicker, narrower and more deeply cut systems toward the northwest and top of the cycles. In addition, there is spatial variation in tributary systems for the upper cycle, from no tributaries associated with the thin, wide valley forms associated with the Horsefly, to narrow and thin tributaries in the BAT south of the Vulcan Low, deeply cut complex tributary systems across the Vulcan Low, and linear deep tributaries north of the Vulcan Low for both the BAT and Ellerslie. The style of depositional fill also changes stratigraphically and spatially, from braided to coarse meandering sheet deposits south of the Vulcan Low associated with the Carmangay and Horsefly, and low accommodation Mesa IV, and higher accommodation Valley and Terrace, Horsefly and BAT; and fluvial-estuarine deposits associated with the Valley and Terrace, BAT and Ellerslie north of the Vulcan Low in higher accommodation setting. The dominant control on the BQ depositional patterns is the interplay between eustasy and heterogeneous basement subsidence. The tectonic influences on sedimentation are most obvious in the sediments immediately overlying long-duration unconformities. Fluvial erosion on the unconformity surface amplifies the tectonic signal by accentuating the tectonically produced relief in a low accommodation setting. The accommodation-limited conditions occurring during BQ deposition resulted in sequence boundaries amalgamating on the interfluve, and it is only by detailed correlation and petrographic analysis that BQ units can be differentiated. End_Page 31-------------------------

High-resolution sequence-stratigraphic correlation between shallow-marine and terrestrial strata: Examples from the Sunnyside Member of the Cretaceous Blackhawk Formation, Book Cliffs, eastern Utah

The Sunnyside Member of the Upper Cretaceous Blackhawk Formation in the Book Cliffs of eastern Utah provides an ideal opportunity to investigate high-resolution sequence-stratigraphic correlation between shallow-marine and terrestrial strata in an area of outstanding outcrop exposure. The thick, laterally extensive coal seam that caps the Sunnyside Member is critical for correlating between its shallow-marine and terrestrial components. Petrographic analysis of 281 samples obtained from 7 vertical sections spanning more than 30 km (18 mi) of depositional dip enabled us to recognize a series of transgressive-regressive coal facies trends in the seam. On this basis, we were able to identify a high-resolution record of accommodation change throughout the deposition of the coal, as well as a series of key sequence-stratigraphic surfaces. The stratigraphic relationships between the coal and the siliciclastic components of the Sunnyside Member enable us to correlate this record with that identified in the time-equivalent shallow-marine strata and to demonstrate that the coal spans the formation of two marine parasequences and two high-frequency, fourth-order sequence boundaries. This study has important implications for improving the understanding of sequence-stratigraphic expression in terrestrial strata and for correlating between marine and terrestrial records of base-level change. It may also have implications for improving the predictability of vertical and lateral variations in coal composition for mining and coalbed methane projects.

Sequence stratigraphy of a coastal-plain incised valley estuary, Lake Calcasieu, Louisiana

ABSTRACT The stratigraphic profile along a 100 km section of the Lake Calcasieu incised-valley fill was reconstructed from 87 bridge borehole records at four sites along the tidal reaches of the Calcasieu River, an additional 18 boreholes aligned along the chenier plain near the estuary mouth, and shallow seismic data collected along 160 km of track line within the Lake Calcasieu central basin. Valley incision occurred during the d18O stage 2 sea-level lowstand, when the paleo-Calcasieu River flowed across the inner continental shelf and incised into older fluvial terraces of the Prairie Formation. The valley floor is 57 m deep beneath the modern chenier plain and rises to -30 m along the upper reaches of the modern estuary. Sea.level rise commenced about 20 ka and has continued episodically throughout the Holocene. Shoreline transgression has proceeded at varying rates in response to sea-level rise, and the incised valley is now occupied by a wave-dominated estuary, fringed by the chenier plain of western Louisiana. Lowstand deposits are poorly preserved within the Calcasieu incised valley. Deposits constituting the transgressive systems tract (TST) lie directly above the lower sequence boundary at the top of the Prairie Formation. The TST shows greatest facies diversity and clearest separation of bounding surfaces toward the present seaward end of Lake Calcasieu. Here, the TST comprises three parasequences that include fluvial, bayhead-delta, and central.basin (estuarine) deposits. Multiple flooding surfaces within the TST relate to the episodic style of sea-level rise. Highstand infilling of the valley has been characterized by nearshore marine deposition and chenier-plain progradation. Along the lower reaches of the modern Calcasieu River, the valley fill comprises transgressive central-basin nd landward-stepping bayhead-delta facies, overlain by the highstand bayhead-delta complex. This study documents the most recent phase of deposition within the compound fill of a coastal-plain incised valley and demonstrates the potential importance of fluvial controls on facies within incised-valley systems.

Morphology and seismic stratigraphy of the inner continental shelf off Nova Scotia, Canada: Evidence for a −65 m lowstand between 11,650 and 11,250 C14 yr B.P.

Abstract Nova Scotias position near the margin of large Pleistocene ice sheets makes the Scotian Shelf region critical for evaluation of glaciation models and sea-level change. The sea floor of the inner Scotian Shelf was mapped using multibeam bathymetry, a combination of conventional seismic and sonar techniques, and sampling. Multibeam bathymetry provides an areal image of the sea floor. Combining this unique image with sub-bottom seismic imaging, the relationships between sea floor topography and the underlying strata were explored. The inner continental shelf of Nova Scotia can be subdivided into five major terrain “zones”. These are the: (1) Truncation Zone, (2) Morainal Zone, (3) Outcrop Zone, (4) Basin Zone and (5) Scotian Shelf End-Moraine Complex. The Scotian Shelf End-Moraine Complex and Basin Zone are glacial-depositional zones at the seaward edge of the inner shelf. Landward of these zones is a region of high relief bedrock, with ridges and valleys largely devoid of surficial sediments extending from 80 to 120 m water depth (Outcrop Zone). This zone is interpreted as a relict bedrock surface, preserved under frozen-bed glacier conditions. To the east of the Outcrop Zone, in similar water depths, are unmodified till ridges overlying bedrock (Morainal Zone). The Truncation Zone is a region of the inner shelf from −90 m to the present shoreline characterized by muted acoustic topography and planar erosional surfaces truncating bedrock and surficial sediments. The Truncation Zone is subdivided into the Valley Subzone, the Transition Subzone, the Platform Subzone and the Estuarine Subzone. The Valley Subzone is typified by sediment-infilled valleys occurring in 75 to 90 m water depths. These valleys contain seismic facies with a ponded style of deposition planed off at the sea floor or by erosional unconformities near the sea floor. The Transition Subzone is marked by a relatively steep “ramp” and terraces. The ramp at the type section extends from 75 to 65 m water depth. The surface of the ramp can be either an erosional unconformity or a depositional surface formed by clinoform beds. The −65 m former shoreline is interpreted as the top of the Transition Subzone ramp surface. At one ramp locality, clinoform beds form part of a progradational sequence called the Sambro Delta. A mussel valve fragment ( Mytilus edulis ) was obtained from a core in the foresets and radiocarbon dated. An age of 11,650 ± 110 yr B.P. was obtained, adjusted for isotopic fractionation. Above −65 m the sea floor forms a low relief, gently sloping erosional surface (Platform Subzone). Estuarine deposits have been found above −50 m. The new sea-level curve constructed with the dated −65 m shoreline and recently published data is at variance with calculated RSL curves based on geophysical models, primarily in the amplitudes and rates of RSL change. Reasons for the discrepancies may be late-melting ice and regional variations in lithospheric strength and thickness.

Stratigraphic Style of Coal and Non-Marine Strata in a Tectonically Influenced Intermediate Accommodation Setting: The Mannville Group of the Western Canadian Sedimentary Basin, South-Central Alberta

ABSTRACT Coal-bearing strata from the Lower Cretaceous upper Mannville Group (south-central Alberta) were investigated in order to evaluate the nature of coal-bearing non-marine to marginal marine sediments developed in an intermediate accommodation setting, located centrally within the Alberta Foreland Basin. Downdip and lateral correlations to the northwest and east link upper Mannville Group strata to the Falher sequences and the Waseca to Lloydminster sequences, respectively, and indicate that a higher order of stratigraphic subdivision, controlled by transgressive-regressive cycles, must also be present in the upper Mannville. Stratigraphic analysis of the upper Mannville Group in the study area, based on over 1200 km of borehole cross-sections and 50 cores, revealed a number of features that can be considered characteristic of intermediate accommodation in non-marine sediments. These include abundant, compound coal seams and numerous incised valleys with even distribution of sediments between incised valleys and adjacent interfluves. The incised valleys may correlate laterally into horizons within the compound coals, indicating that the coal seams contain sequence boundaries within them and therefore span relative sea level cycles. However, the occurrence of coal seam splits in multiple directions suggests that seam splitting in the upper Mannville Group was at least partially controlled by differential subsidence. This is attributed to fault reactivation on the underlying irregular Paleozoic basement, and maps of inferred fault planes indicate a horst and graben style of extensional faulting. Fault planes appear to be associated with relatively steeply dipping basement topography, and areas of thickest cumulative coal preferentially occur above horst blocks. Although these particular features may be unique to the Mannville Group, they suggest that underlying structures and paleotopography are likely to exert a strong influence on sedimentation patterns in intermediate accommodation settings, because the rates of vertical accretion are insufficient to suppress the effects of differential subsidence. Other expressions of basement control are the localization of single and compound incised valleys along structural lows. Coal composition analysis is based on photometric and maceral analyses of three upper Mannville Group coals. The aim was to test various methods of identifying small-scale accommodation trends in the coal and use them to identify a characteristic accommodation signature for each coal. The Glauconite, Medicine River and informally named Hackett coals were shown to be significantly more complex than simple transgressive or regressive style coals. They comprise a number of wetting- and drying-upwards cycles representing repeated episodes of peat deposition under rising or falling accommodation conditions, with or without internal hiatuses between the cycles. These accommodation cycles are driven by changes in groundwater levels that are in turn hydraulically linked to relative sea level, and thus form the basis for identifying a characteristic non-marine sequence stratigraphic style. End_Page 507------------------------

Stratigraphic Style of Coal and Non-Marine Strata in a High Accommodation Setting: Falher Member and Gates Formation (Lower Cretaceous), western Canada

ABSTRACT The Lower Cretaceous Falher Member and Gates Formation of the Western Canadian Sedimentary Basin provide an opportunity to investigate high resolution stratigraphic correlation in non-marine to marginal marine rocks. This is due to the large volume of high-quality data available from subsurface cores and wireline logs in the Elmworth area, corresponding outcrops in the adjacent Rocky Mountains foothills, and outcrop and continuous core data from the Bullmoose Mine area in northeastern British Colombia. A key component of the high resolution interpretation and correlation in this region is the abundant, thick coal seams that occur in fresh mine highwalls and also in cores from the mine sites and the Elmworth area. Using coal petrographic constituent analysis and derived parameters, we are able to identify systematic variations in coal properties that respond to changes in accommodation. In particular, these properties enable us to distinguish two types of peat (transgressive and regressive) characterized by wetting- and drying-upward behaviour linked to variations in the groundwater table. They also enable recognition of a range of non-marine stratigraphic surfaces that record responses to changing accommodation, including accommodation reversal surfaces, flooding surfaces, hiatal surfaces, paludification surfaces and terrestrialization surfaces. A combination of these coal parameters, together with the facies characteristics of the surrounding non-marine and marginal marine rocks, enables recognition of distinctive high-resolution stratigraphic signatures in the rocks. This in turn provides a previously unavailable ability to correlate stratigraphic unit from their downdip marine position, through the shoreline zone and into the terrestrial realm. For the Falher/Gates unit, we recognize nine regionally correlatable cycles over a vertical distance of around 300 m and a lateral distance of 80 km downdip and 150 km along strike. The basis for this detailed correlation is the similarity of the accommodation trends as seen in both the organic and clastic facies. Results show that earlier concepts of parasequences and their flooding surface boundaries in marine rocks need to be significantly modified in the terrestrial realm. Sharp hiatal parasequence boundaries in the marine realm such as flooding surfaces and wave/tidal ravinement surfaces may correlate updip to packages of rocks that pass gradationally from transgressive to regressive units and preserve the transitions between the two. Non-marine sediments may accumulate during and following shoreline regression, and prior to and during shoreline transgression. The exact style and preservation of the non-marine stratigraphic package depends on the local balance between accommodation and sediment flux at the time of deposition. Coals occur in both regressive and transgressive styles and may initiate or terminate parasequences. Coals may also occur as compound coals that span more than one parasequence and contain internal discontinuity surfaces. End_Page 275------------------------

Morphostratigraphy and facies architecture of sandy barriers along the Eastern Shore of Nova Scotia

Abstract Linear southeast trending valleys along the Eastern Shore of Nova Scotia were initially formed by fluvial incision and subsequently scoured during Pleistocene glaciations. Estuarine and barrier sedimentation was initiated when the valleys were drowned during the marine transgression of the early- to mid-Holocene. This coast continues to experience transgression, due to relative sea-level rise of 35–40 cm per century. Eastern Shore barriers have formed under a wave-dominated regime. The modal wave is highly energetic, with a height of 1.5–2 m and a 9–10 s period. Tidal range is low mesotidal with maximum spring range of 2.1 m. Fluvial sediment supply to Eastern Shore estuaries is negligible. Four sandy barriers from the Eastern Shore exhibit erosional and progradational morphologies that result from differences in the balance between relative sea-level rise, sediment supply and depositional processes within coastal compartments. Morphological contrasts represent the cyclic nature of barrier and estuarine evolution along the Eastern Shore. During transgression, sedimentation commences with a unit of fine grained central basin deposits overlain by a bed of fine to medium, massive and cross-bedded flood-tidal delta sands. An erosional surface lies at the contact between the flood-tidal delta facies and the overlying nearshore facies of tabular and cross-bedded gravelly sands. In some cases, the nearshore unit may grade upward into a second, younger flood-tidal delta deposit that is in turn incised, and then filled by sandy channel fill facies. The tidal delta facies is overlain by beach sands at the seaward side and washover deposits on the landward side of the barrier. The uppermost facies in the sequence includes a composite unit of upper beach and dune sands. Where sediment supply is sufficient, a beach ridge plain will form, producing a composite barrier stratigraphy with transgressive and regressive elements.