Abstract

Determining what the controls are on the large-scale (> 10 km) horizontal architecture of coarse-grained fluvial deposits is important for understanding subsurface fluid distribution and extraction from these important aquifers and hydrocarbon reservoirs. This contribution describes the architecture of coarse-grained fluvial sediments from an almost continuous lateral profile 27 km long by 115 m high in the Upper Triassic Wolfville Formation, (Fundy Basin, Nova Scotia). The lateral extent and quality of exposure allows development of a high-resolution correlation scheme and definition of architectural elements.

The lower Wolfville Formation lithofacies, composed of breccias, conglomerates, sandstones, mudstones, and paleosols, comprises alluvial-fan, fluvial, and ephemeral lake-margin facies associations. Alluvial-fan deposits form 15% of the studied section and are developed locally between the overlying fluvial facies and the unconformity with Carboniferous strata, where they infill remnant topography. The succession is dominated (80%) by pebbly sandstones deposited as part of a gravelly braided fluvial system and ends abruptly with a thin ephemeral lake-margin facies association (5%). The large-scale architecture and sedimentology of the pebbly sandstone succession is documented with particular attention focussed on bounding surfaces and stacking patterns.

In the fluvial facies association two scales of development of erosion surface and cycles can be recognized. The “S” surfaces can be correlated across the profile (> 27 km) and show evidence of up to 10 m of erosion, a significant facies or grain-size change, onlap, and association with mature calcic paleosols. The “E” surfaces can be correlated between 5 and 17 km, display erosion of < 3 m, no significant grain size or facies change, and may be associated with incipient paleosols. Five “S” surfaces are recognized and allow subdivision of the fluvial facies association into four mega-units. In these mega-units up to thirteen smaller scale high-resolution fining-upwards cycles (pebble conglomerate to sandstone) bounded by “E” (or “S”) surfaces can be recognized.

The regional extent of the “E” and “S” surfaces indicate that the gravelly fluvial system has a large-scale, sheet-like geometry which comprises a series of stacked, erosion-dominated, multi-story bodies. High-resolution architectural elements are poorly defined in this coarse fluvial system and make it difficult to constrain the scale of the depositional system. An individual high-resolution cycle records braid-plain development and is interpreted as a multi-story channel belt. The braid-plain width is estimated at 19 km but the width of individual active channels is unknown.

The sediments were deposited in a proximal fluvial setting disconnected from marine influences. We interpret the high-resolution cycles and bounding “E” surfaces to record a progressive decrease in runoff and fluvial transport capacity indicative of a drying-upward, climatically driven signal. The origin of the S-surface-bounded packages is more difficult to determine. A variety of mechanisms are considered, including changes in discharge driven by climatic fluctuations, changes in stream power related to drainage capture within the source area, and tectonic tilting.

An important aspect of the study is that the “S” surfaces can be identified and correlated for > 27 km, and provide an example of a robust, high-resolution correlation framework in coarse-grained braided fluvial deposits that could be applied elsewhere. In contrast the “E” surfaces do not display any significant changes in facies or grain size across them, such that many of the smaller scale cycles could potentially be miscorrelated.

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