Depositional and Dispersal Dynamics of Intertidal Sand Bars

Abstract

ABSTRACT The intertidal sand bars of the Minas Basin, Nova Scotia, are asymmetrical in cross-section and linear in plan. They are formed and reworked by tidal currents which are characterized by an asymmetric time-velocity profile. A causal connection between time-velocity asymmetry of tidal currents and bar topography is demonstrated. Average bottom ebb current velocities (90 cm/sec) were found to exceed average bottom flood current velocities (65 cm/sec) over gently-sloping (2 to 3 degree) bar surfaces, whereas over steep-sloping (8 degree) bar surfaces, average bottom flood current velocities (90 cm/sec) were found to exceed average bottom ebb current velocities (65 cm/sec). Both bar topography and zones of flood- or ebb-dominated bottom tidal currents control the distribution of sediment texture and sedimentary structures. On steeper bar slopes, the sediment is fine-grained sand and is characterized by airholes, planar lamination, and surface dunes and ripples. On the gentler bar slopes, the texture of the sediment is medium-grained and coarse-grained sand which has been fashioned into current ripples, simple dunes, complex dunes, simple sand waves and complex sand waves. The internal organization of some dunes and sand waves is extremely complex, inasmuch as thickness of internal sets of cross-stratification is considerably less than dune and sand wave height. A total of 14 sedimentary bar facies are defined from combinations of texture, internal cross-stratification and surface bedforms. Both topography and zones of ebb- and flood-dominated bottom tidal currents control the orientation of directional current structures. Over steep faces of bars, the slip faces of sand waves are oriented in the same direction as flow directions of bottom flood tidal currents, although they do show evidence of reworking by ebb currents. Over gently sloping bar surfaces, slip faces of both dunes and sandwaves are oriented parallel to the direction of flow of bottom ebb current systems. Because dunes and sand waves migrate only during a single phase (ebb or flood) of a tidal cycle, their orientation is generally unimodal. Current ripples are formed in depths of water less than 0.6 meters, during late-stage sheet-like runoff that is controlled by local slope changes on the bar. Indi idual tracer grains are dispersed in a radial-elliptical pattern form a point source. The size of directional properties is hierarchically sensitive to dispersal history. Tracer grains (modeling mineral indicator grains) are dispersed in a radial-elliptical pattern from a point source. Trimodal, highly variant, current ripple slip face orientation develops during late stage sheet runoff at low water depths; this flow is controlled by local bar slope. Unimodal, low variant dune slip faces, sand wave slip faces and associated maximum dip direction of cross-stratification are aligned parallel to tidal current flow and basinal topographic trend. The sand bars are aligned parallel to basinal topographic trend and in turn segregate zones of flood- and ebb-dominated bottom current systems. The tidal sand bars of the Minas Basin are equilibrium forms. Sand is dispersed alternately around the bar through zones of ebb- and flood-dominated bottom tidal currents. As a consequence, the sedimentary facies distribution on the bars has remained essentially unchanged since 1938. Comparison of low-tide airphotos taken in 1938, 1947 and 1963 in one of the study areas shows a slight erosional regime. This erosional regime controls the extreme reworking of sediments, and the formation of an erosional sand wave bedform by wave processes. Physical criteria characteristic of tide-dominated sand bodies include: sharp erosional contacts between sets of cross-stratification; rounded upper set boundaries of cross-stratification; unimodal and bimodal distributions of orientation of cross-stratification; bimodal distributions of set thickness and dip angles of cross-stratification; orientation of dunes, sand waves and cross-stratification in the dominant direction of flood or ebb tidal flow, basinal topographic trend and sand body axis; trimodal orientation of current ripples; oblique or 90° superposition of smaller current ripples on larger current ripples; double-crested current ripples; superposition of current ripples at 90° or 180° on slip faces and crests of dunes and sand waves; complex organization of inte nal cross-stratification in sand waves; etch marks on slip faces of dunes and sand waves; and alignment of the long axis of sand bodies parallel to tidal current flow, basinal topographic trend and basin axis.