AN ANALYSIS OF GRAIN POPULATION TRENDS IN INDIVIDUAL LAMINAE AND RELATIONS TO PHYSICAL PARAMETERS IN A MACROTIDAL FLUVIAL-ESTUARINE ENVIRONMENT

Grain size populations constitute direct data resulting from physical depositional processes. While a large body of research exists exploring many facets of the relations between grain size data and specific depositional processes, relatively few studies have explored grain-size-process dynamics from individual beds or laminae in the field. In this study, deposition of cyclic rhythmites from macrotidal Turnagain Arm, Alaska was monitored closely over three months on tiles placed on gravel-based fluvial-tidal bars. This permitted correlation of individual laminae to tide station data and estimated stream discharge. Accreted foresets were cored and each lamina measured for thickness and sampled for grain-sizing with a small razor blade.

A complication of fluvial-tidal environments is identification of sediment source(s). In the sampled sediments median grain diameter is significantly correlated to lamina thickness. This can be explained by: a) high tidal current velocities persist longer to yield thicker, coarser laminae, and/or b) higher maximum current velocities result in larger maximum and, thus, median grain size in thicker laminae. The data reveal poor correlations between maximum grain size and independent variables (IVs) tidal range, estimated tidal current velocity and stream discharge. This implies a large-clast supply limitation and scenario 'a'; thus, streams/channel bottoms appear not to be a significant sediment source for the tidal bars. Interestingly, no significant correlations existed between any IV and 10th percentile grain size, while lamina thickness and discharge appeared to have a slight relation (r=0.34). Median, mean, and 90th percentile diameters all demonstrate fairly strong correlations to tidal range (and mean velocity) as expected, but all r-values < 0.67, indicating a moderate amount of unexplained variation.

Principal components analysis reveals that dominant-phase daily laminae are clustered along biplot vectors, while thinner laminae from thick-thin, semidiurnal couplets are more dispersed. Dominant laminae more readily reveal size-process relations which can be obscured, especially, during weaker tidal cycles, as sites respond to broad tidal fluctuations and accretion and erosion change the geometry of fluvial-tidal bars and channels.