FACIES ANALYSIS AND HYDRAULIC RECONSTRUCTION OF DEPOSITION IN POINT BARS OF ROWLETT CREEK, PLANO, TEXAS

The scope of this research was focused on facies analysis of point bar sediments based on grain size distributions in Rowlett Creek, Plano. Velocities of currents that deposited the sediments were estimated from the grain size populations and a hydraulic reconstruction of the depositional environment was possible from our quantitative analysis.

The facies model developed in this project includes four facies: (1) upstream gravel facies (Facies A); (2) gravel facies along the channel-ward edge of the point bar (Facies Aa); (3) coarse grained sand in the central part (Facies B); and (3) coarse to medium grained sand at the downstream end of the point bar (Facies C). At the extreme downstream end of the point bar, the sediments become fine grained to shale size. The identification of these facies was based on sieve analysis and MDSK statistics using the Folk nomenclature, including analysis of histograms, frequency curves and cumulative curves. 

Middleton (1976)’s criterion was used to estimate the current velocities that deposited the sediments: the settling velocity of the largest grain size in suspension equals with the shear velocity of the current that deposits the sediments. Cumulative probability curves were used to determine the largest grain size that may be in suspension in our samples. This grain size was found 2 mm in Facies A and 0.5 mm in Facies C; they were plotted on Blatt (1980) and Cheel (2005)’s grain size-shear velocity diagram to find the shear velocities. Our result was that the creek’s current had about 0.16 m/s shear velocity when Facies A was deposited and it progressively slowed down to approximately 0.05 m/s in Facies C. The coarse grained sediments at the edge of the point bar (Facies Aa) were most likely deposited by chute currents of about 0.16 m/s velocity.