Holocene sediments of Mission Bay, Texas

Mission Bay is situated in the Texas Coastal plain near Bayside, Texas. It is one of the smaller bays on the Texas coast, only 14.4 square kilometers. It is situated in the dry subhumid climatic zone where the wind blows predominately from the southeast and the precipitation averages 66.80 centimeters per year. The topography of the Mission Bay area is the result of stream entrenchment by the ancestral Mission River during the Wisconsin Glaciation and subsequent recent erosion and deposition of sediments. Based upon physiographic and geologic characteristics, the Mission Bay area can be divided into four regions. The baymouth region is situated in the southeast quadrant of the bay. The sediments in this region range in age from Pleistocene to Holocene. The sediments are for the most part unconsolidated, of varying colors, and are characterized by the large amounts of shell material found on the beaches and in the inlet channels of this region. The Mission River complex is situated in the northern area of the bay. The sediment of this region is all of Holocene age and consists primarily of sand-sized constituents. The sediment contains large amounts of terrigenous material carried downstream by the river, but also includes large pieces of wood material transported by the river during flood stages. The Mission River complex is comprised of two parts, the Mission River itself and the distributary mouth bar. These two parts, though separate, contain similar river transported sediments. The Pleistocene age cliffs surround the bay proper on the western and northeastern sides. The cliffs range in height from 1.0-6.5 meters. The cliff sediments are part of the Beaumont Formation. These sediments were deposited under conditions not unlike those active along the Texas Coast today. Consequently, the sediments are composed principally of fine sand, silt, and clay-sized constituents. The bay proper region consists of all the area of Mission Bay, excluding the previously mentioned areas, that are covered by water at mean high water. The sediments contained in this region are almost all of Holocene age, except for portions of the Pleistocene Beaumont Formation that outcrop along the bay boundary. The sediments are mostly unconsolidated, light to medium gray in color, and are composed of large amounts of silt-and-clay-sized material. The normalized Q-mode factor analysis based upon the individual weight percent size fractions of the samples, differentiated between the various sources contributing material to the bay proper sediments. Those samples containing appreciable amounts of sediment in the coarse silt and larger grain sizes could have their sediment traced to a possible source. But the samples with abundant fine-grained sediment could not be connected to a possible source.