GEOLOGY OF TERRACES ALONG SPRING CREEK IN RICHARDSON, TEXAS

Spring Creek, a tributary of Rowlett Creek, flows through the Spring Creek Nature Area and Foxboro Park in Richardson, Texas. Spring Creek is bordered by limestone cliffs of the Austin Formation up to 25 feet high and by cliffs of an unconsolidated alluvium below a terrace surface 15 feet above the creek. The exposed alluvium underlying the terrace surface has two distinct facies. The basal layer consists of mottled gray to yellowish-brown limestone-pebble gravel and clayey sand, 0-6 feet thick. Burrows interpreted as crayfish dwelling burrows permeate the basal alluvium. The burrows are typically 2-3 inches wide and filled with brown sandy clay from the overlying alluvium. No body fossils have been identified in the basal alluvium. The upper alluvium consists of 6-14 feet of dark brown to grayish brown clayey sand to sandy clay with some thin layers of gravel. “Fossils” recovered from the upper alluvium consist of shells from several species of bivalves and fresh water and terrestrial gastropods, plus a few pieces of deer bone and turtle scuts. Burrows could not be differentiated in this part of the alluvium.

A radiometric age of 3768 ybp was measured from C14 dating of bivalves from the base of the terrace forming alluvium. Using an average depositional rate of 8.8 feet/1000 years, based on Trinity River alluvium of the same age (Ferring,1990), it is estimated that sediments at the terrace surface were deposited approximately 2228 ybp. Since that time, Spring Creek has eroded through the pre-terrace surface and 4-5 feet into the underlying Austin Formation.

Inspection of the Austin cliffs bordering Spring Creek suggests there may be other older, pre-terrace surfaces at approximately 14 and 22 feet above the creek, each with some alluvium preserved on the surface. Comparison with alluvial sequences described along the Upper Trinity River Basin, suggests they formed during the middle and early Holocene.

Stream piracy may have played a part in the formation of Spring Creek. If so, the process appears to predate formation of the Late Holocene terrace.