By Sid Perkins, Science News
An unexpectedly large canyon carved by a Texas flood may help scientists estimate the size of ancient megafloods believed to have sculpted terrain on both Earth and Mars, new research suggests.
The new canyon has many characteristics similar to those presumed to have been sculpted by larger-scale floods in other terrains, including streamlined features like ones that appear in images taken by Mars-orbiting probes, says Caltech geologist Michael P. Lamb.
In July 2002, record flooding in central Texas caused Canyon Lake, a reservoir held behind a dam about 55 kilometers northeast of San Antonio, to overflow. Water rushing through the dam’s emergency spillway carved a 2.2-kilometer-long canyon downstream of the spillway in just three days, Lamb and colleague Mark Fonstad, a hydrologist at Texas State University in San Marcos, report online June 20 in Nature Geoscience.
“Geology is typically about events that happened long ago and very slowly,” comments Sanjeev Gupta, a field geologist at Imperial College London . “This is the one of the first studies to study the effects of a single canyon-cutting event.”
When the flood began, water flowing through the 365-meter-wide spillway quickly stripped the creek bed of its trees and soil and began chewing into the limestone bedrock below, says Lamb. Over the course of three days, the torrent carved a canyon that ranged between 40 and 60 meters wide and averaged 7 meters deep over much of its length. Overall, the flood excavated about 460,000 cubic meters of material, with almost half of that being rock.
“To be able to document the effects of this event so soon after it occurred, it’s an important contribution,” says Jim E. O’Connor, a geologist with the U.S. Geological Survey in Portland, Ore.
Many researchers view canyon erosion as a process that unfolds over centuries if not millennia. That’s generally true, says Lamb, but he and Fonstad suggest that in this case another factor accelerated the canyon-cutting process — layered limestone bedrock under the creek bed was riven with weaknesses. Individual slabs of rock, which ranged from 20 centimeters to 1.2 meters thick, weren’t strongly bonded to one another, Lamb says. What’s more, many layers had been previously cracked by long-term tectonic movements in the area.
Water flow during the flood was so intense that it plucked large, flat-sided blocks of limestone — some of them 1 meter across — intact from the bedrock. During the peak of the three-day flood, water flowed through the canyon at a rate that would have filled between 12 and 20 Olympic-sized pools each minute.
In the lower half of the canyon, the raging deluge sculpted streamlined islands of material resembling those found in areas stricken by much larger floods, including one that shaped what is now the floor of the English Channel at a time when sea levels there were much lower than they are today (SN: 7/21/07, p. 35), and the cataclysmic torrents that sculpted the Channeled Scablands of Washington state. Similar features, presumably carved by immense and ancient floods, are seen in many images of the Martian terrain taken from orbit.
The Texas flood “is a potential analogue for what we see on Mars,” says Alan D. Howard, a geomorphologist at the University of Virginia in Charlottesville who has studied the geology of Mars. The majority of bedrock on the Red Planet, just like the Channeled Scablands, is basalt deposited by lava flows, each layer of which would have cracked into large blocks as it cooled, he notes. In a large flood, these blocks could have been plucked from the Martian terrain and tossed about like the ones in the Texas flood, excavating a lot of material quickly.
“It doesn’t take millions of years to create an impressive channel,” Howard notes. “Flowing liquid can do a lot of work in a short period of time.”