More to come…
Unlike Minnesota, which sits atop some much older rock, including some 3.5 billion years old, Houston sits atop some Pleistocene layers and lots of sand, the sand being eroded Rocky Mountains. Millennia ago, the Rockies eroded and rivers carried the sand down, opening into alluvial planes that are now our home.
We discuss local geology on some episodes of the CCERP Podcast.
“Houston is a flat, marshy area where an extensive drainage system has been built. The adjoining prairie land drains into the city, which is prone to flooding. Underpinning Houston’s land surface are unconsolidated clays, clay shales, and poorly cemented sands up to several miles deep. The region’s geology developed from river deposits formed from the erosion of the Rocky Mountains. These sediments consist of a series of sands and clays deposited on decaying organic marine matter, that over time, transformed into oil and natural gas. Beneath the layers of sediment is a water-deposited layer of halite, a rock salt. The porous layers were compressed over time and forced upward. As it pushed upward, the salt dragged surrounding sediments into salt dome formations, often trapping oil and gas that seeped from the surrounding porous sands. The thick, rich, sometimes black, surface soil is suitable for rice farming in suburban outskirts where the city continues to grow.”
“The Houston area has over 150 active faults (estimated to be 300 active faults) with an aggregate length of up to 310 miles (500 km), including the Long Point–Eureka Heights fault system which runs through the center of the city. No significant historically recorded earthquakes have occurred in Houston, but researchers do not discount the possibility of such quakes having occurred in the deeper past, nor occurring in the future. Land in some areas southeast of Houston is sinking because water has been pumped out of the ground for many years. It may be associated with slip along the faults; however, the slippage is slow and not considered an earthquake, where stationary faults must slip suddenly enough to create seismic waves. These faults also tend to move at a smooth rate in what is termed “fault creep”, which further reduces the risk of an earthquake.”
In the abstract of “SURFICIAL GEOLOGY OF THE HOUSTON AREA: AN OFFLAPPING SERIES OF PLEISTOCENE (& PLIOCENE?) HIGHEST-SEALEVEL FLUVIODELTAIC SEQUENCES” (pp. 651-666 of Transactions Gulf Coast Association of Geological Societies, Volume XLI, 1991), DeWitt C. Van Siclen writes
“Most near-surface sediments around Houston were deposited by the Brazos and Trinity rivers during Pleistocene interglacial (high sealevel) stages as a series of offlapping, seaward-thickening, fluviodeltaic sequences. During glacial stages, the landward portion of these sequences was exposed to weathering and erosion before the next high-sealevel strata were deposited. Slow southeast regional tilting elevated the exposed sequences enough that some were never covered entirely by later deposits. These updip margins, termed coastal terraces, now form most of the land surface in the Houston area.
“Each coastal terrace south of upper Spring Creek has its own pattern of former natural levees, termed meander-belt ridges (MBRs), which degrade increasingly landward. The MBR pattern on younger terraces truncates those on contiguous older ones and so initially blocked their seaward-flowing drainage. The drainage ponded and broke through the truncated MBs, forming larger streams that closely follow the sequence boundaries, herein termed gathering streams. Practically all the rest of the present-day drainage follows former back-swamps and flood-basins, leaving the MBs as divides, even on the youngest terrace. This inversion of the drainage and development of gathering streams makes it possible to reconstruct the MBR patterns from the drainage, even where ridge preservation is poor.
“The MBR patterns in a 5,700 mi^2 area were mapped from USGS topographic sheets, to produce an improved areal geologic map based on these genetic relationships, i.e., the final sediment transport routes and depositional sequence boundaries. These contrast with traditional mapping criteria which, in the usual absence of outcrops, depend largely on modification by later processes, such as regional tilting and intensity of soil development.
“The new map shows that, since deposition of the Pliocene Goliad Formation, eight coastal terraces separated by gathering streams have built eastward from the present Brazos Valley toward the relatively sediment-starved San Jacinto Valley and its Galveston Bay estuary. Farther east, the Trinity River contributed equivalent terraces directly and by occasionally occupying the San Jacinto Valley. It now appears that stratigraphic sequences representing all the Pleistocene highest sealevel stages of Beard, Sangree, and Smith (1982) may be present at the surface in this area, as well as two “extra” ones in the Plio-Pleistocene transition.”