An Offlapping Series of Pleistocene (and Pliocene?) Highest Sea Level Fluvial/Deltaic Sequences
VAN SICLEN, DeWITT C., Consultant, Bellaire, TX
The near-surface strata around Houston, Texas, were deposited by the local rivers during the highest of high sea level, Pleistocene interglacial stages as an offlapping series of coalescing, seaward-thickening, fluvial/deltaic sequences. During glacial stages a
landward part of each of these sequences was exposed to weathering and erosion before the next highest sea level strata were deposited. Meanwhile, very slow southeast regional tilting elevated the inland margin of the exposed sequence enough that no later highest sea level strata ever covered it entirely. These still-exposed updip margin, termed coastal terraces, now form most of the land surface in the Houston area.
The coastal terraces south of Buffalo Bayou, which flows eastward across the center of Houston to become the (enlarged) Houston Ship Channel, clearly show their depositional origin. Nearest the coast abandoned channels of the latest Pleistocene Brazos River are still quite evident on aerial photographs and topographic maps, but north of Clear Creek (roughly halfway back to Buffalo Bayou) the older channels are rarely recognizable. In both these areas, however, the sandy meander belts show up well on the old USDA Soil Conservation Service maps. These degraded natural levee systems, 2-10 ft high, are referred to as meander-belt ridges (MBRs) and have become the local drainage divides, with all (except part of one) of the modern streams flow through the former backswamps between them. Th s rapid inversion of the drainage enables the MBRs on the more eroded coastal terraces father inland to be reconstructed from the drainage patterns shown on USGS topographic sheets, particularly those of Harris and Galveston Counties surveyed between 1914 and 1926 at the unusual contour interval of 1 ft.
Of primary interest is the area between the Brazos and San Jacinto rivers and the latter's estuary, Galveston Bay, inland for 100 mi. This takes in most of the Pleistocene Brazos deltaic plain, which repeatedly built out toward the east due to that component of regional tilting. These Brazos coastal terraces demonstrate that the most northerly MBR(s) of each new highest sea level sequence blocked the generally southeast-flowing drainage that had formed during the intervening low sea level stage on the next older terrace to the north. This blocked runoff, accumulating along the base of the new MBR, developed into a distinctive type of stream that now heads just outside the Brazos Valley and continues (or once continued) in a net almost due east into the San Jacinto River or Galveston B y. Each of these gathering streams thus truncates the MBR pattern of the preceding high sea level sequence; and from Buffalo Bayou southward it also marks the approximate surface trace of the unconformity beneath the overlapping strata, thereby providing the most objective, mappable boundaries between these adjacent terraces.
The youngest gathering stream in this area is Clear Creek, and Buffalo Bayou is the next older fully developed one. Brays and Sims bayous between these qualify as lesser gathering streams, which divide this area into three separate coastal terrace segments. The runoff contributed by these lesser gathering streams and by the much broader coastal terrace between Buffalo Bayou and the next gathering stream to the north, Cypress Creek, is what made Buffalo Bayou large enough for the early ships to dock at Houston.
AAPG Search and Discovery Article #91006 © 1991 GCAGS and GC-SEPM Meeting, Houston, Texas, October 16-18, 1991 (2009)