Alluvial, lacustrine, and paludal (marsh) sediment is at the surface in about one-third of the Great Sand Dunes area. Alluvium is the dominant non-eolian surficial material present (fig. 1). Thin beds of alluvium consisting chiefly of calcareous silty fine sand and sandy silt cover broad areas. Commonly, a veneer (0.5-1.5 m; 1.6-5 ft) of Holocene alluvium overlies Pleistocene alluvium. The alluvium probably is primarily "river-end" sediment that was delivered to the lower piedmont slope and basin floor by sheetwash and ephemeral streams originating in the mountains or higher on the piedmont slope. In addition, stream-terrace deposits are relatively extensive in the northern part of the area where runoff from drainage basins in the Sangre de Cristo Mountains was sufficient to cut channels all the way across eolian sand to San Luis Creek. Near the mountain front, terrace alluvium is mostly gravel, and it typically includes coarse, commonly bouldery, sediment that was deposited by debris flows. Sand becomes progressively more abundant westward from the mountains until it makes up almost the entire terrace deposit.
|
Stream-terrace deposits are particularly prominent at three different levels in valleys in the northern part of the Great Dunes area (fig. 3). The high terrace is typically 10-12 m (33-39 ft) higher than stream channels and commonly is capped by 2-5 m (7-16 ft) of eolian sand. Degree of weathering and height above stream level suggest that the high terrace alluvium is middle Pleistocene. Deposits of the high terrace appear to extend beneath the Great Sand Dunes south of Sand Creek (fig. 4). The intermediate terrace is the most extensive of the three surfaces (fig. 3). It is typically 6 m lower than the high terrace, not including the eolian-sand cap, and 2-5 m (7-16 ft) higher than nearby stream channels. This terrace deposit probably is equivalent in age to the last glaciation (Pinedale glaciation), which occurred about 35,000-12,000 years ago. The low terrace is only about 1 m (3.3 ft) higher than stream channels and generally is only a few tens of meters wide. It formed during the Holocene.
Figure 2. Locations of the figures shown on this page.
 |
Figure 3. View northeast of the high, intermediate, and low terraces formed by an unnamed creek that is about midway between Deadman and Cottonwood Creeks (see fig. 2 for photo location). Eolian sand, 2-6 m (7-20 ft) thick, overlies alluvium of the high terrace. Here, as elsewhere along valleys in the northern part of the study area, the intermediate terrace is the broadest and best defined terrace. |
 |
Figure 4. View southwest of an 11-m-thick (36 ft) section of thinly bedded alluvium underlying the high terrace on the south side of Sand Creek near the edge of the Great Dunes. The alluvium consists chiefly of sand, but also contains beds of both clast- and matrix-supported gravel and clayey and silty sand. Stratigraphy similar to that shown here also is exposed elsewhere along Sand Creek, Cold Creek, and Deadman Creek (see fig. 2 for locations). |
The alluvial stratigraphy along Big Spring Creek and Little Spring Creek is different than that of streams draining from the mountains. As their names imply, these creeks issue from springs that emerged west of the Great Dunes (fig. 2). Their stratigraphic records do not extend as far back in time as the terraces along streams draining from the mountains, and they record fluctuations in water-table level rather than long-term fluctuations in the magnitude of annual peak flows. The valley floor of Big Spring Creek has been at nearly the same level for most of the past 10,000 years (fig. 5). Terrace deposits ranging in age from earliest to latest Holocene are all within 1.5 m (5 ft) of present stream level. These deposits are products of aggradation that was induced when water-table level rose. Thus far, two periods of higher water table have been identified, although their temporal limits are not yet closely dated. At least part of the interval between about 3,300 and 1,700 years ago was a time of rising water table (fig. 6). During this time, Big Spring Creek aggraded and raised its valley floor to about the same level it was 9,200 calendar years ago (see fig. 9 of Eolian Sand Stratigraphy).
 |
Figure 5. View downstream (southwest) of Big Spring Creek. The valley floor has been at nearly the same level for most of the past 10,000 years. Stream-terrace deposits ranging in age from earliest to latest Holocene are all within 1.5 m (5 ft) of present stream level. The terrace deposits are products of aggradation that was induced when water-table level rose; in other words, they record intervals of wetter climate. |
 |
Figure 6. Stratified alluvium deposited by Big Spring Creek (see fig. 2 for photo location) underlies a terrace that is about 1 m (3.3 ft) higher than the creek. Interbedded sand and organic-rich mud attest to aggradation during a time of rising water table. The calendar-year equivalent of the radiocarbon age shown here is, at 2 sigma (95 percent probability), between 2,980 and 2,760 years before present. |
Most streams entering the closed basin from the San Juan and the Sangre de Cristo Mountains generally disappear within 0.5-3 km (0.3-1.9 mi) from the mountain front, primarily because stream flow is lost to infiltration. However, on the eastern side of the basin, the piedmont slope and the slope of the water table, which is flatter than the piedmont slope, converge toward the basin center. Thus, at some point, water table intersects the piedmont surface and discharges in springs, ponds, lakes, spring-fed streams, and valley-floor wetlands, sometimes referred to as cienegas. Remnants of abandoned channels, once occupied by spring-fed streams, trend into the closed basin from east and west, and a myriad of sinuous relict stream channels trend down the axis of the basin. These channels attest to times in the past that were wetter than the present.
Lake and marsh deposits are widely distributed on the basin floor. During times of more effective moisture, water table rose and lakes and marshes formed in topographic depressions. Hayden (1873) described the floor of the closed basin as "one vast swamp or bog, with a few small lakes". Many ponds and marshes occupy topographic depressions eroded into older lake sediment by wind during times of drought and low water table. The depth and extent to which wind can excavate basins is controlled by water table because wet or moist sand resists deflation. The planar surface of some alluvial flats may mark the level of past or present water table.
Stratigraphic data from wells and boreholes indicate that lakes have occupied the closed basin several times during the Pleistocene and Holocene. The basins of several small present-day lakes were eroded, primarily by wind, into the floors of older, larger Holocene lakes within the past 1000 years. During middle Holocene time, two lakes occupied the floor of the closed basin (fig. 7). The northern lake spilled into a sinuous 5-km-long (3-mi-long) channel that emptied into the southern lake (figs. 7 and 8). Pleistocene lakes were larger than Holocene lakes. The full extent of pre-Holocene lakes has not been determined because former shorelines are buried by eolian sand and alluvium, but older lake beds are exposed in excavations at a few localities (fig. 9). The clay loam texture of the Hooper soil series, which is widespread on the basin floor, is attributed to the lacustrine origin of the parent material rather than to soil-forming processes. Thus, the distribution of the Hooper soils helps define the extent of a Pleistocene lake bottom.
 |
Figure 7. Landsat view of Holocene paleolakes. During middle Holocene time, two lakes occupied the floor of the closed basin. The northern lake spilled into a 5-km-long (3-mi-) channel that emptied into the southern lake. The 5-km-long channel lengthened to 9 km (5.6 mi) as the southern lake shrank. Former shorelines are concealed chiefly by wind-deposited sand. (Click image for a larger view) |
 |
Figure 8. Vertical aerial photograph of the south end of the northern paleolake shown in figure 7 and the relict channel (lower right) leading from the northern paleolake. |
 |
Figure 9. View, looking toward the Great Sand Dunes, of clayey Pleistocene lake sediment exposed in a ditch east of the relict channel shown in figures 7 and 8. Large fragments of iron- and manganese-oxide cemented sediment, some as much as 30 cm (12 in.) across and 10-15 cm (4-6 in.) thick, were excavated from the ditch. The soil formed in this material (not shown in this view) is well developed and includes a relatively thick reddish-brown Bt horizon and a thick white to pinkish white Bk horizon. Click for a larger image. |
Project Home Page | Eolian Stratigraphy | Origin and Conclusions
Hayden, F.V., 1873, Geological report for 1869, Embracing Colorado and New Mexico: U.S. Geological Survey of the Territories, Third Annual Report, 261 p.
U.S. Department of the Interior | U.S. Geological Survey
URL: http://esp.cr.usgs.gov/info/dunes/noneolian.html
Page Contact Information: ESP Web Team
Page Last Modified: Wed 15-Mar-2006 14:38:42 MST
