The Geologically youthful volcanism and extensional tectonics of the Rio Grande Rift that have configured the present topography and drainage patterns of the region are merely the latest phase of a long geological history with direct bearing on the origins of White Sands. The purpose of the summary geological discussion with follows is to review the bedrock geology underlying the park and surrounding regions, with emphasis on those events that have most influenced the origins of the White Sands.

Tectonic setting: The tectonic map (Figure 1-2) of the Tularosa Basin and vicinity illustrates the major uplifts, basins and faults in the region surrounding White Sands. The Tularosa Basin is bounded on it's eastern side by the Mount Blanca and the Carrizozo Volcanic Field, and thence southwards by the Sacramento uplift and the Otero Platform. The Alamogordo Fault Zone, a major basin-bounding fault system, extends along the volcanic field southward to the Sacramento uplift where displacement diminishes. Major faulting has not been identified between the Otero Platform and the eastern portion of the Tularosa Basin to date (Figure 1-3).

A major north-south trending fault separates the deep western portion Tularosa Basin from a shallower platform on the eastern side. It extends from just west of the Jarilla mountains northwards along the axis of the Tularosa Basin, where it may link with a major fault bounding the west side of the Oscura Mountains (Figure 1-4), (Seager, 1984). The shallow, eastern part of the Tularosa Basin has so little subsidence that Permian rocks outcrop near the monument headquarters

The far western side of the Tularosa basin is bounded by a series of normal faults along the eastern side of the San Andres Mountains that down-throw Paleozoic rocks over 15,000 feet on the basin side and bring Precambrian rocks to the surface along the mountains. This fault system, as well as synthetic and antithetic faults within the basin proper, has been active during quaternary times, creating scarps on both the alluvial fans along the mountains, and in the valleys that are plainly visible (Seager, 1984; Blair, et. al., 19__, Buck 1996).

Another significant feature visible on the tectonic map (Figure 1-2) is the Burro Uplift, a Laramide (Upper Cretaceous -Lower Tertiary) feature that has a long history of uplift parallel to the Texas Lineament. Another significant structural trend in the region is the northeast to southwest trending buckles and arches visible east of the Sacramento uplift (Figure 1-2). These trends may have expression in the subsurface of the Tularosa basin and in certain bulges and re-entrants of the ranges bounding it. It may have links to the slightly higher portion of the Tularosa valley floor that forms the southern limits of the dunefield and the lakes Lucero and Otero.

The Lower Paleozoic rocks consist mainly of carbonates, with a trend toward more clastics and evaporites upward in the stratigraphic column, culminating in the extensive evaporites of the Yeso Formation deposited during the Permian (Figure 1-5). The Mesozoic rocks consist of relatively thin intervals of formations such as the Chinle, Mancos and Dakota that are widespread in the Rocky Mountains, but are poorly preserved in the Tularosa Basin due to erosion or non-deposition during Laramide or later times. The Tertiary section consists largely of thousands of feet of volcanics and valley-fill deposits eroded from block-faulted uplifts of the Rio Grande Rift. The White Sands, as well as the Lake Otero and Lake Lucero deposits comprise only the latest (Pleistocene-Quaternary) chapter in this long history, the gypsum of the White Sands being re-cycled into these lakes from much older Pennsylvanian and Permian rocks surrounding the basin.

The configuration of rock units that facilitated formation of the White Sands and other eolian deposits of the basin can be seen on the geologic map shown on Figure 1-4. The Pennsylvanian and Permian rocks that contain large quantities of gypsum (shown in blue on Figure 1-4) outcrop around and within the basin on the west, north and east of the White Sands National Monument. Also shown on the map is the Carrizozo volcanic field and Mount Blanca on the northeast side of the Tularosa Basin, as well as the Organ Mountains and Jarilla Mountains on the south side of the basin - volcanic deposits that are all Tertiary in age. The very young basalt flows of the Malpais (and other basaltic flows) are shown in yellow on the map and are also easily seen on the satellite image of the basin (Figure 1-7).

The San Andres and Sacramento mountains are sites of famous bioherms (reef like carbonate buildups) that constitute some of the best outcrop examples known. They have been studied widely by geologists as analogs for subsurface petroleum reservoirs worldwide because they are such good examples of their types, and because outcrop exposures are so excellent. In the hills just above Alamogordo and southwards, Mississippian Waulsortian mounds consisting of bryozoan and crinoid fragments are exposed virtually in 3-Dimensional views in the Lake Valley Formation. The best example is the Muleshoe mound south of Alamogordo (Figure 1-6A). North of Alamogordo in Dry Canyon on the north side of U.S. highway 82 are Pennsylvanian Phylloid algal reefs that are classic examples of the type (Colpitts, et. al., 1991). In Hembrillo Canyon in the San Andres Mountains Soreghan and Giles (1999) have described bioherms in the Panther Seep (uppermost Pennsylvanian) Formation. Cyclic sedimentation and additional bioherms have also been described in the underlying Lead Camp (San Andres Mts.) and Gobbler (Sacramento Mts.) formations by Algeo, et. al., (1991).

As shown in summary form on the geologic map, the gypsum dunes of White Sands are not the only aeolian deposits in the region. There are widespread quartzose eolianites both north and south of the White Sands, and on the western side of the San Andres Mountains, as shown on Figure 1-4. Additionally, there are some inactive gypsum dunes, commonly in the form of lake-bordering lunettes, north and south of highway 82/70 between the Park Service Visitor Center and the Missile Range Headquarters.

Some of these quartzose dunefields are visible on the satellite image of the Tularosa Basin as shown in Figure 1-7. They appear as bright yellow areas on the false-color image. The black mass of the Malpais basalt flow is clearly visible, along with reddish hues (in false color of the image) of strings of vegetation along drainages that enter the basin from surrounding mountains. Note the large number of drainages entering the basin from the northeast side.

Historical Geology: It is important to summarize the geological history prior to any discussion of the Quaternary geology of the Monument because this history has a direct bearing on the stratigraphy of the dunefield and lakes Otero and Lucero. The uniqueness of White Sands as the largest gypsum dunefield in the world is directly related to the peculiarities of stratigraphic and structural evolution that preceded it_x2019;s very recent formation by wind. Some of these relationships, while subtle in the context of the present, become clear when ancient basins and uplifts are considered.

Lower Paleozoic rocks: Summary maps showing the distribution of lower Paleozoic rocks are shown in Figure 1-8, which depicts isopachs of the Cambrian, Ordovician, Silurian, Devonian and Mississippian systems (Kottlowski, 1963, Kottlowski, et. al 1956). In general, these isopach maps record the northward thinning of all the Lower Paleozoic units (Figure 1-9 and Figure 1-10). The erosional/depositional edge of the Cambrian Bliss Sandstone is just north of Alamogordo. In the Tularosa Basin, the Bliss Sandstone, which rests on the Precambrian, is anywhere from 150 feet thick to zero. The Ordovician rocks, mainly dolomites and sandstones, thin northward due to Pennsylvanian and Permian erosion, averaging about 400 feet thick in the San Andres Mountains opposite White Sands. Silurian rocks are thin in Tularosa Basin, but thicken rapidly southward toward a depocenter in southernmost New Mexico, Texas and Mexico. The northern limit of these rocks is marked by a truncation attributable in part to Devonian erosion (Figure 1-8C).

Detritus from erosion of these and other Paleozoic rocks on the east side of the San Andres range has made it_x2019;s way into the older alluvial fans bounding lake Lucero, as well as the very young detrital fans that have built into the modern lake, and thence into the sand dunes.

The isopachs of the Devonian and Mississippian rocks reveal the beginnings of what will later become the Orogrande and Carrizozo Basins of Pennsylvanian and Permian age (Figure 1-8A,B and Figure 1-11). Depocenters that first appear in northern Donna Anna County are precursors of the much deeper Orogrande and Carrizozo basins. The shelf-to-basin transition near Alamogordo during the Mississippian (Figure 1-8A) was the ideal geological setting for the formation of the Mississippian (Waulsortian type) bioherms that presently outcrop just east of the city. These unusual bioherms developed on a marine shelf that was shallow enough for sunlight of penetrate to the reefs. The Pedernal landmass north of Alamogordo formed the landward portion of this ancient carbonate shelf, which was flooded by deeper water associated with subsidence of the Orogrande Basin of the Pennsylvanian Period following formation of the mounds (Figure 1-11D).

Upper Paleozoic rocks: The Upper Paleozoic sequences have the greatest importance to the history of White Sands, since these formations, mainly the Yeso of Permian age, have contributed the bulk of the evaporites to the basin. By Pennsylvanian time, the Orogrande basin, which strikingly mirrors the modern Tularosa Basin, had begun rapid subsidence. Over 3000 feet of Pennsylvanian sediments were deposited in this basin between Alamogordo and Las Cruces (Figure 1-11D). The Laramide (Upper Cretaceous-Lower Tertiary) Burro Uplift has thinned these deposits locally (Figure 1-11D). These rocks are mainly biohermal, shelfal carbonates.

During the Permian, aridity increased and the seas shallowed or withdrew completely at times, leading to deposition of shallow-water carbonates, redbeds and gypsiferous evaporites. By far the bulk of these evaporites are in the Yeso Formation (Figure 1-11b). Moreover, the gypsum content in the Yeso thickens northeastward into the Carrizozo Basin depocenter near the town of Carrizozo, where the formation exceeds 4200 feet in thickness. This northward increase in gypsum content of the Yeso Formation, and thickness of the formation itself was observed in measured sections from south to north along the San Andres Mountains by Kottlowski (1956), (Figure 1-12). Figure 1-13 shows the isopachs of the Yeso Formation superimposed on the present day Tularosa Basin, with drainages and uplifts highlighted, as well as the position of the present day White Sands. It is clear from these data that much of the gypsum in the present day lake basin may have been flushed southward toward the low point of the basin at Lake Lucero from the northern and eastern portions of the Tularosa Basin. This is due to wide exposure of gypsum bearing rocks, great thickness of gypsum, and a broad catchment area that for surface and subsurface runoff. Sinkholes due to gypsum solution by percolating groundwater have been reported northwest of the Malpais lava flow (Meinzer and Hare, 1915). Although large amounts of gypsum may not be moving down surface and subsurface drainages toward Lake Lucero at the present time, large amounts would have been dissolved and flushed into Lake Otero during Pleistocene glacial pluvials. Final evaporation and disappearance of lake Otero following the last Pleistocene (Wisconsin) glaciation reprecipitated the gypsum. It is also worth observing that much of the Yeso Formation outcrop that is in the San Andres Range directly opposite the White Sands outcrops on arroyos that drain into the Jornada del Muerto Basin. It is only in the northern part of the San Andres Range that major contributions from the Yeso are likely, at least with the present configuration of surface and subsurface flow.

Because the Orogrande basin was very similar in shape to the Tularosa Basin, the Mississippian, Pennsylvanian and Permian rocks have similar lithofacies in outcrop around the basin margins. Thus, they have been correlated from one side to the other with some confidence, as illustrated in Figure 1-14, with a few name changes from one side to the other.

Mesozoic system and the Laramide uplifts: Most of the Mesozoic rocks that are exposed in and around the Tularosa basin occur near Carrizozo. They are shown in green on the geologic map (Figure 1-4). These rocks include the Triassic Chinle and Sarton Formations, and the Cretaceous Mesaverde and Dakota Formations along with andesite flows and breccias near Hillsboro (Figure 1-5).

The southern limit of Triassic strata is in part depositional, but also reflects truncation of this system beneath lower Cretaceous conglomerates and sandstones (Figure 1-11A). As pointed out by Kottlowski, southwestern New Mexico and southeastern Arizona seem to have been positive areas that shed much clastic material northwards (Kottlowski, 1963). No Jurassic rocks are known from the Tularosa Basin outcrops.

Much of the Cretaceous record has been stripped from the region of the Tularosa Basin by Laramide and Rio Grande Rift uplift, with the exception of some thick deposits near Carrizozo, and remnants of Late Cretaceous rocks found from El Paso northward into the Jornada Del Muerto Basin. The Mancos shale, Eagle Ford Formation and lower part of the Colorado Shale change from a black shale and argillaceous (clay-rich) limestone facies southward into an arenaceous (sand-rich), calcareous clastic facies containing coal beds. These lithologies suggest fluctuating shorelines along the southern margin of the Tularosa basin during these times (Kottlowski, 1963). Kottlowski notes that near Capitan, above Carrizozo, Allen and Jones (1951) measured 135 feet of Dakota Sandstone, 390 feet of Mancos Shale and more than 535 feet of the Mesaverde Formation. On the crest of the Sacramento mountains Pray and Allen measured a 200 foot thick remnant of the Dakota Sandstone resting on the Permian San Andres Formation (Kottlowski, 1963; Pray and Allen (1956), Figure 1-11B.

In summary, much of the Mesozoic rock record, if were ever deposited at all, has been removed by erosion due to either Laramide compress ional tectonics or later Tertiary extensional uplift associated with formation of the present Rio Grande Rift. Oil tests in south Otero County cited by Kottlowski suggest that a widespread compressional uplift that that roughly followed the outlines of the Orogrande basin (e.g.; inversion of the Paleozoic basin) was responsible for stripping off the Mesozoic rocks, or for their non-deposition. This may have been the direct result of continent-wide Laramide compressional events that affected the entire western United States at the close of the Cretaceous into the early Tertiary (Middle Eocene, Chapin and Seager, 1975)(Figure 1-16).

Tertiary Rocks and the Rio Grande Rift: The Tertiary deposits of the Tularosa Basin record the onset of volcanism and extensional tectonics in late Oligocene time, and the renewed subsidence of the much of the area encompassed by the old Orogrande Basin to form the much younger Tularosa Basin. Throughout the Rio Grande rift, beginning perhaps in the Oligocene and accelerating in the Late Miocene (Seager, et.al., 1984), rising mountains have shed coarse detritus into rapidly subsiding extensional basins by means of meandering sandy rivers and alluvial fans. These processes have all operated in a climate that has grown increasingly arid. The Love Ranch Formation and the various formations of the Santa Fe group including the Camp Rice Formation deposited by the Rio Grande River are the principal valley fill clastics in and around the Tularosa Basin (Figure 1-5). In many of the young intermontane basins coarse clastic fluvial and sheetflood sedimentation was interrupted at various times by lacustrine episodes that deposited shales, marls and siltstones.

The tectonic events that formed the Tularosa Basin are ongoing, as evidenced by numerous Quaternary fault scarps. These scarps are both antithetic and synthetic to the main bounding fault on the West side of the basin. They occur on the valley floor and along the mountain front. Indeed, much of the present narrow, basin and range topography may have evolved during a late period of rapid extension from 7 to 4 million years ago (latest Miocene and Pliocene) (Seager, et.al., 1984).

List of Illustrations:

Figure 1-1: The extent of the Rio Grande Rift in the southwestern United States, showing the locations of White Sands National Monument and Great Sand Dunes National Monument within the rift.

Figure 1-2: Tectonic map of Southern New Mexico and Texas.

Figure 1-3: Structural cross-sections of the Tularosa Basin; (a) Alamogordo to San Andres Mountains, (b) Jarilla mountains to San Andres Mountains.

Figure 1-4: Geologic map of the Tularosa Basin and vicinity.

Figure 1-5: Stratigraphic column for the Tularosa Basin and vicinity, including the San Andres and Sacramento ranges and Otero Platform.

Figure 1-6: Photographs of bioherms in Paleozoic Rocks of the Tularosa Basin. (A) Muleshoe mound, Sacramento Mountains, on the east side of the Tularosa Basin just south of Alamogordo. This mound formed following the post Devonian extinctions, and is comprised mainly of bryozoan and crinoid fragments and carbonate mud. (B) Pennsylvanian age filloid-algal mound on north side of Highway 82 just north of Alamogordo.

Figure 1-7: Landsat image of the Tularosa Basin and vicinity, showing aeolian deposits in the vicinity of White Sands and surrounding uplifts and drainages.

Figure 1-8: Map showing isopachs of Lower Paleozoic Rocks in Southern New Mexico; (a) all Mississippian strata, (b) all Devonian strata, (c) Silurian Fusselman Dolomite, (d) Ordovician Montoya Dolomite and Cable Canyon Sandstone, (e) Ordovician El Paso Limestone - uppermost zones beneath Montoya Dolomite, (f) Cambrian Bliss Sandstone.

Figure 1-9: Lithologic cross section along the San Andres Mountain from north to south, showing lower Paleozoic rocks thinning northwards from Ash Canyon to Rhodes Canyon.

Figure 1-10: Index map of Tularosa Basin for cross sections.

Figure 1-11: Map showing isopachs and facies of Upper Paleozoic (Pennsylvanian and Permian) rocks in Southern New Mexico, from youngest to oldest; (a) Glorieta, Sand Andres, Scherrer, and Concha (including Rainvalley), (b) Yeso Formation and Epitaph Dolomite, (c) all Wolfcampian (Lower Permian) strata, (d) all Pennsylvanian strata.

Figure 1-12: Lithologic cross section along the San Andres Mountains from north to south showing the Yeso Formation thickening and increasing in gypsum content northwards.

Figure 1-13: Isopach of the Yeso Formation in the Tularosa basin and vicinity, and present surface drainage.

Figure 1-14: Correlation diagram of Permo-Pennsylvanian rocks in the San Andres and Sacramento Ranges, after Algeo, et. al. al (1991).

Figure 1-15: Map showing isopachs of early Cretaceous formations, Southern New Mexico: (A) Early Cretaceous, (B) Late Cretaceous.

Figure 1-16: Index maps of Laramide Tectonics: (a) Extent of the Laramide orogeny in North America (b) Laramide uplifts (Late Cretaceous-lower Tertiary) near the Tularosa Basin.