Online guide to the continental Cretaceous-Tertiary boundary in the Raton basin, Colorado and New Mexico

Description of Geology

RATON BASIN

The Raton basin (Fig. 1) is a large asymmetric syncline (2,500 sq. mi. area) that extends from Huerfano Park, Colorado, to Cimarron, New Mexico. The Cretaceous and Tertiary rocks dip steeply and form hogbacks along the western margin of the basin on the east flank of the Sangre de Cristo Mountains. These rocks dip more gently inward along the other margins of the Raton basin and are highly dissected.

Stratigraphy
The sedimentary rocks of the Raton basin are shown in Figure 2. The marine Pierre Shale (Campanian to Maastrichtian) and overlying marginal marine Trinidad Sandstone (Maastrichtian) underlie the nonmarine Upper Cretaceous and Tertiary rocks in the basin. Nonmarine sedimentary rocks in the Raton basin, from oldest to youngest, include the coal bearing Vermejo Formation (Maastrichtian) and Raton Formation (Maastrichtian and Paleocene), and the non-coal bearing Poison Canyon Formation (also Maastrichtian and Paleocene), which overlies and intertongues with the Raton Formation.

Figure 2. Click image to see a larger version.

Depositional and tectonic history
The formations present in the Raton basin are the Pierre Shale and Trinidad Sandstone, the Vermejo and Raton Formations, and the Poison Canyon Formation as portrayed in the stratigraphic section (Fig. 2). Their depositional and tectonic history is shown in Figure 3. The Cretaceous epeiric sea covered the area during most of Late Cretaceous time. The sea was filled by a thick sequence of calcareous, deep marine or basinal shales, overlain by the Pierre Shale, 1,800 to 1,900 feet of shallow marine or shelf shales and siltstones. These beds range in age from Cenomanian to Maastrichtian as defined by ammonite fossils studied by G.R. Scott and W.A. Cobban (G.R. Scott, written commun.). Most of the Pierre consists of gray noncalcareous shale that coarsens upward into silty shale and siltstone, reflecting the influx of silt as the paleoshoreline of the sea retreated to the east across Colorado and New Mexico in late Campanian and early Maastrichtian time. The upper part of the Pierre grades into the overlying Trinidad Sandstone

Figure 3. Click image to see a larger version.

The Trinidad Sandstone is a tabular body, in most places about 80-100 ft thick, composed mostly of fine to very fine-grained sandstone that contains Ophiomorpha, Diplocraterion, and other trace fossils. It forms persistent, conspicuous, light-colored cliffs at the east and south edges of the basin. The Trinidad was deposited in contemporaneous delta front and interdeltaic barrier bar environments as the sea continued to regress eastward. The delta front deposits include distributary mouth bar and distributary channel sandstones (Flores and Tur, 1982). The barrier bar deposits consist of middle shoreface, river estuarine inlet, and beach sandstones (Leighton, 1980) that are overlain by the fluvial deltaic and back barrier deposits of the Vermejo Formation.

The Vermejo Formation consists of interbedded sandstone, siltstone, shale, carbonaceous shale, and coal that together form steep, generally debris-covered slopes above the cliffs of the Trinidad Sandstone. The Vermejo varies in thickness from 370 to 390 ft along the western border of the Raton basin to 0 ft in the eastern part of the basin, south and east of Raton, New Mexico. It contains coal beds as thick as 10-13 ft near the top and bottom of the formation (Pillmore, 1976). The sediments and coals of the Vermejo were deposited in contemporaneous fluvio-deltaic and back-barrier coastal plains fronted by barrier-bar and delta-front sandstones of the Trinidad (Flores and Tur, 1982). Lower alluvial plains dissected by meandering streams separated by flood basins characterized the upper part of the Vermejo. Here, sand filled stream channels and fine grained sequences of silt and mud were laid down in floodplains associated with crevasse splay and minor crevasse-channel sandstone. Coal beds in the lower part of the formation formed in poorly drained back barrier coastal swamps and in swamps adjacent to distributary channels of delta plains. These alluvial deposits grade upward into the more landward deposits of the Raton Formation.

The Raton Formation contains the K-T boundary interval. The formation consists of sandstone, siltstone, mudstone, coal, carbonaceous shale, and conglomerate. It ranges in thickness from more than 2,100 ft in the west-central part of the basin to 1,100 ft in the eastern part. A basal pebble conglomerate bed commonly rests unconformably directly on the Vermejo Formation, but in the vicinity of Raton and Trinidad, the conglomerate is commonly absent and no unconformity is evident. This scour-based sandstone forms a persistent cliff throughout much of the Raton basin, especially in the western and southern parts. Lee (1917) originally divided the Raton into a basal conglomerate, a lower coal zone, a barren series, and an upper coal zone. A similar subdivision that better fits our purposes, especially in the area around Raton and Trinidad where the basal conglomerate is lacking, includes the basal conglomerate in the lower coal zone. The three subdivisions are mostly consistent and identifiable throughout the Raton basin and thicken from east to west.

The Raton Formation was deposited on an upper alluvial plain (Flores, 1984) characterized by various modes of aggradation and erosion. Deposition on the alluvial plain was interrupted by several styles of fluvial sedimentation. Following the deposition of fluvio-delta plain and back-barrier sediments of the Vermejo Formation, rapid uplift of the source area to the west (the present San Luis valley, termed the San Luis highland, Tweto, 1987) caused widespread erosion of the highland and concurrent deposition of the basal conglomerate of the Raton. The basal conglomerate, which consists of medium- to coarse-grained channel sandstones with lenses and stringers of conglomerate, was probably deposited in basin-margin braided streams merging basinward into meandering streams. The uplift of the highland during Laramide time may have moved along thrust faults that bordered the western margin of the basin (Flores and Pillmore, 1987 and Woodward and Snyder, 1976). After deposition of the basal conglomerate, more stable tectonic conditions returned and sandstones were deposited mainly in meandering streams at the same time that interbedded thin coal, carbonaceous shale, mudstone, and sandstone beds accumulated in floodplains and backswamps. Crevasse splays periodically interrupted and infilled low lying floodplains during river floods. Though some thick coal beds were deposited locally in the lower coal zone, most coal swamps that formed in the flood basins were well drained, small, and shallow, which limited peat accumulation and resulted in thin lenticular coal beds mostly less than 8-12 in thick. At the close of the Cretaceous, the extensive alluvial plain was an ideal environment for deposition and preservation of the impact ejecta and fallout resulting from the K-T boundary asteroid impact.

Shortly after the close of the Cretaceous, tectonic conditions in the Raton basin changed as uplift of the source area was reinitiated in the west (Fig. 3c). It is probable that episodic upthrusting along the fault belt to the west created extensive erosion and sediment input into the basin. Sediment load increased and a fluvial system characterized by braided streams merging basinward into meandering streams and well-drained flood basins once again characterized the depositional basin. Streams aggraded broad belts across the alluvial plain, resulting in sheetlike to vertically stacked channel deposits. These deposits are locally interbedded with and laterally grade into floodplain mudstone and siltstone deposits. Associated carbonaceous shale and thin, lenticular coal beds formed in backswamps. Along the eastern and southern margins of the basin, cliffs of the barren series commonly stand high above slopes of the lower coal zone and the Vermejo Formation. The upper coal zone of the Raton supports the hills and ridges above the barren zone in the interior part of the basin.

The barren series was succeeded by the upper coal zone, which was deposited on a low-gradient alluvial plain environment, resulting from a decrease in tectonic movement in the source area. This tectonic pause, perhaps combined with basin subsidence, led to aggradation of the alluvial plain by a meandering fluvial system. The system was accompanied by floodplains that developed poorly drained backswamps in which coal beds as thick as 12 ft accumulated. The floodplains were locally filled by overbank and crevasse splay detritus during episodes of floods. These deposits extended into the backswamps and caused splits of coal beds where the peat swamps reestablished after detrital influx. Deposits of this setting grade upward into deposits of more landward environments of the Poison Canyon Formation.

The Poison Canyon Formation overlies the Raton Formation throughout most of the Raton basin; but to the west and southwest, it intertongues with the Raton. The Poison Canyon consists of thick to massive, lenticular, ledge-forming beds of coarse grained to conglomeratic arkosic sandstone intercalated with beds of nonresistant, yellow-weathering, sandy, micaceous mudstone and siltstone. The contact with the underlying Raton generally is indefinite and gradational. The Poison Canyon was deposited in a high-gradient alluvial plain environment (Strum, 1984), characterized by non-coal-bearing floodplain and braided to meandering stream alluvial fan deposits. The high sediment input into the alluvial plain indicated by these high bedload streams and fans probably reflects intensive erosion of a rapidly rising source area to the west. These tectonic and depositional conditions probably correspond to a pulse of upthrusting that exposed core rocks as indicated by the abundance of potassium feldspar grains in the channel sandstones. The Poison Canyon deposits coarsen to the west, forming the alluvial fans of the piedmont environment marginal to the rising ancestral San Luis highland.

Back to Online Guide Home Page