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

Sites to See

Stop 2b: Starkville South K-T boundary site

Just a ¼ mile further south is the Starkville South K-T boundary site (Fig. 11). This site has probably been the most heavily sampled site in the Raton basin. In 1984, a team from the Smithsonian Institution collected a 2½-ton sample of the boundary interval and shipped it back to Washington, D.C., to archive this significant rock sequence. The Starkville South section shown in Figure 1 represents several sites in roadcuts along I-25. At the Starkville South site, the strongest Ir anomaly ever measured in continental rocks (56 ng/g) was measured in the flaky shale at the top of the ejecta layer of the boundary claystone (Pillmore and others, 1984). The Ir is concentrated at the top of the K-T boundary claystone in this thin bed now known as the fireball layer (Fig. 12).

Figure 11. Photograph of the Starkville South site, looking east, showing the position of the K-T boundary claystone interval about 6 ft above a 23-ft-thick point-bar sandstone. Arrow points to the K-T boundary claystone layer.

Figure 12. Photograph of the K-T boundary sequence at the Starkville south site showing the light gray boundary claystone and overlying thin coal bed that contains the fern-spore spike.

Palynology of the Starkville South K-T boundary site

The K-T boundary at Starkville South is placed at the level of the disappearance of the Cretaceous palynomorphs of the Proteacidites assemblage. These fossils are present in the Cretaceous rocks below but disappear at the top of a kaolinitic claystone, which here is located about 6 ft above the top of the point bar sandstone (Fig. 11). Cretaceous palynomorph assemblages in mudstones below the boundary are diverse and include many species of fossil pollen and spores that also occur in the lower Paleocene (e.g., Pandaniidites radicus, Kurtzipites spp., and Ulmipollenites sp.).

The relative abundance of fern spores just below the kaolinitic claystone is about 30 percent and comprises several species. Just above the kaolinitic claystone is a 2-cm-thick interval that to date has not yielded any palynomorphs and has been called the barren interval. Palynomorph assemblages recovered from the coal just above the barren interval are dominated by fern spores -- this is the "fern-spore spike" interval (see section on the Cretaceous-Tertiary Boundary for a discussion of the significance of the fern-spore spike). Fern spores constitute 80 percent of the assemblage from the lower half and 77 percent of the assemblage from the upper half of the coal; a single species of fern spore dominates the assemblages. Angiosperm (flowering plant) pollen in the fern spore spike interval is greatly reduced in relative abundance and diversity. Above the coal, angiosperm pollen gradually increases in relative abundance and diversity until it again dominates.

A 3-ft-thick, olive-gray mudstone just above the K-T boundary coal bed contains fossil planktonic green algae (Pediastrum and Scenedesmus), which also occur in modern freshwater lakes and ponds. The presence of these algae indicates that the mudstone was deposited in a floodbasin lake or pond (Fleming, 1986, 1987). This interpretation is corroborated by the presence of Pandaniidites radicus in the brown mudstone immediately overlying the olive gray mudstone. The plants that produced P. radicus are attributed to modern Pandanus (screw pine), which grows in coastal and marshy areas of the tropics and subtropics. At Starkville South, the presence of Pandaniidites radicus indicates deposition in a marshy area, probably on the margin of the floodbasin lake that contained planktonic green algae. The palynological and sedimentological evidence suggest that the K-T boundary layers and associated sediments were deposited in a floodbasin environment that included peat forming swamps (represented by the coal), marshes (represented by brown, carbonaceous mudstone containing Pandaniidites radicus), and floodbasin lakes or ponds (represented by olive-gray mudstone containing Pediastrum and Scenedesmus).

The depositional framework at Starkville South is somewhat different from that at Starkville North. At Starkville North, the boundary lies near the top of a sequence of mudstone beds 20 ft thick that rests on a channel sandstone 7 ft thick; at Starkville South, the mudstone sequence has thinned to 10 ft, and the channel sandstone has thickened to 23 ft.

To continue, go back up the service road, cross the Interstate, and turn south.

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