Search And Discovery
After the Chicxulub impact crater core’s analyses revealed that this impact predates the KTB, similar to the findings of NE Mexico, we renewed our efforts on finding the primary or original impact spherule ejecta deposit. We expanded our search for impact evidence well into the Maastrichtian sediments and discovered numerous localities with impact spherules below the sandstone complex or so-called impact-tsunami deposit (Keller et al., 2002; Schulte et al., 2003) (Fig. 12). In most localities the impact spherules were clearly reworked into Maastrichtian sediments, in others the origin was uncertain.
These NE Mexico KTB localities served as prime outdoor hands-on teaching laboratories for Princeton undergraduates for many years. Here they could examine the evidence pros and cons for the impact-tsunami hypothesis, collect samples for study and reach their own conclusions.
Our hypothesis was that the primary Chixculub impact spherule layer should be below the reworked impact spherules at the base of the submarine channels, it should be free of reworked and transported shallow water debris and reflect the relatively rapid settling through the water column.
To test this hypothesis we chose El Penon, a series of low-lying hills capped by the 8-12 m thick sandstone complex and gently dipping to the northeast (Figs. 30, 31). Eager students dug a trench at the most expanded exposure of Maastrichtian sediments below the sandstone complex.
Figure 31. Eager students digging a trench in search of the primary Chicxulub impact spherule layer at El Penon. The dig was successful; 4 m below the sandstone complex a 2 m thick impact spherule layer was discovered. Keller et al., 2002.
Figure 32. Stratigraphic column of the trenched section below the sandstone complex and reworked impact spherule layers.
The dig was successful. About 4 m below the sandstone complex a 2 m thick impact spherule layer was discovered (Fig. 32). Sediments exposed between the reworked impact spherule layers at the base of the sandstone complex and the new spherule layer consists of marls, two marly limestone layers and two thin red layers (Fig. 33B, C). The red layer C marks volcaniclastic influx. The stratigraphic layering is parallel to the overlying sandstone complex and shows normal marine sedimentation with abundant burrows. There is no evidence of tectonic disturbance or slumps (Keller et al., 2009, Fig. 33). These sedimentary features reveal normal sediment deposition over a long time interval with no significant disturbance.
Figure 33. Marls and marly limestone exposed between the reworked impact spherule layer at the base of the sandstone complex and the newly discovered primary impact spherule layer. Normal sedimentation occurred between these two spherule deposits. See text for description. From Keller et al., 2009.
Primary Impact Spherule Layer
The search for the Chicxulub impact ejecta layer in its primary or original position on the seafloor marking the exact time of the impact on Yucatan was discovered in a 2m thick spherule layer 4 m below the sandstone complex at El Penon (Keller et al., 2009). This spherule layer differs markedly in many aspects from the two reworked upper spherule layers with their abundant shallow water debris that indicates erosion from shallow nearshore waters and transport via submarine channels into the deeper slope at 500 m depth where they came to rest. The 4 m of sediments that separates the primary and reworked spherule layers indicates that tens of thousands of years lapsed between the Chicxulub impact and the reworked episode that deposited the two upper spherule layers and the sandstone complex.
The primary spherule layer can be traced over about 100 m along the El Penon hillside parallel to the sandstone complex at the top of the hill and then dips below ground level (Fig. 34). There is no significant disturbance in the marls along the hillside. The spherule layer is 2m thick in the submarine channel and laterally thins gradually to 20 cm. Very rare marl clasts can be found. The largest two (10-20 cm long) were found in the thickest part of the spherule layer and hence the deepest part of the channel where current erosion is strongest.
Unlike the reworked spherules at the base of the sandstone complex, the primary spherule layer lacks any sandstone or other clastic deposits above it. Instead, normal marl sedimentation resumed after spherule deposition. Thus there is no evidence of significant disturbance at the seafloor and no impact generated tsunami deposit can be invoked.
Figure 34. El Penon hillside with the primary impact spherule layer parallel and 4 m below the reworked spherule layers at the base of the sandstone complex. B,C and D show spherule layer at the locations marked on the hillside. Images of the impact spherules from the primary deposit; from left to right: spherules melted and welded, spherules with concave-convex contact due to deposition while still hot, oval compressed and calcite cement. From Keller et al., 2009.
Characteristics of Primary Impact Spherule Layer
The spherule layer contains no shallow water foraminifera or debris. Spaces in between spherules are infilled with calcite well after deposition. Most spherules are round, oval or compressed, some show concave/convex surfaces indicating compression while glass was still hot and malleable.
Figure 35 shows a variety of Chicxulub impact spherules, glass shards and melt rock in a calcite matrix from the primary spherule deposit near the base of uppermost Maastrichtian zone CF1 at El Peñon 1B in Fig. 34. Spherules range from 2 mm to 5 mm in size. A-C shows vesicular spherules; D-F, compressed vesicular spherules; G-I, dumbell shaped spherules (G), and compressed vesicular spherules with concave-convex contact (I); K-M, vesicular glass shards. N-O shows melt rock glass of welded, amalgamated spherules where original spherules can rarely be recognized (N). P shows a foraminifer in melt rock (Keller et al., 2009).
These spherule characteristics indicate that deposition of the Chicxulub impact spherules occurred rapidly, possibly by raft-like accumulation of hot spherules at sea surface and rapid sinking. The compressed spherules, concave-convex contacts and melt rock indicate deposition within hours of the impact. The calcite matrix between the spherules precipitated well after settling of spherules.
The near absence of clastic grains and absence of shallow water debris indicates that this deposit was not derived from erosion of nearshore deposits, and there was relatively little seafloor disturbance at the time of deposition, in contrast to the reworked two spherule layers at the base of the sandstone complex. Rare large clasts are present near the deepest part of the channel (El Penon 1B, Fig. 34).
Figure 35. Thin section micrographs of the 2m thick primary impact spherule layer near the base of zone CF1. Keller et al., 2009.