1. Chicxulub predates the KT boundary and is not the cause for the end-Cretaceous mass extinction: Evidence from NE Mexico

1.1. Chicxulub KT AGE?
1.2. Proximity to KT
1.3. KT Boundary in NE Mexico
1.4. Tsunami?
1.5. Bioturbation negates tsunami
1.6. Multiple spherule ejecta layers
1.7. Spherule ejecta layers predate KT by 300 kyr

The evidence that shows Chicxulub predates the KT boundary and is not the causer for the end-Cretaceous mass extinction is diverse and from various independent sources, including stratigraphy, sedimentology, paleontology, mineralogy and geochemistry. Critical evidence summarized here include:

1. Chicxulub impact ejecta layers in late Maastrichtian sediments throughout northeastern Mexico within planktic foraminiferal zone CF1, which spans the last 300 ky of the Maastrichtian.

2. The impact ejecta layer beneath the siliciclastic deposit (“tsunami”) in northeastern Mexico separated by a 15-20 cm thick burrowed limestone bed.

3. Multiple burrowed horizons by invertebrates in the siliciclastic deposits near the top of the Maastrichtian, which invalidates the common “impact generated tsunami” interpretation for these deposits.

4. Impact ejecta layers reworked within early Danian deposits of zone Pla (P. eugubina) in central and southern Mexico, Haiti, Guatemala and Belize.

5. In the Chicxulub crater core Yaxcopoil-1, late Maastrichtian planktic foraminiferal assemblage of zone CF1 in the 50 cm thick laminated micritic limestone between the suevite breccia and the KT boundary.

6. Deposition of this 50 cm thick laminated micritic limestone interrupted five times by glauconite formation, indicating normal low energy deposition with prolonged pauses, winnowing and small-scale localized transport between the impact breccia and the KT boundary.

7. Prior reports of late Maastrichtian limestones with planktic foraminifera overlying the impact breccia in Yucatan cores T1, C1 and Y6, as also supported by e-log correlations.

1.1. Chicxulub KT AGE?
Ever since the discovery of the Chicxulub subsurface crater in the early l990’s many scientists have assumed that this is the crater of doom that caused the demise of the dinosaurs and many other animal groups at the end of the Cretaceous. This very attractive theory was supported by:

(a) 39Ar/40Ar ages of about 65 ± 0.2 Ma of melt glass in the Chicxulub breccia and impact ejecta in the form of glass spherules (microtektites) in Haiti and NE Mexico (2-4).
(b) The geochemical similarity of microtektites with melt rock from Chicxulub (5-7).
(c) The stratigraphic proximity to the K-T boundary in localities throughout Mexico, Guatemala, Belize and Haiti (8-11).
(d) The assumption that the siliciclastic unit that separates the spherule layers from the overlying K-T boundary in NE Mexico represents impact-generated tsunami deposits (9-11).

Detailed stratigraphic investigations of numerous localities in Mexico, Haiti, Guatemala and Belize (Fig. 2) challenged (c) and (d) and revealed that Chicxulub ejecta is in proximity to the KT boundary - either above or below it - but cannot be determined to be stratigraphically at the KT boundary.

Figure 2. Localities with Cretaceous-Tertiary boundary sequences that contain impact ejecta (microtektites) from the Chicxulub crater on northern Yucatan.

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1.2. Proximity to KT
The impact spherule deposits are variable in their stratigraphic positions relative to the K-T boundary, which is globally identified by a specific set of criteria that includes:
1) a boundary clay and thin red layer with an Ir anomaly
2) a negative d13C shift and
3) the mass extinction of tropical and subtropical planktic foraminifera (l2).
4) The first appearance of Danian species immediately above the KT red layer and Ir anomaly.

For example, in Haiti, Belize, Guatemala and central and southern Mexico, the spherule deposits occur above the K-T boundary in the (early Tertiary) Danian zone Pla (named for Parvularugoglobigerina eugubina) (11, 13-16). They consist of spherules mixed with reworked late Maastrichtian marl clasts and planktic foraminifera. We consider these spherule deposits to be reworked, as shown in the Beloc sections of Haiti, Coxquihui in central Mexico, and Actela in eastern Guatemala (Figs. 3-5). In all three localities an Ir anomaly of chondritic origin (l7) is present above the spherule layer in zone Pla and suggests an early Danian impact (see summary in (1)).

Figure 3. The KT transition at Coxquihui, Central Mexico, where the KT boundary is marked by a hiatus and a thin (2cm) spherule layer. A 60cm thick spherule deposit is present in the early Danian zone Pla. Reworked Maastrichtian planktic foraminifera within this spherule deposit indicate reworking and redeposition from an older deposit. The Ir anomaly in zone Pla above the spherule layer is unrelated to the spherule ejecta event and may represent an early Danian zone Pla impact event (13).

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Figure 4. The KT transition in Beloc, Haiti, where the KT boundary is marked by a short hiatus and the spherules are interbedded with limestones of the early Danian zone Pla. Reworked Maastrichtian planktic foraminifera and clasts with spherules within the spherule and limestone layers indicate reworking and redeposition from an older deposit. The Ir anomaly in zone Pla above the spherule layer is unrelated to the spherule ejecta event, or the KT boundary, and may represent an early Danian zone Pla impact event (11, 17).

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Figure 5. The KT transition in Actela, Guatemala,and Santa Theresa, Belize, where the KT boundary is marked by a short hiatus and the spherules are interbedded with limestones of the early Danian zone Pla. Similar to Coxquihui and Beloc reworked Maastrichtian planktic foraminifera and clasts with spherules indicate reworking, transport and redeposition from an older deposit. The Ir anomaly in zone Pla above the spherule layer is unrelated to the spherule ejecta event, or the KT boundary, and may represent an early Danian zone Pla impact event as also observed in Haiti and central Mexico (14-15).

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Figure 6. Locations of KT sections with spherule-rich deposits below the sandstone-siltstone complex. Stars mark localities of sections; at each locality several sections were examined, samples collected and analyzed. Location map of the La Sierrita area showing the low laying mesas and the sections analyzed.

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All early investigations of the KT transition in NE Mexico have centered on the siliciclastic deposits and the underlying spherule layer. We searched for KT boundary sequences that would be unencumbered by the siliciclastic unit in order to evaluate the KT transition. Such sections can be found by tracing the siliciclastic deposits laterally until they disappear.

At La Sierrita a 3-5 cm thick silty sandstone layer marks the remnant of the siliciclastic deposit at a distance of about 300 m from the center of the channelized unit. A 2-3 mm thick red layer overlies this sandstone layer and is enriched in iridium. No spherules are present. Planktic foraminifera show the extinction of all tropical and subtropical species with only a few ecological generalists surviving into the early Tertiary. The early Tertiary is characterized by the evolution of new species, all of which are very small and of simple morphology (Fig. 7). Within one meter the sandstone layer disappears and the KT transition is marked only by the thin red layer that separates the Maastrichtian Mendez Fm marls from the early Danian shales of the Velasco Fm. (Fig. 8). Similar sequences have been documented at La Parida and El Mimbral (18, 19). These sections indicate a KT boundary red layer and mass extinction that is similar to Tethyan locations worldwide.

a) Multiple disconformities within the siliciclastic unit, which indicate repeated interrupted deposition and erosion (Fig. 8, (21-22).

b) Multiple horizons of bioturbation within the siliciclastic unit, which indicate repeated colonization of the ocean floor by invertebrates followed by erosion and rapid deposition (Fig. 8, (20, 23).

c) A 15-20 cm thick sandy limestone layer, which is bioturbated and truncated by erosion and separates the spherule deposit at the base of the siliciclastic unit into two layers (Fig. 9, (23).

d) A thin layer with Maastrichtian planktic foraminiferal assemblages between the top of the siliciclastic unit and the KT boundary in several sections (24).

e) Well defined KT red clay and Ir anomaly above the siliciclastic unit (Fig. 7, (18, 19).

Figure 9. The KT transition at El Mimbral, NE Mexico, showing the siliciclastic deposit with the spherule unit 1 at the base, the thick sandstone unit 2, alternating shale, sand and siltstone layers of unit 3, and the KT boundary above it. Erosive contacts separate these units. Bioturbtion is common particularly by Chondrites in the fine layers of unit 3 (Fig. 9a). In general, Thalasinoides, Zoophycos are common in the upper layers of unit 3, and J-shaped burrows are found in the sandstone unit 2 and the sandy limestone layer of spherule-rich unit 1 (Fig. 9b).

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Figure 9c. Deposition of the siliciclastic deposit at El Mimbral most likely occurred during the latest Maastrichtian sea-level lowstand about 100,000 years prior to the K-T boundary. At this time, sediments exposed by the lower sea level were eroded, transported seaward and redeposited in the deep submarine canyons at depths of more than 500 m.

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If the Chicxulub impact is KT in age, then the spherule ejecta at the base of the siliciclastic unit must be coeval with the KT boundary and Ir anomaly above it. To explain this discrepancy, the siliciclastic unit was interpreted as impact generated tsunami deposit. However, bioturbation, disconformities and sandy limestone layer within the spherule deposit are all indicators of long-term deposition unrelated to the impact event.

The sedimentologic features of this siliciclastic deposit and the bioturbated horizons are consistent with deposition in a submarine canyon during a sea level lowstand and early transgression. At the onset, sediments exposed in nearshore areas by the sea level regression are eroded, transported seaward and redeposited in submarine canyons. These sediments are characterized by wood and plant fragments and glauconite, which forms in shallow shelf environments. The presence of abundant Chicxulub impact spherules mixed with this shallow water debris indicates that these were also eroded from nearshore areas.

Figure 10a. Sandy limestone layer separates the spherule unit 1 at El Mimbral and marks this ejecta unit as two events separated by a period of normal pelagic limestone sedimentation. J-shaped burrows truncated at the top of this limestone layer have been observed at El Penon.

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Figure 10b. Limestone layer within spherule deposit at El Mimbral

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Figure 11. The classic El Penon I outcrop showing the spherule unit 1 at the base of the siliciclastic deposit separated by a 10-15 cm thick sandy limestone layer. J-shaped burrows infilled with spherules are present in this limestone layer (Fig. 11a) as well as near the base of the sandstone unit 2 above the spherules (inset). This indicates a long-term depositional history, rather than rapid fallout ejecta followed by a tsunami wave.

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Figure 11a. Close up of the two spherule layers of unit 1 separated by the sandy limestone layer and the sandstone unit 2 at El Penon I.

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Figure 11b. J-shaped burrow infilled with spherules and truncated by erosion from the top of the sandy limestone layer at El Penon I.

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Figure 11c. J-shaped burrow infilled with spherules and truncated by erosion from the base of the sandstone unit 2 at El Penon I.

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Burrowing organisms (e.g. worms, crabs, crustaceans, bivalves etc.) leave traces of their presence in the sediments. These trace fossils are important indicators of conditions on the ocean floor during sediment deposition. If trace fossils are present, this means that sediment accumulation was normal. If sediment deposition was rapid, as during tsunami events, slumps, or gravity flows, then the burrowing ceased as the animals are buried and suffocated by the sudden influx of thick sediment deposits. Toni Ekdale’s discovery and documentation of burrowing organisms in several horizons through the siliciclastic units 3 and 2 reveals repeated times of colonization of the ocean floor by invertebrates (20). The subsequent discovery that J-shaped burrows are also present within the sandy limestone that separates the spherule unit 1 negates a single event deposition for the impact ejecta.

Each colonization by invertebrates would have taken an amount of time significantly exceeding the duration of a tsunami event. Each colony was in turn wiped out during rapid influx of sediments from shallower environments (indicated by the presence of shallow water benthic organisms (19-20)), probably by gravity flows. This process probably accounts for the erosion and truncated burrows. Thus sedimentation alternated between rapid and normal deposition. This means not only that deposition of the siliciclastic deposit (units 2 and 3) occurred over an extended time period, but also that this particular spherule unit 1 was deposited in two events, and that deposition of each siliciclastic unit (units 1, 2 and 3) in turn far exceeded the duration of a tsunami event.

Similarly, this interrupted sedimentation pattern is also reflected by the multiple erosional disconformities within the siliciclastic unit, some of which truncate the burrows. Each erosional disconformity indicates temporary cessation of sediment deposition and erosion of the underlying sediments prior to resumption of sedimentation. Thus both the disconformities and burrowing horizons reveal pulses of rapid sedimentation alternating with normal sedimentation during which the ocean floor was colonized by invertebrates.

This pattern of sedimentation not only negates the tsunami interpretation, but there is also no way that the spherule ejecta below the siliciclastic deposit can be coeval with the KT boundary and Ir anomaly above it.

1.6. Multiple spherule ejecta layers
The original spherule ejecta deposit was discovered at El Mimbral, NE Mexico, at the base of the siliciclastic unit (8-10), and was followed shortly thereafter by similar discoveries at El Penon, La Lajilla, El Mulato, La Sierrita and eventually over 40 localities in NE Mexico (Fig. 2). A problem with the interpretation of these spherule ejecta layers as single event deposit surfaced early on due to the presence of the 10-20 cm thick burrowed, sandy limestone layer within the spherule unit 1, as discussed above. With the discovery of the J-shaped burrows, the sandy limestone layer can no longer be dismissed as diagenetic and postdepositional; it clearly represents normal pelagic sedimentation.

Limestone deposition occurs slowly (2-4cm/1000yrs) in normal pelagic environments with little detrital influx. The presence of the burrowed sandy limestone layer sandwiched between two spherule beds indicates that spherule deposition occurred at two different times and separated by normal limestone deposition and burrowing colonies on the ocean floor. These two spherule ejecta layers could therefore not represent deposition during a single event, and they could not be co-eval with the KT boundary – as assumed commonly assumed (8-10), The abundance of shallow water debris and benthic foraminifera indicates that these spherule layers are reworked and transported from shallow water environments into the deeper bathyal environment.

More evidence of multiple spherule ejecta layers was discovered in the late l990’s by five masters students from the Universities of Neuchatel and Karlsruhe. These students mapped and analyzed the KT boundary, siliciclastic units, spherule ejecta deposits, and underlying late Maastrichtian Mendez marls over an area spanning about 60km2. This first detailed investigation of the late Maastrichtian Mendez marls revealed the presence of three additional spherule layers interbedded in 10-12 m of pelagic marls (1, 25)(Fig. 12). The lowermost spherule layer consists of almost pure glass spherules, indicating rapid deposition and little or no transport. Some spherules are fused indicating settling while glass was still hot (Fig. 12a-b).

In 2002, Princeton University undergraduates trenched the hillside of El Penon in NE Mexico and discovered the original spherule ejecta layer 4 m below the spherule-rich layers at the base of the siliciclastic deposit (Fig. 12A). The 4 m of pelagic marls between the reworked and original spherule layers are horizontally bedded and represent normal undisturbed marine sedimentation. The original spherule ejecta layer is 1.8 m thick and consists of almost pure impact spherule glass at the base and few clasts from the underlying sediments. The fused spherules indicate rapid settling after the impact. The absence of shallow water debris indicates that these spherules were not derived from erosion of nearshore areas (Fig. 12B).

Figure 12. The longest late Maastrichtian records have been recovered to date at Loma Cerca, Mesa Juan Perez and El Penon. Correlation of this record is shown for El Penon and Loma Cerca. Zone CF1 spans the last 300 ky of the Maastrichtian. A. Closely packed glass spherules. B. Fused spherules indicating deposition while melt rock was still hot.

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Figure 12A. In search of the elusive original Chicxulub impact glass spherule ejecta layer, Princeton undergraduates trenched the late Maastrichtian pelagic sediments below the reworked spherule layers at the base of the siliciclastic deposit.  At 4 m below, they discovered the original spherule ejecta layer in a 1.8 m thick spherule deposit, which contains no reworked shallow water debris and the base consists of closely packed and partially fused impact spherules (Fig. 12B). This spherule layer is near the base of zone CF1, which dates this impact at about 300,000 years before the KT boundary.

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Figure 13. Multiple impact glass sperule layers are present in late Maastrichtian marls of zone CF1 in NE Mexico and can be correlated across the entire region. Zone CF1 spans the last 300 ky of the Maastrichtian.

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1. The KT boundary and Ir anomaly overlie two siliciclastic units which represent deposition over a very long time period (see #1 and #2).

2. The siliciclastic deposits (units 2 and 3) contains multiple horizons of burrowing by invertebrates that indicate intermittent colonization of the ocean floor in pelagic environments.

3. The siliciclastic deposit contains multiple erosional disconformities, frequently terminating burrowing, which indicates pelagic sedimentation alternating with rapid deposition by gravity flows, slumps, etc.

4. The spherule ejecta (unit 1) at the base of the siliciclastic deposit, which is generally considered the direct ejacta fallout from the Chicxulub impact, represents two events separated by thousands of years during which limestones accumulated and invertebrates burrowed on the ocean floor.

5. Additional glass spherule layers are interbedded in the uppermost 12 meters of the late Maastrichtian marls of the Mendez Formation below the spherule ejecta unit 1.

6. The lowermost glass spherule layer at El Penon is 4 m below the base of the siliciclastic and reworked spherule deposit. This layer consists of closely packed spherules with only rare clasts of marls or foraminifera. Many spherules are fused, indicating rapid deposition while still hot. This appears to be the original Chicxulub ejecta fallout.

7. All stratigraphically higher glass spherule layers contain common to abundant clasts and reworked planktic foraminifera and shallow water benthic foraminifera. These layers appear to be reworked and transported from shallower environments. The most abundant reworked shallow water debris is present in the spherule layers just below the siliciclastic units – and commonly considered the original fallout.

8. To date, the multiple spherule layers within the late Maastrichtian Mendez marls have been documented in over 40 sections spanning more than 100 km.

9. Only a few minor localized slumps spanning a few meters have been observed in these late Maastrichtian sequences. There is no evidence of large-scale slumping, mass wasting or margin collapse.

10. In all sections, the spherule deposits are within planktic foraminiferal zone CF1, which spans the last 300 ky of the Maastrichtian.

11. The stratigraphically oldest spherule deposit, and original ejecta fallout, consistently occurs near the base of zone CF1. This indicates an age of about 65.3 Ma for deposition of this spherule ejecta.

12. The glass spherule ejecta is geochemically linked to the Chicxulub impact.