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
2. Chicxulub Crater

3. Conclusions: Chicxulub impact predates KT by 300 kyr

References

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1. Chicxulub predates the KT boundary and is not the cause for the end-Cretaceous mass extinction
This conclusion was announced by Gerta Keller, Wolfgang Stinnesbeck and Thierry Adatte at the April (2003) EGU-AUG meeting in Nice, France, based on over 10 years of KT research in Mexico, Guatemala, Belize and Haiti (1), and culminating with the new drill core Yaxcopoil-1 in the Chicxulub crater. This announcement has triggered a renewed debate over the cause of the KT mass extinction and the role of Chicxulub. (Public debate sponsored by Geological Society of London, November 2003, GEOSCIENTIST). Here we summarize the major evidence in support of a pre-KT age for the Chicxulub impact.

Figure 1. Gerta Keller, Wolfgang Stinnesbeck and Thierry Adatte at the El Mimbral outcrop during the NASA LPI sponsored field trip in l994. This long-term investigation and slow unraveling of the Chicxulub history has greatly benefited from collaboration with Doris Stueben and her research group, Zsolt Berner, Utz Kramar and students at the Geochemistry and Mineralogy laboratory of the University of Karlsruhe. Our team has trained six Masters students (Marc Affolter, Ives Houriet, Falk Lindenmaier, Lionel Schilli, Peter Schulte, and) and five Ph.D. students (Lupe Lopez-Oliva, Alfonso Pardo, Peter Schulte, Markus Meudt, Markus Harting) all of which have significantly contributed to unraveling the history of Chicxulub and the KT mass extinction. We have also led numerous field trips to Mexico to provide undergraduate students with hands-on experience to investigate the evidence that has contributed to one of the greatest controversies of the last Century (link to Sept. 6, Schultz article on field trip). Field expeditions for professionals were led in l992 and l994.


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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Detailed examination of 12 sections in Haiti, Belize, Guatemala, central and southern Mexico revealed the same pattern of reworked glass spherule ejecta in early Danian zone Pla sediments above the KT boundary. In a few sections some spherules were also found at the KT boundary (e.g. Coxquihui, Fig. 3), but the bulk always occurred well above it. The presence of marl clasts with spherules and Maastrichtian planktic foraminifera within these spherule-rich early Danian zone Pla sediments indicates that they are reworked from older deposits.
Clay deposition marks the KT boundary worldwide. The spherules in the reworked marl clasts were originally deposited in marls, rather than the KT clay, which rules out original deposition at the KT boundary. Some of the marl clasts contain late Maastrichtian planktic foraminifera which suggests that spherule deposition preceded the KT boundary, though this evidence is not conclusive. The sediments underlying the KT boundary in Belize, Guatemala and southern Mexico consist of shallow water platform limestones and limestone breccias of non-impact origin (14-16). They yield no age control for the late Maastrichtian and no further information on the age of the spherule-producing event. However, such information is currently available from numerous sections in NE Mexico.

1.3. KT Boundary in NE Mexico
In NE Mexico,1m to 8m thick siliciclastic deposits formed by sandstone, shale and silt units, cap low mesas due their resistance to weathering. These deposits are underlain by altered impact glass spherules (microtektites) of variable thickness ranging from a few cm to more than 1 m. Both the siliciclastic deposits and spherules are usually lenticular in shape and infill paleochannels cut into the underlying late Maastrichtian marls of the Mendez Formation. The KT boundary is above the siliciclastic deposits and frequently eroded along with any Tertiary sediments, though good KT boundary sediments have been analyzed at El Mimbral II, La Lajilla, El Mulato, La Parida, La Sierrita among others. Our team has analyzed over 40 sections throughout northeastern Mexico over the past 10 years (Fig. 6).

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.




Figure 7. The KT transition at La Sierrita, Mexico, in a section where the siliciclastic deposit is very thin (3-5cm) and no spherules are present. Planktic foraminiferal species ranges show an abrupt change at the boundary and the basal Danian is enriched in iridium (unpublished data).

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Figure 8. The red layer at the La Sierrita section, Mexico, at a distance of about 301 m beyond the channelized siliciclastic deposit. At this locality the red layer overlies the tan colored marls of the Mendez Fm and underlies the gray shales of the Velasco Fm.

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1.4. Tsunami?

In l992 the first evidence of impact ejecta (glass spherule deposits) in NE Mexico was discovered at El Mimbral underlying a thick siliciclastic unit, which in turn underlies the KT boundary marked by an Ir anomaly and the first Danian species. To reconcile the ejecta with the KT iridium anomaly as a single impact origin, Smit et al. (8-10) interpreted the siliciclastic unit as impact-generated tsunami deposits. In this scenario the glass spherules settled out first, followed by a megatsunami depositing the siliciclastic unit and finally settling of fines depositing the Ir anomaly. All this would have happened within hours to days.

This hypothesis became very popular even though there was contrary evidence from the beginning – namely, bioturbation (churning caused by burrowing organisms) and erosional disconformities within the siliciclastic unit and the position of the K-T boundary and Ir anomaly above. The tsunami hypothesis was challenged during the l994 LPI-sponsored field trip by trace fossil expert Toni Ekdale, who discovered bioturbation within the siliciclastic unit. Although he was effectively booed for this observation, he later returned to Mexico to study many of the classic K-T localities and document several horizons of bioturbation (20).

Thus the impact-generated tsunami hypothesis could not be reconciled with a host of critical evidence, which effectively disproved it.
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).
These features are well displayed at the classic sections of El Mimbral and El Penon.

At Mimbral, the siliciclastic deposit represents a channel fill that is about 3m thick at its maximum and thins to 20 cm over about 150 m where only the topmost bioturbated sandy limestone is present. Unit 3, the alternating sand, shale and silt layers, are strongly burrowed by Chondrites within the finer grained beds and Thalassinoides and Zoophycos in the upper coarser layers (Fig. 9a). Erosional disconformities are present between the units, as well as within them. The KT boundary and Ir anomaly is above the siliciclastic deposit. The spherule deposit is variable in thickness, but reaches a maximum of about 1m. A 15-20 cm thick sandy limestone separates this spherule deposit into two layers (Fig. 10), as also observed at El Penon. This sandy limestone layer within the spherule unit at the base of the siliciclastic deposit has been traced over 300km (Keller et al. 1997). These features indicate multi-event and long-term deposition unrelated to the impact event.


 

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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At El Penon, five outcrops have been examined over an area of about 500 m. The siliciclastic deposit is variable over this area and thickest at the classic Penon I section where the sandstone unit 2 reaches a thickness of about 4m. In most other outcrops unit 2 is missing and the topmost unit 3, the alternating sand, shale and silt layers, overlies the spherule deposits. The spherule deposits are also of variable thickness as well as variable stratigrapahic horizons (discussed below).


1.5. Bioturbation negates tsunami

At the classic El Penon I outcrop the spherule unit 1, which overlies the Mendez marls, is separated into two layers by the 10-15 cm thick sandy limestone layer, similar to El Mimbral (Figs. 11, 11a). During a recent field trip Princeton undergraduates discovered J-shaped burrows in this sandy limestone layer. The burrows are infilled with spherules and truncated by erosion at the top (Fig. 11b). Similar J-shaped spherule infilled burrows have been observed from near the base of the sandstone unit 2 (Fig. 11c). This indicates a multi-event depositional history for the spherule unit 1, including rapid deposition of the lower spherule layer followed by a period of normal limestone sedimentation and burrowing invertebrates, erosion followed by rapid deposition of the upper spherule layer.


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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1.7. Spherule ejecta layers predate KT by 300 kyr
To date, these late Maastrichtian spherule layers have been examined in more than three dozen sections and correlated over more than 100 km (Fig. 13). In all sections, the spherule deposits are within planktic foraminiferal zone CF1, which spans the last 300 ky of the Maastrichtian. The lowermost spherule layer is near the base of this zone and we consider it to represent the original ejecta layer because it consists of nearly pure spherule debris with only very rare clasts or foraminifera (Fig. 12a). All subsequent layers contain marl clasts with spherules and reworked foraminifera, suggesting that these layers are reworked from the original ejecta deposit.

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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Only some small local slumps spanning a few meters were observed by us or others (26, 27). Marls are normally stratified and contain typical late Maastrichtian zone CF1 planktic foraminiferal assemblages. Impact triggered slumps, mass wasting, or earthquakes cannot account for these normally stratified Mendez marls or foraminiferal assemblages (1, 25).
The evidence against the KT impact-tsunami scenario and for a pre-KT age of the Chicxulub impact ejecta is thus multi-faceted and extremely robust, including:
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.
We conclude that the evidence from NE Mexico KT sequences negates the commonly quoted Chicxulub impact-generated tsunami scenario to explain the siliciclastic deposit that separates the impact ejecta and the KT boundary. In contrast, the evidence strongly supports a pre-KT age for the Chicxulub impact, predating the KT boundary by about about 300 ky.

These observations and impact ejecta deposits in Haiti, Belize and Guatemala are summarized in Figure 14 along with climate change from the mid-latitude South Atlantic DSDP Site 525 and Deccan Traps volcanism. The oldest impact layer is dated at about 65.3 Ma and is linked to the Chicxulub impact based on glass geochemistry. This impact event coincides with the global climate warming between 65.2 and 65.4 Ma (28) and peak intensity of Deccan volcanism (29, 30). Younger impact glass spherule layers in the late Maastrichtian zone CF1 and early Danian zone Pla may be repeatedly reworked as a result of sea level fluctuations. The KT boundary event is frequently absent in the region due to tectonic activity and erosion. A widespread Ir anomaly in the early Danian subzone Pla(l) (e.g., Guatemala, Mexico, Haiti) is tentatively identified as an early Danian impact event, and a Pd anomaly and minor Ir anomaly at the Pla(l)/Pla(2) transition may be related to a regional volcanic event (Fig. 14).
  Figure 14. Multiple impact scenario based on impact glass spherule layers and Ir anomalies surrounding the Gulf of Mexico, Caribbean and Central America. Note that the Ir anomaly at the KT boundary is found worldwide. The Pla(l) has so far been documented from Guatemala, Haiti, Mexico and possibly ODP Site 1049.

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