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Old Evidence, Partial Interpretation - Part 2

(B) Siliciclastic rocks and the myth of impact-tsunamis

1. Channelized siliciclastic deposits

It has been known from the very beginning that the siliciclastic deposits are channelized. In many outcrops the sandstone (unit 2) and alternating sand-silt-shale layers (unit 3) can be traced laterally a few hundred metres to where they disappear. We have shown this at El Mimbral, El Penon, La Parida, La Sierrita and many other areas (Adatte et al., l996). Where Tertiary sediments are present, the K-T boundary clay, red layer and Ir anomaly are above the mass extinction of planktic foraminifera and the thin remnant of the clastic deposit, as shown for El Mimbral in Figure 11 (Keller et al., 1994b). The presence of the red clay and Ir anomaly is evidence that the K/T boundary is present in these sections and well above the multiple spherule layers of the Maastrichtian Mendez marls.
Figure 11. Sketch of the Mimbral outcrop based on measured section at 2m intervals. The siliciclastic deposit at El Mimbral is channelized, where at the edge of the channel (Mimbral II) only the thin rippled sandy limestone of the top of unit 3 is present and underlying the K/T boundary red layer and Ir anomaly.

At La Parida, the channel deposits can be traced over about 150m where they thin and disappear, as shown in Figure 12. However, most of the late Maastrichtian zone CF1 is eroded in this area.
Figure 12.

The siliciclastic deposits of all channelized deposits overlie disconformities where erosion of the underlying Mendez Formation marls occurred upon deposition. Although erosion was variable, it is generally minor since in most outcrops the underlying Mendez marls contain at least part of zone CF1, which spans the last 300,000 years of the Maastrichtian. However, in some sections most or all of zone CF1, which contains multiple spherule layers, is eroded.

2. Grain size and tsunamis

Unit 3 of the siliciclastic deposit is characterized by alternating sand-silt-shale layers. Frequently, the finer grained layers contain burrows, though in many sections various sand layers are also burrowed, as discussed below. We have interpreted the finer grained layers as indicating normal hemipelagic deposition as opposed to the more rapid sedimentation of the coarser grained layers.

Smit argues that the finer grained layers are more silty than Mendez marls and therefore do not represent normal hemipelagic sedimentation. As evidence he shows a histogram of grain sizes comparing Mendez marls with silt layers of unit 3 (Fig. k). This is a no-brainer and demonstrates the obvious. Mendez marls are by definition finer grained than silts or shales.

The critical question is whether the coarser and finer grained layers represent rapid deposition via tsunami waves, as Smit argues, or whether periods of rapid deposition (turbidites) alternate with normal sedimentation during which invertebrates colonized the ocean floor, as we argue. We can demonstrate the latter, not from a single outcrop as Smit does, but from over 40 outcrops throughout the region and we show four sections in Figure 13. The grain size patterns were obtained from insoluble residues by a laser particle counter.
Figure 13. Granulometric analyses of the siliciclastic units 1 to 3, the Mendez marls and Velasco shales at El Penon, El Mulato, La Lajilla and El Mimbral.

Spherule unit 1 characterized by alternating finer and coarser grain sizes, with the former corresponding to Mendez marl matrix and the latter to the spherules. Note that the grain size increases in the sandy limestone layer due to the sand component. The sandstone unit 2 is characterized by more constant grain-size spectra. The alternating sand-silt-shales of unit 3 are reflected in the alternating coarser and finer grain-size pattern. The small size of the clay minerals present within these finer grained layers precludes settling through the water column within a few hours after a tsunami wave, as Smit suggests.

Contrary to the grain size pattern shown from one outcrop by Smit, our regional granulometric analysis reveals that the fine grained intervals of unit 3 are not really significantly different from the underlying Mendez marls or the overlying Tertiary Velasco Formation. Moreover, the various units and sub-units of the siliciclastic deposit have granulometric spectra which can be correlated from one outcrop to another. This type of pattern does not support chaotic deposition via impact-tsunami.

3. Burrows and Tsunamis

Smit insists that experts are wrong - burrows don't exist in the siliciclastic deposit, except at the top. This denial of evidence is understandable. Smit et al's entire K/T impact-tsunami hypothesis is dependent on the interpretation that the siliciclastic deposit was laid down within a matter of hours to days by the tsunami waves generated by the Chicxulub impact. Burrows within this deposit are irrefutable evidence that this interpretation is wrong and deposition occurred over a long time period during which invertebrate communities repeatedly colonized the ocean floor.

Admitting the existence of burrows thus means that the entire house of cards that holds up the impact-tsunami hypothesis and ties Chicxulub to the K-T boundary comes tumbling down. It means that:
  • The siliciclastic deposits between the spherule layers and the K-T boundary are not tsunami deposits, but represent sedimentation over a long time period.
  • The K-T boundary, mass extinction and Ir anomaly above the siliciclastic deposits are not coeval with the spherule layers in the Mendez marls below.
  • The Chicxulub impact predates the K-T boundary
It is therefore no surprise that Smit insists that the burrows don't exist, that trace fossil experts are mistaking scratches, wasp nests and mud-filled root tubes for burrows, and that evidently only he knows what real burrows are.

(a) Burrows at the top of unit 3:

Smit agrees that there is a heavily bioturbated top of unit 3 that forms the top of the Penon outcrop. It would be hard to deny this fact. But he interprets these as having colonized the ocean floor after the K/T boundary event and after deposition of the 'tsunami'. By this interpretation the K/T impact-tsunami hypothesis is not threatened. But where is the evidence for burrowing after the K/T impact event? Smit provides none.

There is a simple test that can establish whether the burrowing community lived in Tertiary sediments and burrowed downward, as Smit claims. By downward burrowing, the animals drag some of the sediments from the top into the burrows. In fact, most burrows are infilled entirely with sediments of the overlying strata in which the animal lived. If the animals lived in the post-K/T environment, at least some of the microfossils within the burrows should be of early Tertiary age. If they lived in the late Cretaceous, all microfossils should be of late Maastrichtian age. Our analysis of the microfossils within the burrows on top of unit 3 shows them to be infilled with late Maastrichtian sediments. This means that even the heavily burrowed horizon at the top of unit 3 represents colonization during deposition prior to the K/T boundary event.

(b) Burrows within unit 3:

Smit denies that there are multiple horizons of burrowing within the fine-grained layers of unit 3 - calling them scratches, wasp nests and mud-filled rootlets. Nevertheless, back in the early l990s he responded to our observation of multiple burrowed horizons with a standard block diagram (see Smit fig. I), explaining that these are due to only one type of organism? Ophiomorpha, which could burrow downward up to 1m from the top. This old interpretation is repeated here in a feeble attempt to explain the presence of several horizons of burrows made by various animals within the alternating sand-silt-shale unit 3 of El Penon.

We have shown burrows from El Penon in the last round of this debate and burrows from many other sections have been discussed by Ekdale and Stinnesbeck (l998) and Keller et al. (l997). Here we show burrows from the Rancho Canales section where at least two truncated Ophiomorpha borrowing horizons occur within unit 3 (Fig. 14).
Here we show burrows from the Rancho Canales section where at least two truncated Ophiomorpha borrowing horizons occur within unit 3 (Fig. 14).

Figure 14. There are at least two truncated Ophiomorpha burrowing horizons within unit 3 of the Rancho Canales section. This indicates repeated colonization during sedimentation over an extended time period.

At Rancho Canales the siliciclastic deposit is 110cm thick with a sandy limestone layer in the spherule unit 1. Unit 3 consists of sandstones that indicate at least 3 depositional events. The lower 40cm are laminated and contain rare, truncated Ophiomorpha burrows (Figs. 14, 15). In the overlying sandstone, Ophiomorpha burrows, lined with glauconite are abundant (Fig. 16). The burrows are truncated by the overlying sandstone, which is not bioturbated at its base. This indicates that this colony could not have originated from the top of the unit.
The third burrowing horizon is at the top of unit 3 and includes Ophiomorpha, Zoophycos, and Chondrites burrows, which do not penetrate more than 2cm into the underlying sands.
Figure 15. Rancho Canales outcrop showing the bioturbated sandstone of unit 3 overlying a thin spherule unit 1 with a sandy limestone layer.
Figure 16. Rancho Canales sandstone unit with green, glauconite filled Ophiomorpha burrows.

Forty kilometers east of Monterrey is the Los Ramones section with common U-shaped Rhizocorallium burrow sticking out of the lower surface of a sandstone layer of the siliciclastic deposit (Fig. 17). This indicates interruption of sedimentation over an extended time periods between deposition of two sandstones. Above this bioturbated interval, there are no burrows. Los Ramones, Rancho Canales, El Penon and many other sections contain undisputed burrowing horizons within the siliciclastic deposits. These burrowing horizons rule out deposition via impact-tsunami.
Fig.17 Figure 17. Bioturbation in the Los Ramones siliciclastic deposit showing U-shaped Rhizocorallium burrows. These burrows rule out deposition via impact-tsunami.

(c) J-Shaped burrows in unit 1 and unit 2:

Smit is obviously confused about the J-shaped spherule infilled burrows in units 1 and 2. Contrary to his claim, we never found these burrows in unit 3. Clearly, the animal that made them lived on the ocean floor during deposition of units 1 and 2, and this rules out rapid deposition via impact-tsunami - burrows and tsunami deposition are incompatible.

Smit states: "I strongly question the presence of burrows in the lower part of the clastic beds - my students, colleagues and I were not able to find burrow structures in several fieldtrips." We have good news for Smit. Figure 18 shows the classic El Penon outcrop with two burrows, one near the base of unit 2 and one in the sandy limestone layer in spherule unit 1. The latter was discovered by a Princeton undergrad during a short field trip last spring.
Fig.18 Figure 18. El Penon outcrop showing spherule unit 1 with the sandy limestone layer, and sandstone unit 2. J-shaped burrows are present in both units as indicated.

We earlier illustrated J-shaped spherule infilled burrows from near the base of unit 2 and from the sandy limestone layer of unit 1 in both black and white and colour. Smit reproduced these images and absurdly claims that they represent mirror images of one and the same burrow, one from the hand specimen and one from the field outcrop. This is nonsense. These burrows are still in the field. We did not cut them out for hand specimens. Moreover, the burrow in the sandy limestone layer of unit 1 is about half the size of the one in unit 2.

What possesses Smit to make such an absurd claim? He argues that by reducing the burrows to at most one, there is no problem for the impact hypothesis. He suggests that this animal could have hurriedly made its burrow between the arrivals of tsunami waves, which he estimates to be about 1 hour apart. Fertile imagination, indeed, but for science to progress we need hard data.

4. Currents and Tsunamis

Smit argues that current directions in the sandstone layers of unit 3 at the Lajilla outcrop support impact-tsunami deposition with the main direction coming from Chicxulub. In science, any interpretation based on a single datapoint is suspect. Smit's use of data from a single outcrop to make such a wide sweeping interpretation is highly suspect. The database has to be regional to rule out local anomalies in the current directions. In addition, Smit is basing his analysis on unit 3, for which we have already demonstrated several horizons of burrowing as well as finer grained layers representing normal hemipelagic deposition (see above). An impact-tsunami interpretation is therefore inconsistent with the empirical evidence. Here we show that it is also inconsistent with the regional database of current directions.

In the last debate round we have shown, based on region-wide measurements in units 1, 2 and 3 of the siliciclastic deposit, that the main current direction is from the opposite side, namely from the landward side with sediments derived from the Sierra Madre Oriental and transported via submarine channels. Based on this database we can model the channel axis directions of the various outcrops as shown in Figure 19.
Figure 19. Modeling of channel axis direction in the La Sierrita area shows a general E-SE direction.

It is clear that all channels in the La Sierrita area trend toward the same direction (E-SE). This direction is consistent with the regional palaeogeography and parallels the Burgos Basin uplift of the Sierry Madre Oriental. Note that the current direction of the spherule unit 1 is slightly different from the clastic units. Such a feature would not be expected from rapid deposition of all units via impact-tsunami.

Channel axis directions in the Mesa Juan Perez area trend from E-SE to a more southerly direction (Fig. 20).
Figure 20. Modeling of channel axis direction in the Mesa Juan Perez area.

These regional data do not support Smit's sweeping interpretation of impact-tsunami deposition based on a single isolated outcrop and measurements from only unit 3 (note that unit 2 is not present at Lajilla, Adatte et al., l996; Stinnesbeck et al., l996). In addition, we have already demonstrated the presence of several horizons of bioturbation in unit 3, as well as bioturbation in units 1 and 2, which effectively rules out tsunami deposition. Moreover, whole rock, clay mineral and granulometric analyses from sections covering the entire region indicate that units 1 contains a bioturbated sandy limestone, and unit 3 contains several fine grained layers, all of which indicate clastic deposition was interrupted by periods of normal hemipelagic deposition. The vast empirical database we have accumulated over the past 12 years simply does not lend support to the Smit et al (l992, l996) impact-tsunami hypothesis.