Reworked K/Pg ejecta in early Paleocene deposits, Difunta Group, NE Mexico
Ejecta del K/Pg retrabajados en depósitos del Paleoceno temprano, Grupo Difunta, NE de México
David Cadena-González1, Ricardo Barragán2, Francisco A. Vega-Sandoval3, Criseida Ruiz-Aguilar4, Francisco J. Vega2,*
1 Posgrado en Ciencias Biológicas, Universidad Nacional Autónoma de México, Unidad de Posgrado, Edificio D primer piso, Ciudad Universitaria, 04510, CDMX, México.
2 Instituto de Geología, Universidad Nacional Autónoma de México, Circuito de la Investigación Científica, Ciudad Universitaria, 04510, CDMX, México.
3 Facultad de Ciencias, Universidad Nacional Autónoma de México, Circuito de la Investigación Científica, Ciudad Universitaria, 04510, CDMX, México.
4 Escuela Nacional de Estudios Superiores, Unidad Juriquilla, Universidad Nacional Autónoma de México, Bulevar Juriquilla 3001, 76230, Juriquilla, Querétaro, México.
Corresponding author: (F.J. Vega) This email address is being protected from spambots. You need JavaScript enabled to view it.
How to cite this article:
Cadena-González, D., Barragán, R., Vega-Sandoval, F.A., Ruiz-Aguilar, C., Vega F.J., 2025, Reworked K/Pg ejecta in early Paleocene deposits, Difunta Group, NE Mexico: Boletín de la Sociedad Geológica Mexicana, 77(2), A190625. http://dx.doi.org/10.18268/BSGM2025v77n2a190625
Manuscript received: June 9, 2025; Corrected manuscript received: June 11, 2025; Manuscript accepted: June 20, 2025
ABSTRACT
The Chicxulub impact triggered massive sediment removal and deposit of ejecta around the world. The impact must also have triggered large tsunamis, whose geological record is nevertheless scarce. Some of the tsunamite backwash deposits were reworked during early Paleocene, and evidence of severe earthquakes and ejecta fall are preserved along the tsunamite deposits at top of Cretaceous and the base of lower Paleocene sediments in the Difunta Group, within Parras and La Popa basins, NE Mexico. Tsunamite backwash deposits were previously studied near El Papalote diapir (La Popa Basin, Nuevo León), where pseudotektites and bioclasts are ubiquitously abundant, along with diverse bioclasts and intraclasts of different lithologies that include igneous, metamorphic and sedimentary rocks from outcrops located to the N of the Difunta Group. We report here on three localities with lower Paleocene tempestites at La Popa Basin, all interpreted as storm deposits. These outcrops are found at the top of the K/Pg Delgado Sandstone Member and also include altered pseudotektites and bioclasts. Although ejecta and bioclasts were already reported from two localities (El Papalote and Amargos), we also illustrate deformed quartz and uncommon bioclasts such as crab cuticle, ammonite nuclei and shark teeth.
Keywords: K/Pg, tsunamite, tempestites, ejecta, bioclasts, Difunta Group.
RESUMEN
El impacto de Chicxulub disparó una remoción masiva de sedimentos y depósitos en todo el mundo. Seguramente, el impacto debió generar grandes terremotos y tsunamis cuyo registro geológico es escaso. Algunos de estos depósitos fueron retrabajados durante el Paleoceno temprano, posiblemente debido a intensas tormentas, consecuencia de alteraciones en la atmósfera, con evidencias de terremotos intensos y caída de eyecta preservadas a lo largo de depósitos de tsunamita que se encuentran entre la cima del Cretácico y la base del Paleoceno inferior en el Grupo Difunta, en las cuencas de Parras y La Popa, NE de México. Depósitos que representan el regreso de la tsunamita hacia la cuenca del golfo de México han sido estudiados cerca del diapiro El Papalote (Cuenca La Popa, Nuevo León), en donde pseudotectitas, bioclastos son localmente abundantes, junto con diversos bioclastos e intraclastos de diversas litologías que incluyen rocas ígneas, metamórficas y sedimentarias, acarreadas de afloramientos ubicados al N de la Cuenca La Popa. Previamente, dos localidades con eyecta han sido reportadas para las cuencas de Parras y La Popa. Aquí reportamos otras tres localidades con tempestitas del Paleoceno inferior, ubicadas en la Cuenca de La Popa. Estos afloramientos se encuentran en la porción superior de Miembro de Arenisca Delgado, cuya edad se estima alrededor del límite, incluyendo pseudotectitas. Eyecta y bioclastos han sido reportados previamente para dos afloramientos (El Papalote y Amargos). Adicionalmente, aquí reportamos cuarzo deformado y bioclastos como cutícula de cangrejo, núcleos de ammonite y dientes de tiburón.
Palabras clave: K/Pg, tsunamita, tempestitas, eyecta, bioclastos, Grupo Difunta.
1. Introduction
The Chicxulub impact on what is now the NE of the Yucatan Peninsula sent into the atmosphere large amounts of ejecta, among them as tektites whose composition, size, and shape are related to the distance from the crater, with localities distributed along the Caribbean, Central and South America, as well as the East coast of Mexico, and the SE of the USA. Other nearly immediate consequences of the impact included severe earthquakes, ejecta fall, large tsunami(s), an ignited atmosphere and subsequent ash and Iridium fall. The East coast of Mexico includes several KPB localities reported by several authors, mainly at relatively deep-water settings (Izett et al., 1990; Sigurdsson et al., 1991; Smit, 1999). Here, we reinforce the presence of lower Paleocene reworked ejecta-bearing deposits in the Difunta Group, NE Mexico.
The Difunta Group, located in NE Mexico in the S and SE of Coahuila (Parras Basin) and the E of Nuevo León (Figure 1), is a succession of mainly siliciclastic sedimentary rocks deposited in shallow marine and transitional environments during late Campanian to early Eocene time (McBride et al., 1974; Vega et al., 1989; Lawton et al., 2001, 2009; Eberth et al., 2004). Based on mollusk biostratigraphy, the K/Pg boundary was placed between the Middle Siltstone Member and the Upper Mudstone Member of the Potrerillos Formation in the La Popa Basin (Vega et al., 1989; Vega and Perrilliat, 1995).
Most of the sedimentary history of lithostratigraphic units at La Popa Basin was controlled by salt tectonics, including 3 diapirs (El Papalote, El Gordo, La Popa) and a weld (La Popa weld; Giles and Lawton, 1999, 2002; Lawton et al., 2001, 2021). To the SE of El Papalote diapir (Figures 2 and 4), ejecta-bearing deposits were first recognized in deposits of the Delgado Sandstone (Aschoff et al., 2001; Aschoff, 2003; Shipley, 2004; Lawton et al., 2005; Schulte et al., 2010; Figure 3), formally described by Lawton et al. (2001) as a shoreface and shelfal 40-60 m thick sandstone deposit, lying erosively over the Middle Siltstone Member (upper Maastrichtian) and conformably underlying the Upper Mudstone Member of the Potrerillos Formation (lower Paleocene). In this unit tsunamite backwash was deposited as valley-fill accumulations in a bathymetric break that separated Maastrichtian upper shoreface from lower shoreface and prodelta depositional settings (Lawton et al., 2001; Aschoff and Giles, 2005).
This bathymetric break might be a pre-existent feature, possibly emphasized by salt-withdraw triggered by the Chicxulub earthquake (Lawton et al., 2005). The ejecta-bearing strata represent an S-SE tsunamite backflow which probably reached also deeper settings toward the Gulf of Mexico basin (Aschoff and Giles, 2005; Lawton et al., 2005). Schulte et al. (2010) provided detailed study of the tsunamite backwash deposits at several outcrops S and SE El Papalote diapir found at the upper portion of the Delgado Sandstone of the Potrerillos Formation (between 26°02”44.95’ N, 100°43”14.49’ W and 26°03”05.64’ N, 100°43”52.63’ W; Figure 4). The light-gray tsunamite deposits are conformed by sandstone with hummocky cross-bedding at the base (Aschoff, 2003; Shipley, 2004; Lawton et al., 2005; Schulte et al., 2010). The El Papalote K/Pg ejecta-bearing deposits range in thickness from a few centimeters up to 4 m and include large reworked boulders with the same lithology as the basal sandstones of the Delgado Sandstone Member, interpreted as fragmented remains removed and transported basinward by the tsunami backwash (Lawton et al., 2005; Schulte et al., 2010). Tektite pseudomorphs are more abundant at the base of the tsunamite, becoming scarce toward the top of the deposit (Lawton et al., 2005; Schulte et al., 2010). Lawton et al. (2005) reported tektite pseudomorphs as composed mainly by Fe and Mg, ooids with quartz and calcite nuclei and other clasts represented by quartz, chert, micrite, calcareous siltstone, and very fine-grained sandstone (Figure 3C). Detailed compositional analyses of the pseudotektites are found in Schulte et al. (2010).
Previously reported bioclasts include foraminifera, corals, gastropods, ostreids, ammonites, bryozoa and echinoderms, most of them transported from shallow marine and estuarine environments to the N and NW of La Popa Basin (Lawton et al., 2005). Schulte et al. (2010) also reported tektite pseudomorphs composed of calcite, Fe-Mg as well as Si-K and mention bioclasts as red algae, wood, foraminifera, gastropods, bivalves, bryozoa, brachiopods, echinoderms, and vertebrates.
To the W, at the NE portion of the Parras Basin, Stinnesbeck et al. (2016) reported an ejecta-bearing, channel-fill deposit found at the top of the Las Encinas Formation near Amargos, Coahuila (25°56”36.90’ N, 101°01”28.64’ W; Figures 2 and
5). The K/Pg boundary at the N and NE portion of the Parras Basin was previously placed within the upper third of the Las Encinas Formation, but no evidence of ejecta was found (Murray et al., 1959; Hasseltine, 1968; Wolleben, 1977; Vega et al., 2007). Pseudotektites are relatively ubiquitous at the Amargos locality, where light-gray sandstone-conglomeratic deposits are interpreted to be a tempestite deposit that occurred during early Paleocene storm events (Stinnesbeck et al., 2016).
The presence of the early Paleocene (Danian) ostreoid Gorizdrella gorizdroae Vyalov, 1937 as a relatively abundant and well-preserved Paleocene mollusk included into the ejecta-bearing sediments, reinforces the interpretation of Paleocene storm deposits. Stinnesbeck et al. (2016) defined the Amargos reworked tsunamite as a bioclastic oligomict microconglomerate to contain devitrified, weathered tektite pseudomorphs. This 1 m thick channel-like structure is laterally continuous for about 100 m includes both late Maastrichtian and early Paleocene macrofossils such as the ammonite Sphenodiscus pleurisepta (Conrad, 1857) (phragmocone filled with spherules), the Maastrichtian echinoid Hardouinia mortonis (Michelin, 1850) and the early Paleocene (Danian) small ostreid Gorizdrella gorizdroae (Stinnesbeck et al., 2016; Cadena-González et al., 2023). The ammonite was interpreted to be carried along with the tsunamite, but the echinoids are thought to be eroded from the underlying upper Maastrichtian shallow marine beds of the lower Las Encinas Formation during the storm that deposited reworked tsunamite matter. The relatively small early Paleocene ostreids, although represented mostly by the left concave valve, preserve delicate fine ornamentation, suggesting low energy and/or protection by the fine sandstone matrix during transport in shallow marine settings. The purpose of this research is to report ejecta and bioclast contents of four ejecta-bearing localities (two of them new) in the La Popa Basin: El Papalote, Amargos, Puerto Luis and Delgado Norte. From these, only El Papalote includes K/Pg tsunamite backwash deposits in situ. The other three localities include reworked tsunamite deposited during early Paleocene storms, including tektite pseudomorphs, bioclasts and other microscopic grains of diverse lithology of igneous, metamorphic and sedimentary rocks. Bioclasts, ejecta and microclast contents are commented and illustrated for each locality, with emphasis on other bioclasts.
2. Material and method
The four outcrops—El Papalote (K/Pg boundary), Puerto Luis, Delgado Norte, and Amargos (Lower Paleocene tempestites)—were systematically sampled at 10 cm intervals from base to top in order to obtain a relatively uniform dataset. From each sampling point, rock fragments measuring approximately 5×5 cm were collected to ensure consistency in sample size and facilitate subsequent petrographic and microtextural analyses. These rock fragments were processed to obtain both polished surfaces and thin sections. Polished surfaces were prepared using progressively finer abrasives combined with diesel as a lubricant to achieve an optimal finish for macroscopic and mesoscopic observations. Thin sections were produced using an automated thin section machine to maintain uniform thickness and minimize preparation artifacts.
Microscopic observations and image acquisition were carried out using a Zeiss Axiozoom V.16 stereomicroscope equipped with a high-resolution Zeiss digital camera, which allowed for detailed documentation of textures, grain relationships, and sedimentary structures. For transmitted light petrography, a Zeiss polarizing microscope was employed, coupled with a Canon EOS camera to capture high-quality images. This combination facilitated both routine petrographic description and the documentation of key features relevant to the depositional and diagenetic history of the samples. All captured images were processed and edited using Adobe Photoshop Elements 2.0
3. K/Pg and lower Paleocene ejecta-bearing localities
3.1. EL PAPALOTE BACKWASH TSUNAMITE
In this study, we sampled the main outcrop at El Papalote, known as Hornet’s Nest (26°02”14.50’ N, 100°43”26.01’ W; Figure 5A). At the base of the tsunamite deposit there is a layer of sandstone with numerous pseudotektites with no apparent intraclasts or bioclasts, this layer may represent the first pulse of ejecta out fall, or just the base of the tsunamite back deposit (Figures 5B and 5C). As reported by Lawton et al. (2005), other large sandstone boulders are found at the base and within the tsunamite backwash, with evidence of soft deformation (Figure 5E). Some gastropods include a slightly bended shell of the gastropod Haustator trilira (Conrad, 1860) (Figure 5F). Shark teeth and gastropods are embedded into the middle portion of the tsunamite backwash deposits (Figures 5G and 5H). Along with the bioclasts previously mentioned, olive-green, orange and black tektite pseudomorphs are found here (Figures 5I-5N). Bubbly calcite spherules along with altered glass are relatively abundant on the observed thin sections and are similar to those reported for the El Papalote tsunamite by Lawton et al. (2005) and Schultze et al. (2010). Red algae, foraminifera, ooids, echinoid plates and crustacean cuticles are also present (Figures 5O, 5P and 6A-6E).
To the NW (1.6 Km) of the Hornets Nest outcrop, we also sampled a thin (30 cm) layer of light-gray conglomeratic sandstone (26°03”14.30’ N, 100°43”58.25’ W; Figures 6F and 6G), directly overlying the top of the Delgado Sandstone Member. This layer is here interpreted as a lower Paleocene tempestite that also includes diverse reworked clasts such as bryozoa, crustacean cuticle and microscopic shark teeth (Figures 6H-6K), along with bubbly pseudotektites (Figure 6L).
3.2. AMARGOS
The Amargos lower Paleocene reworked tsunamite is found in an approximately 100 m wide and up to 1 m thick channelized structure, erosively overlying the top of the Las Encinas Formation (Stinnesbeck et al., 2016; Figures 2-4 and 7A). An ammonite shell, possibly Sphenodiscus pleurisepta (Conrad, 1857), includes numerous pseudotektites inside the phragmocone (Figure 7B), suggesting that the shell was empty and filled with fine-grain sediment that transported the ejecta. Although nearly as abundant as the ones found at El Papalote, tektite pseudomorphs are here replaced by calcite and/ or smectite (Figures 7F and 7G). The thin sections include ooids, fragments of gastropods, oysters, ammonites, crustaceans, bryozoa, echinoderms, and chert clasts (Figures 7B-7O). The small ostreids belong to the species Gorizdrella gorizdroae, which indicates an early Paleocene age for this deposit (Figure 7E). Cadena-González et al. (2023) also reported on the presence of numerous specimens of the Maastrichtian echinoid Hardouinia mortonis (Michelin, 1850) (Figures 7C and 7D), interpreted to be exhumed and carried along the Amargos area.
3.3. PUERTO DE LUIS
Located at 26°06”19.’ N, 100°48”47.14’ W, 800 m SW of Puerto de Luis ranch, there is an outcrop of lower Paleocene reworked tsunamite that erosively overlies the Delgado Sandstone, with an approximate thickness of 30 cm (Figures 8A and 8B). This outcrop appears to be equivalent to that observed near El Papalote diapir. Thin sections reveal a content of clasts similar to those observed at Amargos and El Papalote lower Paleocene reworked tsunamite (Figures 8C-8P).
3.4. DELGADO NORTE
The Delgado Sandstone type section is found near the Paleocene upper Gordo limestone lentil, NE of the Delgado syncline (26°02”09.48’ N, 100°47”40.30’ W; Lawton et al., 2001, p. 231, fig. 12; Figure 9A). A small outcrop of a light-gray monomictic breccia that contains medium-sized sandstone clasts with angular edges (Figures 9B and 9C). Ammonites with pseudotektites inside the fragmocone are scarce but present in the lower Paleocene reworked tsunamite (Figure 9D).
It is interesting to note that the fragmocone of the ammonite found in this outcrop is unfilled also with diverse lithologies such as mudstone and coarse sandstone. This reveals that the ammonite shell was possibly empty by the time it was transported by the tsunami to be reworked along with tsunamite during an early Paleocene storm. The matrix includes pseudotektites replaced by calcite, some with bubbly internal structure (Figures 9E-9G).
Stratigraphically, this outcrop lies within the upper portion of the Delgado Sandstone, and includes scarce but recognizable left valves of the early Paleocene ostreoid Gorizdrella gorizdroae.
4. Discussion
Four ejecta-bearing localities have been recognized, two of them previously reported: El Papalote series of outcrops (Lawton et al., 2005; Schulte et al., 2010) and Amargos (Stinnesbeck et al., 2016); two other ejecta-bearing localities are here reported (Puerto Luis and Delgado N). The El Papalote localities include in situ K/Pg tsunamite deposits in the most shallow K/Pg marine environmental settings found in NE Mexico (Smit, 1999), at an approximate distance of 800 km NE from the Chicxulub impact area.
Two new ejecta-bearing localities are here documented (Puerto de Luis and Delgado Norte), and along with the previously reported two localities (El Papalote and Amargos) include relatively abundant ejecta, igneous, metamorphic and sedimentary clasts, transported during the tsunami backwash from transitional and continental rock exposures located to the NW and
N. The source of these microclasts is uncertain, but some can be compared with Cretaceous sedimentary units such as the Cenomanian-Turonian Eagle Ford Group, the Indidura Formation and upper Campanian Parras Shale, all cropping out to the NW and N of the study area. Bioclast diversity is illustrated, with some important additions to the previously mentioned bioclasts (Lawton et al., 2005; Schulte et al., 2010; Stinnesbeck et al., 2016), such as ammonite nuclei, crustacean cuticle and shark teeth. Active since the early Cretaceous, salt-tectonics along with the initial pulses of the Laramide Orogeny might have created shallow marine and surficial topographic traps from where tsunami backwash deposits were reworked by early Paleocene strong storms that transported sediment basinward.
Comparison between morphology and composition of ejecta from La Popa Basin and that found in deeper marine K/Pg settings reported to the East in Nuevo León and Tamaulipas, might help understand if early Paleocene storms carried reworked tsunamite basinward, resulting in multiple ejecta-bearing strata.
5. Conclusion
New outcrops of reworked ejecta are reported from the lower Paleocene Difunta Group, La Popa Bain, Nuevo León. Two previously reported localities (one NW of Parras Basin, Coahuila) were sampled and interpretations of reworked Chicxulub ejecta are confirmed. Although it is not possible for the moment to state if the reworked ejecta were accumulated by a single storm during the early Paleocene, differential contents in fossils, bioclasts and ejecta might suggest either different depositional scenarios or/and different storm events. In all sections, ejecta-bearing deposits are found just above the K/Pg Delgado Sandstone Member of the Potrerillos Formation (La Popa Basin). Some interesting and relatively abundant microscopic bioclasts include crustacean cuticle, ammonite shell fragments and shark teeth. Further fieldwork and sampling is still necessary in order to evaluate if the reworked ejecta layers are part of a discrete, continuous layer across the base of the lower Paleocene Upper Mudstone Member of the Potrerillos Formation, including a detailed map of outcrops. Most of the K/Pg localities reported from the Caribbean and Gulf of Mexico belong to relatively deep marine and continental deposits, the ejecta-bearing localities here mentioned belong to shallow marine environments.
Contributions of authors
DCG: field work, images and final manuscript review; RBM: interpretation of microclasts and some illustrations; FAVS: field work, polished samples and thin sections; CRA: laboratory experiments; FJV: field work, structure of manuscript, location figures.
Financing
DGAPA, UNAM for the economic support from PAPIIT project IN109424.
Acknowledgements
Our sincere gratitude to Dirección General de Asuntos del Personal Académico (DGAPA, Universidad Nacional Autónoma de México) for the economic support from PAPIIT project IN109424 to FJV. Aid of Fernando Núñez-Useche in recognizing bioclasts is much appreciated. José Flores-Ventura (Museo del Desierto, Saltillo, Coahuila) offered assistence during fielwork. Field observations and advice was given by Hermann D. Bermúdez (Department of Earth and Environmental Studies, Montclair State University, Montclair, New Jersey 07043, USA). Advise and fieldwork support by Michelangelo Martini, Hermann Bermúdez and Fernando Núñez-Useche was very important and so we thank these specialists. Comments and suggestions of two anonymous reviewers are highly appreciated.
Conflicts of interest
We declare no conflict of interest in this contribution.
Handling editor
Antoni Camprubí.
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