Evidencias e implicancias paleoambientales de paleoterremoto y tsunami en el límite marino transicional de la formación Horcón entre los 32°41’ Y 32°42’S.
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Fecha
2023
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es
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Universidad Andrés Bello
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Licencia CC
Resumen
La Formación Horcón es una secuencia de areniscas, conglomerados y fangolitas de finales
del Neógeno que aflora entre los 32°42’ y 32°38’S en las costas de Chile central. Esta se
divide en un miembro inferior caracterizado por la presencia de glauconita y uno superior
definido por la abundancia de fósiles marinos. En el límite entre los miembros, se identifica
un depósito asociado a un evento de paleo terremoto (sismita) y uno relacionado a un
evento de tsunami (tsunamita), este último conformado por 4 niveles, que van desde una
arenisca media, a una capa de conglomerados con restos de vertebrados, un estrato de
arenisca con un 2% de moldes de gastrópodos y culmina en un nivel superior de arenisca
media con un 80% de moldes de gastrópodos y bivalvos bien preservados. A partir del
hallazgo de la sismita y tsunamita y de los fósiles que esta última contiene, surgen ciertas
incertidumbres acerca del proceso tsunamigénico de formación de este depósito y cómo el
evento de paleo terremoto afectó al desarrollo de la Formación Horcón. El presente estudio
busca comprender el impacto y evolución de ambientes sedimentarios asociados al evento
de paleo tsunami en el límite marino transicional de la Formación Horcón. Para esto se
realizó un levantamiento de información paleontológica y estratigráfica en terreno, junto con
la generación y correlación de columnas, asociaciones de facies y análisis de muestras,
tanto a través de lupa binocular como por DRX en roca total, este último enfocado en
determinar la abundancia y grado de madurez de la glauconita (mineral utilizado para la
reconstrucción paleo ambiental de ambientes marinos). Lo anterior permite vislumbrar las
diferencias de facies del miembro inferior y superior. El miembro inferior presenta
variaciones laterales y verticales de facies, desde ambientes costeros, deltaico-fluviales a
uno de plataforma interna. La transición hacia el miembro superior está marcada por un
cambio brusco desde una plataforma interna, a uno de costa arenosa, manteniendo un
dominio costero tanto vertical como lateralmente. Este notorio cambio, se evidencia además
en la presencia dominante de glauconitas maduras en el miembro inferior, a diferencia del
miembro superior con escasa glauconita y mayor presencia de montmorillonita e illita. Los
organismos presentes en el depósito de tsunami fueron transportados y depositados en
base a su tamaño y flotabilidad. Los de mayor tamaño (vertebrados) fueron transportados
por arrastre en el fondo de la secuencia, mientras que los invertebrados (Incatella chilensis
y Retrotapes sp.) fueron llevados en suspensión y dispuestos en la capa superior de la
tsunamita.
The Horcón Formation is a sequence of sandstones, conglomerates and mudstones from the late Neogene that outcrops between 32°42' and 32°38'S on the coasts of central Chile. It is divided into a lower member characterized by the presence of glauconite and an upper one defined by the abundance of marine fossils. At the boundary between the members, a deposit associated with a paleoearthquake event (sismite) and one related to a tsunami event (tsunamite) are identified, the latter consisting of 4 levels, ranging from mediumgrained sandstone to a layer of conglomerates with vertebrate bones, a sandstone level with 2% of gastropod molds and culminating in an upper level of medium-grained sandstone with 80% of well-preserved gastropod and bivalve casts. From the finding of the sismite and tsunamite and the fossil content of the last one, certain uncertainties arise about the tsunamigenic process of formation of this deposit and how the paleoearthquake event affected the development of the Horcón Formation. The present study seeks to understand the impact and evolution of sedimentary environments associated with the paleo-tsunami event at the transitional marine boundary of the Horcón Formation. For this purpose, paleontological and stratigraphic information was collected in the field, along with the generation and correlation of columns, facies associations and sample analysis, both through binocular loupe and XRD patterns of total rock, the latter focused on determining the abundance and degree of maturity of glauconite (mineral used for the paleoenvironmental reconstruction of marine environments). This allows to identify key differences between the lower and upper member. The lower member shows lateral and vertical facies variations, from coastal, deltaic-fluvial to inner shelf environments. The transition to the upper member is marked by an abrupt change from an inner shelf to a sandy coast, maintaining coastal dominance both vertically and laterally. This notorious change is also evidenced by the dominant presence of mature glauconites in the lower member, in contrast to the upper member with scarce glauconite and greater presence of montmorillonite and illite. The organisms within the tsunami deposit were transported and deposited based on their size and buoyancy. The larger ones (vertebrates) were transported by dragging on the bottom of the sequence, while the invertebrates (Incatella chilensis and Retrotapes sp.) were carried in suspension and deposited in the upper layer of the tsunamite
The Horcón Formation is a sequence of sandstones, conglomerates and mudstones from the late Neogene that outcrops between 32°42' and 32°38'S on the coasts of central Chile. It is divided into a lower member characterized by the presence of glauconite and an upper one defined by the abundance of marine fossils. At the boundary between the members, a deposit associated with a paleoearthquake event (sismite) and one related to a tsunami event (tsunamite) are identified, the latter consisting of 4 levels, ranging from mediumgrained sandstone to a layer of conglomerates with vertebrate bones, a sandstone level with 2% of gastropod molds and culminating in an upper level of medium-grained sandstone with 80% of well-preserved gastropod and bivalve casts. From the finding of the sismite and tsunamite and the fossil content of the last one, certain uncertainties arise about the tsunamigenic process of formation of this deposit and how the paleoearthquake event affected the development of the Horcón Formation. The present study seeks to understand the impact and evolution of sedimentary environments associated with the paleo-tsunami event at the transitional marine boundary of the Horcón Formation. For this purpose, paleontological and stratigraphic information was collected in the field, along with the generation and correlation of columns, facies associations and sample analysis, both through binocular loupe and XRD patterns of total rock, the latter focused on determining the abundance and degree of maturity of glauconite (mineral used for the paleoenvironmental reconstruction of marine environments). This allows to identify key differences between the lower and upper member. The lower member shows lateral and vertical facies variations, from coastal, deltaic-fluvial to inner shelf environments. The transition to the upper member is marked by an abrupt change from an inner shelf to a sandy coast, maintaining coastal dominance both vertically and laterally. This notorious change is also evidenced by the dominant presence of mature glauconites in the lower member, in contrast to the upper member with scarce glauconite and greater presence of montmorillonite and illite. The organisms within the tsunami deposit were transported and deposited based on their size and buoyancy. The larger ones (vertebrates) were transported by dragging on the bottom of the sequence, while the invertebrates (Incatella chilensis and Retrotapes sp.) were carried in suspension and deposited in the upper layer of the tsunamite
Notas
Memoria (Geólogo)