Sedimentología y paleoecología de un estuario del litoral Pacífico del Istmo Centroamericano Primordial (Mioceno Medio, Costa Rica)
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[1] Universidad de Costa Rica
Universidad de Costa Rica
Hospital, Costa Rica
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- Localización: Revista geológica de América Central, ISSN 0256-7024, Nº. 3, 1985, págs. 1-68
- Idioma: español
- Enlaces
- Dialnet Métricas: 2 Citas
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Resumen
- español
En el área de Punta Judas, en el litoral del Pacifico de Costa Rica, se estudió una sección de 1300 m de espesor que documenta la evolución de un estuario durante el Mioceno Medio bajo. Este estuario se orienta hacia el Pacifico y estudio delimitado por un retropais montañoso. Fue alimentado a partir de un sistema potamal, el que suministró grandes volúmenes de sedimentos volcaniclisticos, predominantemente arenosos. Además de la influencia potamal, se verificó un menor aporte fluvial, a partir de rios que desembocaron lateralmente al estuario.
Se identifican megaciclos de programación. Estas secuencias de colmatición se inician por depositos de estuario abierto y culminan con sedimentos de mang!ar, así como de estuario interno, de aguas salobres a hiperhalinas. Fue posible diferenciar 10 tipos mayores de facies, que reflejan los cambios ambientales y las sucesiones ecológicas correspondientes. Las evaluaciones paleoecológicas están basadas, en lo principal, en ricas tafocenosis de moluscos, echinoideos e ichnofósiles.
El estuario analizado tuvo un carácter mixto con respecto a los actuales golfos estuarinos del Pacífico de Centroamérica, de origen tectónico, y aquél los que bordean al Atlántico en latitudes medias, de tipo epicontinental, los que han sido clásicamente referidos por la literatura biológica y sedimentológica.
- English
The rise of the Central American land-bridge is characterized by a Campaaian to Eocene island are stage, an Oligocene to Miocene svell stage, and a Pliocefte to Recent aountain chain stage, During the Oligocene to Miocene svell eta¡e the volcanic edifices and insular shelves of the foregoiog stage gradually grev toaether, foraing the primordial Central American isthaus. Thus, extensive tidal flat areas establiahed which occupied, especially during the Middle Miocene, large areas of the actual territory of Costa Rica.
Tbe present study deals vith a Middle Miocene section si tuated at Punta Judaa on tbe Pacific coast of Costa Rica. This section provides a nearly complete record of the depositional enviroa.ents that characterize the Pacific estuaries of the primordial Central American isthmus.
Tbe 1300 • of aainly litharenitic volcaniclastic sediments are extremely fossiliferous, especially in mollusks, echinoids, and trace fossils . Thus, sedimentological interpretationa can be choroughly controlled and complemented by palecological data. 10 different facies typea could be identified that reflect a great variety of depositional environments rangina froa open estuary through intertidal shoals and mangrove svamps to mesohaline marginal shallovs, aod even slightly hypersaline lagoons.
Facies character and pattero in the subtidal area vas largely controlled by the teaporarinesa and volwae of the terrigenous input vhich was transferred serosa the deltaic depocenter and overtaken by the estuarine circulatory regi me Ecoloaical auccesaiona that devélopped in response to the interaittent sediment suply commonly are cbaracterized by native background communities ( supension feedera - molluske) and opportuniatic after-event communitie (sediment feederscefossils) senau SEILACHER (1980). · · .
In the intertidal area lower flats are typified by cross-laminated sandstones, whereas in mid and upper flats mangrove swamp deposita are predominant .
This facies unit frequently interfingers with granule and pebble conglomerates that enterered from minor lateral river mouths . The coarse river bed load is redistributed both in meandering tidal channels and in straight beach-parallel channels. The latter show an unidirecional filling pattern, the size ranges being fashioned into large graded sigmoidal foresets that end up in well-developped toe sets; mud drapes prove that these features have to be considered as ti· dal bundles. Tempestites are fairly frequent within the mangrove swamp deposita, but occur also in nearly all other facies types.
Folloving these criteria it mustbe concluded that the Punta Judas estuary was laterally bounded by a mountainous hinterland and headed by a stream system that supplied large amounts of fine- to medium-sized volcaniclastic sediments.
Despite the lov salinities in its uppermost sector, the estuary vas strongly controlled by aesotidal currents which obviously folloved an asymmetrical circulatory pattern. It vas an essentially sandy estuary and must have reached dimensiona comparable to the actual Gulf of Nicoya. There is no evidence for the existence of an outer bar, of barrier islands, or ebb- and flood-tidal deltas. Hence, the Midle Miocene Punta Judas estuary has to be considered as an open embayent.
The Punta Judas section documenta at least 2 complete filling cycles ( progradations).
The return from shoreline to open estuarine environments always case off very abruptly. This suggests rapid sea-level rises, followed by gradual lowerings. The absence of both angular unconformities and hardgrounds provea that subsidence was fairly constant, Most probably, the Punta Judas estuarine embayment vas a direct graben-like tectonic feature.
Theae data coincide vith the tectonical and geographical framevork of the actual Pacific estuaries of Central Aaerica. Differences exist, hovever, in that the Punta Judas estuary reached salinities as lov as the B mesohaline range.
This is not cmmon to the actual ones vhich are essentially normal marine to high polyhaline. Thus, ve propose an environmental model which falls midvay between the strongly tectonically controlled Pacific centroamerican type and the epicontinental type vhich characterizes aid latitude Atlantic coastal plains.
- español
