Polinomorfos del Triásico superior resedimentados en los materiales marinos eocenos de Sant Llorenç del Munt (cuenca del Ebro, NE de España)

MIGUEL LÓPEZ BLANCO, N. SOLÉ DE PORTA

Abstract


The Eocene Sant Lloren$ del Munt fan-delta complex is located in northwestern Spain, attached to the southeastern margin of the Ebro basin. The Ebro basin was developed mostly during Tertiary time in response to the building of three mountain chains, the Pyrenees (N), the Iberian Chain (SW) and the Catalan Coastal Ranges (SE).

During Paleogene time the SE margin of the Ebro basin was subjected to transpressive conditions which led to the developement of a series of stnictures including folds, thrusts and strike-slip faults. This tectonic activity uplifted a structural block located SE of the Ebro basin. This uplifted block comprises mainly Paleozoic basement and Mesozoic cover rocks, and was the source area of a series of alluvial fans and fan deltas like the Sant Lloren$ del Munt complex. After this contractional-transpressive stage, under Neogene extensional conditions, the uplifted block developed a horst-and graben structure, preserved today in the Catalan Coastal Ranges. The Sant Lloren$ del Munt complex has well-developed subaerial and submarine portions, in whicb five main facies associations can be distinguished (proxirnal aüuvial-fan, dista1 alluvial-fanlfan-delta plain, fan-delta front, fan-delta slope/offshore and carbonate platfonn). These deposits have been subdivided into three orders of transgressive-regressive cyclicity, expressed in three different kinds of sequences. Thirty-two samyiles have been taken from the Bartonian marine facies in order to study their palynological content. These samples come from the lower part of the regressive megaunit of the lowest frequency transgressive-regressive sequence distinguished (Milany megasequence).

In 19 of these samples Triassic-derived pollen has been found. Most of these samples are from facies and parts of the highest frequency sequences which indicate high sediment supply episodes. More than fifteen different Tnassic palynomorphs, most of them never before found in uppeir Triassic samples near the studied area, have been identified. Palynornorphs are typical of Tuvalian and Norian stages, and upper Tnassic (Classopollis, Patinasporites densus, Vallasporites ignacii, Samaropolle~tites speciosus, Triadispora verrucata and Camerosporites secatus ).

Given the high content of Triassic pollen, present in most of the samples and abundanit in some of them, the source area of this complex should be nch in upper Triassic rocks with abundant flora. The tetrads of some of these Tnassic-denved pollen grains indicate a short transport distance fro:m the source area. They probably came from the present Valles area. 14s deduced from the proximal alluvial fan conglomerate composition, the source area of the Sant Llorenq del Munt complex was not exclusively constituted of Mesozoic cover rocks. It was a mixed source a;rea with also the presence of Paleozoic basement rocks, which are the rnost important clast-component of the proximal alluvial fan conglomerates (laterally equivalent to the sampled section). Based on the palynological study, we consider the source area richer in upper Tnassic rocks than tpreviously tbought from study of the alluvial fan conglomerate-clasts composition. There is a certairi relationship among the sequences, the content of resedimented Tnassic-denved poiien and the degree of preservation of the pollen grains. Rere is also a relation between the sampled facies and the content o.F resedimented pollen.


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