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| Natura: | Artículo científico |
| Lingua: | en |
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Universidad Nacional Autónoma de México
2006
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| Accesso online: | https://www.redalyc.org/articulo.oa?id=57230207 |
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- Chronostratigraphy and paleomagnetism of the Balsas Group in the Tuzantlán-Copalillo basin, northern Guerrero state, Mexico Roberto S. Molina Garza Amabel Ortega Rivera Ciencias de la Tierra Mexico Cenozoic Guerrero paleomagnetism magnetostratigraphy The Cenozoic sedimentary fi ll of the Tuzantlán-Copalillo basin, in northeast Guerrero state, Mexico,is assigned to two distinct stratigraphic units. The fi rst unit consists of a ~200 m thick fi ning upwardsection that grades eastward from massive cobble and boulder conglomerate into interbedded coarseconglomeratic sandstone and mudstone. The sandstone and mudstone facies of this unit has been referredto as the Zacango beds, but together with the conglomeratic section to the west it is here assigned to theBalsas Group based on demonstrable correlation and lateral facies changes that link these strata. Thesecond unit disconformably overlies the Balsas Group, and comprises a tuffaceous fl uvio-lacustrine unitthat has been mapped as the Oapan Formation. These strata are lithologically dissimilar to the formallydefi ned Oapan Formation elsewhere in the Balsas river basin. We do not consider the fl uvio-lacustrinetuffaceous unit part of the Balsas depositional cycle; we note, however, that including these strata inthe Oapan Formation thus requires a re-defi nition of this unit. Volcanic rhyolitic rocks were emplacednear the end of the Balsas depositional event, and locally they are interbedded in the sequence. Mineralseparates of biotite and sanidine for one of these volcanic units produced concordant 40Ar/ 39Ar dates ofca. 33 Ma. We hypothesize that emplacement of volcanic rocks changed the depositional style in the basinby reducing stream gradient and closing drainages. Oapan strata were thus deposited under differentconditions than the Balsas Group. Magnetostratigraphic sampling of a ~130 m thick composite sectionfrom two localities, suggests that that the Balsas Group sequence in the Tuzantlan-Copalillo basinwas deposited in the interval between chrons C17n and C12r, between about 37 and 32 Ma. The meanpaleomagnetic direction for the Amacuzac river locality (D=340.8º, I=34.3º; k=62.6, n=9, α95=6.6º) isslightly discordant, whilst at the Acapulco highway locality (D=351.0º, I=39.9º; k=70.4, n=9, α95=6.2º)the direction is concordant. The Amacuzac river result indicates a 13.2° ± 8° counterclockwise rotation withrespect to the reference direction, and it is attributed to transpression in the region. However, the BalsasGroup does not record rotations as high as those observed in Cretaceous and Paleocene rocks, whichdemonstrates that the process responsible for the rotations was waning by Early-Oligocene time. 2006 artículo científico 1026-8774 https://www.redalyc.org/articulo.oa?id=57230207 en http://www.redalyc.org/revista.oa?id=572 Revista Mexicana de Ciencias Geológicas application/pdf Universidad Nacional Autónoma de México Revista Mexicana de Ciencias Geológicas (México) Num.2 Vol.23