M. A. Rodriguez-Pascua | Geology Reports

USGS2010

Palaeoenvironmental significance of diatom and vertebrate fossils from Late Cenozoic tectonic basins in west-central México: A review

Pronounced lacustrine sedimentation developed in west-central México during the late Miocene, between approximately 11 and 7 Ma. This was in response to tectonic extension associated with the initial emplacement of the late Miocene substrata of the Trans-Mexican Volcanic Belt. Climatic conditions in west-central México during this interval were relatively warm and humid based on the widespread distribution of interpreted lacustrine beds. Following a latest Miocene (8.0–5.4 Ma) stage of arid conditions and greatly reduced deposition of fine-grained lacustrine sediments, extensive, east–west oriented, relatively deep, perennial lakes ensued. They mark the early Pliocene (5.3–4.0 Ma). Lower Pliocene diatomites contain the same diatom species (e.g., Stephanodiscus carconensis and Tertiarius aff. baikalensis) found in rocks of this age in the western United States. The relatively warm and humid conditions that characterized this interval in central México coincide with a period of high-latitude warming, higher global sea level, and a reduction in size of the Antarctic Ice sheets. Because the Central American Seaway persisted until at least the latest Miocene, it might have acted to increase precipitation in central Mexico. This could have continued into the earliest Pliocene. Mexican Pliocene mammalian faunas also support a savanna setting with moist and warm conditions prevailing at the time. Shallow lakes and fluvial conditions dominate after 4.0 Ma, until the end of Pleistocene. A combination of reduced precipitation, due to general global cooling and drying, as well as volcanic and tectonic processes, are presumed to have been the cause for this mid-Pliocene reduction in lake size and extent in central México.

Chincua, Ixtlahuaca↗

USGS2010

Ancient earthquakes from archaeoseismic evidence during the Visigothic and Islamic periods in the archaeological site of “Tolmo de Minateda” (SE Spain)

The ruins of the ancient settlement of “El Tolmo de Minateda” are one of the best representative archaeological sites within the Albacete Province (SE Spain), characterized by a well-preserved record for the last ~3800 yr. The present ruins record an almost continuous of occupation from the Late Bronze Age (Iberian Culture, from ca. third century B.C.) to the High Middle Ages, including intervening and successive Roman, Visigoth, and Muslim city remains. The eventual Muslim settlement was abruptly abandoned and destroyed during the ninth–tenth centuries A.D., leaving a lack of any archaeological evidence of war or decay. Another previous anomalous archaeological episode of abrupt city abandonment and destruction is recorded during the Visigothic Period (seventh century). The archaeological record of this city supports evidence for earthquake damage linked to both periods of city abandonment and destruction, including oriented collapse of walls, watchtowers, and columns, oriented cracking of walls and column drums, as well as in situ broken pottery, abrupt abandonment of kilns, and anomalous sedimentary infilling of canals and water-supply facilities. Additionally, large-scale rockfalls containing Visigothic carved tombs are also apparently associated with both episodes, constituting one of the few instances of combined geoarchaeological evidence of earthquake ground effects ever reported. As a means of testing the theoretical archaeoseismic potential of this site, we obtained an archaeoseismic quality factor (AQF) value of 0.074.

Tolmo de Minateda↗