________________________________________________________________________The ephemeral nature of most sedimentation processes and the fragmentary character of the sedimentary record are of first-order importance. Despite a basic uniformity of external controls on sedimentation resulting in markedly similar lithologies, facies, facies associations and depositional elements within the rock record across time, there are a number of secular changes, particularly in rates and intensities of processes that resulted in contrasts between preserved Precambrian and Phanerozoic successions. Secular change encompassed (1) variations in mantle heat, rates of plate drift and of continental crustal growth, the gravitational effects of the Moon, and in rates of weathering, erosion, transport, deposition and diagenesis;(2) a decreasing planetary rotation rate over time; (3) no vegetation in the Precambrian, but prolific microbial mats, with the opposite pertaining to the Phanerozoic; (4) the long-term evolution of the hydrosphere-atmosphere-biosphere system. A relatively abrupt and sharp turning point was reached in the Neoarchaean, with spikes in mantle plume flux and tectonothermal activity and possibly concomitant onset of the supercontinent cycle. Substantial and irreversible change occurred subsequently in the Palaeoproterozoic, whereby the dramatic change from reducing to oxidising volcanic gases ushered in change to an oxic environment, to be followed at ca. 2.4-2.3 Ga by the "Great Oxidation Event" (GOE); rise in atmospheric oxygen was accompanied by expansion of oxygenic photosynthesis in the cyanobacteria. A possible global tectono-thermal "slowdown" from ca. 2.45-2.2 Ga may have separated a preceding plate regime which interacted with a higher energy mantle from a ca. 2.2-2.0 Ga Phanerozoicstyle plate tectonic regime; the "slowdown" period also encompassed the first known global-scale glaciation and overlapped with the GOE. While large palaeodeserts emerged from ca. 2.0 -1.8 Ga, possibly associated with the evolution of the supercontinent cycle, widespread euxinia by ca. 1.85 Ga

The volcaniclastic Tepoztlán Formation (TF) represents an important rock record to unravel the early evolution of the Transmexican Volcanic Belt (TMVB). Here, a depositional model together with a chronostratigraphy of this Formation is presented, based on detailed field observations together with new geochronological, paleomagnetic, and petrological data. The TF consists predominantly of deposits from pyroclastic density currents and extensive epiclastic products such as tuffaceous sandstones, conglomerates and breccias, originating from fluvial and mass flow processes, respectively. Within these sediments fall deposits and lavas are sparsely intercalated. The clastic material is almost exclusively of volcanic origin, ranging in composition from andesite to rhyolite. Thick gravity-driven deposits and largescale alluvial fan environments document the buildup of steep volcanic edifices. K-Ar and Ar-Ar dates, in addition to eight magnetostratigraphic sections and lithological correlations served to construct a chronostratigraphy for the entire Tepoztlán Formation. Correlation of the 577 m composite magnetostratigraphic section with the Cande and Kent (1995) Geomagnetic Polarity Time Scale (GPTS) suggests that this section represents the time intervall 22.8-18.8 Ma (6Bn.1n-5Er; Aquitanian-Burdigalian, Lower Miocene). This correlation implies a deposition of the TF predating the extensive effusive activity in the TMVB at 12 Ma and is therefore interpreted to represent its initial phase with predominantly explosive activity. Additionally, three subdivisions of the TF were established, according to the dominant mode of deposition: (1) the fluvial dominated Malinalco Member (22.8-22.2 Ma), (2) the volcanic dominated San Andrés Member (22.2-21.3 Ma) and (3) the mass flow dominated Tepozteco Member (21.3-18.8 Ma).

123 ResumenLa estratigrafía del pozo profundo San Lorenzo Tezonco registra una intensa actividad volcánica en la Cuenca de México y alrededores durante los últimos 20.1 Ma. La columna litológica de 2008 metros de profundidad está dominada por material volcánico, ya sea como flujos de lava o depósitos piroclásticos (97 %) y solamente los 70 metros más superficiales están constituidos por depósitos lacustres (3 %). Con base en descripciones de campo y análisis de laboratorio (petrografía, geoquímica de roca total y fechamientos radiométricos), fue posible reconocer cuatro paquetes litológicos a lo largo del pozo, los cuales fueron correlacionados con rocas expuestas en superficie. La parte inferior del pozo está representada por rocas pertenecientes a la Formación Tepoztlán (876 -2008 m de profundidad) que varían en composición de andesita basáltica a riolita y con edades de 13 a 20.1 Ma. En superficie esta formación aflora alrededor de los poblados de Malinalco y Tepoztlán. Entre las profundidades de 581 y 875 m, se encuentran rocas interpretadas como parte de la Sierra de las Cruces, con una composición de andesita a dacita y edades entre 0.9 y 5 Ma. Hacia la parte superior del pozo (510 -580 m) aparecen rocas volcánicas interpretadas como parte del Cerro de la Estrella, de composición andesítica y fechadas en 0.25 Ma. El último paquete de material volcánico encontrado en el pozo fue interpretado como parte de la Sierra Santa Catarina (70 -120 m) de composición andesítico-basáltica, con edades inferiores a 0.25 Ma, probablemente del Holoceno. Los depósitos lacustres coronan la columna estratigráfica, con edades de hasta 34 mil años.Adicionalmente, se concluye que las rocas pertenecientes a la Formación Xochitepec, que aflora alrededor de Xochimilco, en la ciudad de México, son de 1.23 a 1.66 Ma de edad, y no del Oligoceno como se había propuesto en trabajos previos. Estas nuevas edades junto con la composición química permiten correlacionar a rocas de Xochitepec con la Sierra de las Cruces.Palabras clave: Pozo profundo San Lorenzo Tezonco, Cuenca de México, estratigrafía volcánica, geocronología. Abstract The underground stratigraphy of the San Lorenzo Tezonco deep well records intense volcanic activity in the Mexico

The Singhbhum Craton has a limited Palaeoproterozoic supracrustal record, which suggests a three-part history, comprising: a long period of high freeboard and palaeosol formation on granitoids; subsequent rift-related mafic–ultramafic volcanism and subordinate sedimentation (c. 2.25–2.1 Ga: Dhanjori and Jagannathpur basin-fills; possibly also Simlipal, Malangtoli and Ongarbira basin-fills), which overlapped locally with mafic soil formation; and a major regression at around 2.0 Ga. Following a long hiatus, the approximately 1.6 Ga Dhalbhum–Dalma succession was laid down, probably under continental conditions. This rather truncated record stands in contrast to the chronologically and geographically much more widespread supracrustal basin-fills of the Kaapvaal Craton, and there appears to be an overall poor comparison between these two early Precambrian crustal blocks. However, on Kaapvaal, three analogous events are identified: widespread approximately 2.2 Ga mafic volcanism, followed by a well-developed palaeosol and a major transgression prior to 2.05 Ga. The three shared events between the two cratons are compatible with the postulate of a global, approximate 2.45–2.2 Ga shutdown of magmatic and tectonic geodynamics, with the origin of the triumvirate directly reflecting its resumption again after about 2.2 Ga. We recognize here that a large diversity of views on Singhbhum's geodynamic history exists, predicated on a lack of precise geochronology and commonly poor outcrops, and the current hypotheses are presented with these factors in mind.

No abstract

No abstract

The reservoir potential of volcanic and associated sedimentary rocks is less documented in regard to groundwater resources, and oil and gas storage compared to siliciclastic and carbonate systems. Outcrop analogue studies within a volcanic setting enable to identify values followed by tuffs, conglomerates, sandstones and tuffaceous breccias. On the contrary, the highest permeabilities can be found in the conglomerates, followed by tuffs, tuffaceous breccias, sandstones and lavas. The knowledge of these petrophysical rock properties provides important information on the reservoir potential of volcanic settings to be integrated to 3D subsurface models.

Volcanic terrains such as magmatic arcs are thought to display the most complex surface environments on Earth. Ancient volcaniclastics are notoriously difficult to interpret as they describe the interplay between a single or several volcanoes and the environment.