Giorgio Basilici

Sedimentary facies in an extensional and deep-lacustrine depositional system: the Pliocene Tiberino Basin, Central Italy

Abstract The Tiberino Basin was formed during Plio-Pleistocene extensional tectonic activity in Central Italy. The Pliocene sediments that fill this structure consist predominately of fine-grained clastic lithofacies deposited in a lacustrine system; four facies associations have been distinguished. Facies association A was deposited in a deep-offshore lacustrine environment and consists of massive, laminated, bluish-gray marly clays that lack benthonic fauna but have preserved organic matter (leaves, wood). This facies association is the main lacustrine deposit, whereas the other three represent marginal facies. Facies association B is interpreted as a delta system deposit; it has Gilbert-type sandy gravel bodies with foresets up to 50 m high and prodelta bodies consisting of laminated to massive marly clays that alternate with sandy and gravelly mud strata. Association C corresponds to a coastal environment, where two sub-environments are recognized: (1) a wave-dominated coastline characterized by interbedded muddy and sandy strata with sedimentary structures attributed to hummocky cross-stratification; and (2) clayey silts and lignites interpreted as a coastal wetland; a spectacular fossil forest is preserved in situ within this unit. Association D, formed on the distal part of a muddy alluvial fan, consists of sheet-flood clayey sandy silts, that alternate with incipient paleosols; rare, ribbon-channel fills are also present. Paleoenvironmental reconstruction indicates that the Tiberino Basin was occupied by a narrow, deep meromictic lake during the Pliocene. The size, shape, depth and sub-environment distribution of this lake were controlled by tectonic phenomena, such as the high subsidence rate, which caused the lake to be large and deep. Only in the few coastal areas that had high sedimentary input were wetland or alluvial fan deposits formed.

Arquitetura e mecanismos deposicionais em sistemas aluviais holocênicos de clima árido no oeste argentino e comparação com depósitos neocretáceos brasileiros

The Pre-Cordillera in the northwest of Argentine is characterized by geomorphological features formed during Miocene Andes Cordillera uplifting. The arid weather and compressive tectonic regime propitiated the origin of countless sedimentary basins filled by alluvial fans and playa-lakes deposits. Because of the outcrops abundance and their qualities these basins are important sites for direct observation of depositional alluvial architectures and facies analyses. The description of some deposits in the Rio Jachal valley (neighborhood of San Jose de Jachal), and Pie de Palo desert alluvial plain in San Juan, allowed the recognition of main depositional mechanisms in these environments. The analyses were based in the longitudinal cross sections that show intertonguing of the alluvial fans, playa-lakes and eolian deposits. It was characterized the internal and external geometries of the bodies by hierarchical surfaces and macroforms analyses to make the interpretation of the sedimentary process and the depositional dynamics. The facies variation from proximal to distal deposits recorded the evolution of confined or unconfined non-cohesive debris flow to turbulent and laminar flows, resulting by the transport stream energy decrease of ephemeral torrential currents associated with storm periods. Due to the annual low pluviosity (70 mm/y), the sedimentation rate is very low, mainly suggested by the vegetation and soil developed. Although this study has been made in a restrict area with a very recent sedimentary history initiated only in the Late Pleistocene to Holocene, the facies distribution and the depositional mechanisms conditioned by geomorphologic positions in the alluvial fan systems and the sedimentologic characteristics conditioned by the arid climate allow to compare these deposits with the Brazilian neocretaceous, in the Bauru Basin, that shows similar sedimentologic and architecture characteristics.

Intermontane eolian sand sheet development, Upper Tulum Valley, central-western Argentina

The intermontane Upper Tulum eolian sand sheet covers an area of ca. 125 km² at north of the San Juan Province, central-western Argentina. The sand sheet is currently an aggrading system where vegetation cover, surface cementation and periodic flooding withhold the development of dunes with slipfaces. The sand sheet surface is divided into three parts according to the distribution of sedimentary features, which reflects the variation in sediment budget, water table level and periodic flooding. The central sand sheet part is the main area of eolian deposition and is largely stabilized by vegetation. The sedimentary succession is 4 m thick and records the vertical interbedding of eolian and subaqueous deposits, which have been deposited for at least 3.6 ky with sedimentation rates of 86.1 cm/ky. The construction of the sand sheet is associated with deflation of the sand-graded debris sourced by San Juan alluvial fan, which is available mainly in drier fall-winter months where water table is lower and wind speeds are periodically above the threshold velocity for sand transport. The accumulation of sedimentary bodies occurs in a stabilized eolian system where vegetation cover, thin mud veneers and surface cementation are the main agents in promoting accumulation. The preservation of the sand sheet accumulations is enabled by the progressive creation of the accommodation space in a tectonically active basin and the continuous burial of geological bodies favored by high rates of sedimentation.

A prodeltaic system controlled by hyperpycnal flows and storm waves: reinterpretation of the Punta Negra Formation (Lower-Middle Devonian, Argentine Precordillera)

The Punta Negra Formation (FPN) constitutes a Lower-Middle Devonian sandstone unit located in the Argentine Precordillera. Based on detailed facies and ichnological analyses, this unit, which had been previously interpreted as a deep-water system, has been reinterpreted in this study as a prodelta system, in which the distribution and sedimentation of the clastic deposits were controlled by hyperpycnal flows associated with storms. Five lithofacies attest the deposition by unidirectional hyperpycnal flows combined with storm-generated oscillatory flows. Fossil traces testify variable energetic conditions of the depositional system with the colonization of opportunistic organisms immediately after the deposition of high energy combined flows and the colonization of more specialized organisms during the long low-energy periods. Three architectural elements, deposited from coastline to offshore, have been recognized: 1) Channelized sandstone, composed of the thickest and coarsest sandstone beds and interpreted as the proximal portion deposited close to the mouth of the distributaries of the delta system; 2) Tabular sandstone, which are composed of combined and oscillatory flow deposits with lesser thickness, grain size and spacing of the undulated bed forms; 3) Tabular sandstone interbedded to mudstone, which represents the most distal architectural element and is composed of thin sandstone and thicker mudstone strata, deposited by low-energy combined flows.

Unusual thick eolian sand sheet sedimentary succession: Paleoproterozoic Bandeirinha Formation, Minas Gerais

Some present-day eolian sand sheets have small and width dunes, called zibars, as common type of depositional morphology. Their formation is related to different stabilizing factors, which reduce the availability of clastic materials for eolian processes. In fact, zibars are dunes which do not have time to develop a larger dune with slipface (proto-dune). Dunes in sands sheet areas generally are constructed by wind ripple laminations and commonly generate sedimentary succession less than 20 m thick, which are the consequence of low sand supply. This study deals with an uncommon eolian sand sheet sedimentary succession more than 50 m thick, Proterozoic in age, known as Bandeirinha Formation. This paper tries to explain the anomalous thick of this sand sheet sedimentary succession. High thickness was probably the result of a high input of sand material, combine with low availability of sand, thus allowing the only construction of proto-dunes (zibars). Early cementation, due to near-surface evaporation of saline water, has been proposed as main factor that reduced the sand availability into this eolian system. Finally, the subsidence processes related to the first stage of rift Espinhaco Basin must have generated the accommodation space to preserve the sand sheet succession.

ARCHITECTURAL REINTERPRETATION AND ACOUSTIC FACIES OF THE LOWER PORTION OF THE APIÚNA UNIT (CAMBRIAN DEEP-WATER DEPOSITIONAL SYSTEM, SANTA CATARINA STATE, BRAZIL)

An architectural model of an ancient sand-rich and mixed mud-sand ramp deep-water sedimentary environment, outcropping near Apiuna, Santa Catarina State, Brazil, is presented. This research contributes in the following way to the expl oration sector of the petroleum industry: i) with a conceptual model of reservoirs in ramp deep-water systems and ii) with synthetic seismograms, based on the outcrops. Two types of ramp are recognized. One type, sand-rich, is characterized by extensive sandy- sheet reservoirs, not linked to proximal channels and showing an apparent casualness of construction. The mechanisms that gener ated the sandy-sheets were bipartite: a granular flows (debris flows s.l.), in the lower part, and a turbulent flow (turbidity currents), in the upper part. The different velocity and efficiency of the two flows produced a distinct lithological and architectural distribut ion of the resultant deposits: medium grain size, amalgamated and thick sandstone in the proximal part; fine - very fine grain size and th in sandstone in the distal part. (2) The second type, mixed sand-mud, is characterized by a channel-levee complex and overbank deposits. The two ramp systems represent a vertical variation of the depositional environment, controlled probably by tectonic- morphologic factors. Linked to the architectural model we produced synthetic seismograms. The properties needed for constructin g the synthetic seismograms (densities and P-wave velocities) are evaluated from theoretical petrophysical tables. The synthetic seismograms allow defining a seismic forward model and characterizing the acoustic facies of ramp systems.