The lower Cretaceous Springhill sandstones are the major hydrocarbon reservoirs in the Austral basin. In the La Tehuelche block (south-central part of the Santa Cruz province, Argentina, Fig. 1) these sandstones show a transgressive and backstepping pattern from west to east (Fig. 3). They are generally thin, thickness ranging from 5 to 90 m and is absent over some Tobífera highs. To understand the depositional origin of these reservoirs in the La Tehuelche area, we did a detailed sedimentological study using 120 m of conventional core from 9 wells and electric logs from 19 exploratory wells. This study reveals 11 sedimentary facies and 2 main facies associations: 1) an inner estuary association and 2) an estuary mouth and shallow shelf association. The first association is located in the lower part of the unit and consists of 5 sedimentary facies: 1) Sigmoidal cross-bedded sandstones (subtidal channel-bar complex): this facies is composed of mediumto- very-coarse-grained, cross-bedded sandstones with coaly mud drapes and double mud layers, reactivation surfaces and subordinated heterolithic stratification (Figs. 4a and 4b). 2) Rhythmites sandstones (tidal channels): it consists of fine-to-coarse-grained cross-bedded sandstones with abundant mud drapes, rhythmic alternation of the sandstone and mudstone layers, fining-upward trends, with erosional bases and basal lags (Fig. 4c). 3) Cross-bedded and shaly sandstones (tidal creeks): this facies is composed of coarse-to-very-coarse-grained cross-bedded to massive sandstones and fine-grained-toshaly sandstones. They show fining-upward trends 40 to 60 cm with erosional bases and basal lags. It is very frequent the presence of heterolithic stratification, mud drapes, coal and root marks to the top of the individual sequences (Figs. 4d and 5a). 4) Heterolithic sandstones (mixed sand flat): this facies comprises medium-grained sandstones and subordinated shales with abundant flaser and wavybedding. 5) Coaly shales (marsh): it consists of dark coaly shales, with abundant root marks and carbonaceous fragments (Fig. 5b) and occasionally thin fine-grained sandstone bed. This facies association conforms fining-upward and shallowing-upward sequences, typical of the tidal flats succesions (Weimer et al., 1982). The estuary mouth and shallow shelf facies association is predominantly sandy and rests directly either on a major flooding surface or on a thin transgressive interval with Glossifungites ichnofacies (Fig. 5d) generated during erosional shoreface retreat. Except when the former facies association is absent it rests on Tobífera rocks. This association comprises 6 sedimentary facies: 1) Bioturbated fine-to-very-fine-grained sandstones (lower shoreface): this facies consists of fine-to-very finegrained sandstones with abundant glauconite, frequent valves and carbonaceous fragments. The primary sedimentary structures are masked by the intense bioturbation (Fig. 5c). 2) Bioturbated coarse-grained sandstones (sandy shoals): it is composed of medium-to-very-coarse-grained bioturbated sandstones, with glauconite, valves, carbonaceous and log fragments, diffuse stratification and occasionally flaser bedding (Fig. 6a). 3) Sandstones with fine-grained drapes (marginal marine bar): this facies is composed of medium-to-coarsegrained cross-bedded sandstones with glauconite and fossil fragments. It is common at the toe of the sets wavy bedding with some degree of bioturbation (Fig. 6b). 4) Sandstones with hummocky cross-stratification (lower to middle shoreface, storm-dominated): it consists of fine-to very-fine-grained glauconitic sandstones with hummocky cross-stratification and valve fragments (Fig. 6c). 5) Cross-bedded fine-grained sandstones (upper shoreface): this facies is composed of fine-grained glauconitic cross-bedded sandstones (Fig. 6d). 6) Green shales (offshore): this represents the Palermo Aike Formation deposits and consists mainly of glauconitic shales. This association is characterized by thinned, incomplete facies successions that conform retrogradational sequences. Based on the sedimentary facies types in the two associations and their vertical and lateral relationships we interpret the studied interval to represent tide-dominated estuarine facies. According to the models of Dalrymple et al. (1992) and Allen & Posamentier (1994), we recognize the two main facies associations that characterized the estuarine deposits: 1) the inner estuary facies and 2) the estuary mouth and seaward and lateral shelf facies. The most proximal facies in the inner estuary are represented by the tidal creeks and marsh facies, and the most distal by the subtidal channel and bar complexes. The estuary mouth deposits are represented by the sandy shoal and the marginal marine bar and the shallow shelf facies comprises mainly lower shoreface (fair weather and storm weather) and upper shoreface facies. In the study area we could identify 8 cycles or sedimentary units bounded by major marine flooding or ravinement surfaces. Each cycle represents a transgressive pulse in the overall succession. As transgression proceeded, the paleoshoreline had migrated to the east. The lateral and vertical facies relationships are shown in figure 9. The relative position of the facies in each cycle are in the paleogeographic maps of figures 10 to 17.
La Formación Springhill en el ámbito del área La Tehuelche (centro de la provincia de Santa Cruz) conforma una sucesión retrogradante-transgresiva en sentido oeste-este de areniscas y pelitas en un ambiente estuarino. En la misma se identificaron ocho ciclos o unidades limitadas por superficies de "ravinement" o de inundación marinas. Un detallado estudio de testigos-corona y perfiles eléctricos de pozos permitió reconocer las facies que caracterizan al complejo estuarino. Se identificaron dos asociaciones de facies, de interior de estuario y de desembocadura de estuario-plataforma somera. La asociación de facies de interior de estuario comprende cinco tipos de depósitos: 1) complejos de barra-canal submareales, compuestos por areniscas medianas a muy gruesas, con estratificación cruzada sigmoidal a tangencial y heterolítica, superficies de reactivación y restos carbonosos, 2) canales de marea intermareales a submareales, conformados por areniscas cuarzosas finas a gruesas, ordenadas en secuencias granodecrecientes de base neta, con estratificación entrecruzada rítmica, recubrimientos pelíticos y abundantes restos carbonosos, 3) canales de marea pequeños en la zona intermareal a supramareal, 4) planicie mixta y 5) pantano, compuesta por pelitas carbonosas con restos de troncos, marcas de raíces y delgados niveles de carbón. En esta asociación se desarrollan secuencias de facies de planicies de marea. En contacto neto o erosivo se desarrolla sobre estos depósitos la asociación de facies de desembocadura de estuario-plataforma somera. El contacto entre las dos asociaciones de facies principales suele estar indicado por un nivel de retrabajo de areniscas gruesas muy bioturbadas, en ocasiones con desarrollo de ichnofacies de Glossifungites, la cual indica la presencia de discontinuidades erosivas (límites de secuencia o superficies de "ravinement"). Los depósitos de desembocadura comprenden dos tipos de facies: 1) cuerpos someros de areniscas medianas a muy gruesas intensamente bioturbadas, con estratificación difusa, fragmentos de valvas y restos de troncos carbonizados y 2) barras marinas marginales afectadas por mareas, compuestas por areniscas medianas con estratificación entrecruzada y restos de valvas de pelecípodos .Dentro de los depósitos de plataforma somera se diferenciaron facies de "shoreface" alto y de "shoreface" bajo. Las primeras están constituidas por areniscas finas con glauconita y estratificación entrecruzada de mediano a bajo ángulo. Las facies de "shoreface" bajo comprenden areniscas glauconíticas finas a muy finas intensamente bioturbadas, con abundantes bioclastos y en ocasiones, niveles con estratificación "hummocky". Esta facies representa la posición más externa del complejo estuarino, muestra una amplia distribución areal y aparece en el tope de la sucesión transgresiva de la Formación Springhill. Las mismas son cubiertas en contacto neto por pelitas de costa-afuera de la Formación Palermo Aike.
