Luca Ragaini

Basin physiography and tectonic influence on sequence architecture and stacking pattern: Pleistocene succession of the Canoa Basin (central Ecuador)

Facies, shell bed features, and sequence stratigraphic framework for the shallow-marine Pleistocene upper Canoa and Tablazo Formations are presented, based on outcrop data from the southern coast of Cabo San Lorenzo, Ecuador. Sediments of this succession exhibit a distinct cyclic pattern, consisting of a stack of eight depositional sequences (cyclothems) likely developed under the main control of orbitally induced sea-level changes. As a rule, within the studied interval an idealized cyclothem is composed of a transgressive systems tract (TST) and a highstand systems tract (HST), whereas deposits attributable to the lowstand and falling-stage systems tracts are not present. Transgressive lithosomes may be defined by estuarine deposits interposed between the sequence boundary and the ravinement surface (back-barrier wedge) and by upward fining shoreface to inner-shelf facies successions above the ravinement (backstepping shelf wedge). Separated by an expanded siliciclastic core, hiatal shell concentrations occur at the base (onlap shell beds) and the top (backlap shell beds) of the transgressive shelf wedges, and some occur at the base of highstand systems tracts (downlap shell beds). On the basis of sedimentary facies, geometry, taphonomy, and paleoecology of shell beds, and the nature of the transition between siliciclastic and mollusk-bearing sediments, cyclothems were classified into two main types that show dependence upon paleoshoreline morphological configuration: sheltered (in the upper Canoa Formation) and exposed (in the Tablazo Formation). Notwithstanding the different synsedimentary tectonic and climatic regimes, the Ecuadorian cyclothems share basic patterns of condensation and facies assemblages with other roughly coeval cyclothemic successions around the world. This suggests that (1) hiatal shell bed development is not just a temperate-latitude phenomenon; (2) a global process, such as glacio-eustatic sea-level change, is the primary mechanism of control for the general architecture of sequences; and (3) specific paleogeographic settings play an important role by determining the taphonomic and paleoecologic characteristics of key shell beds, the nature of their contacts with the encasing sediments, and the type of the component set of facies. At a multicycle time scale, tectonics influenced the long-term trend of the relative sea-level changes and consequently the large-scale stratigraphic organization. Owing to the continued tectonic uplift of the area, successive high-frequency depositional sequences are nested to form a longer-order falling-stage sequence set.

Shell concentrations as tools in characterizing sedimentary dynamics at sequence-bounding unconformities: examples from the lower unit of the Canoa Formation (Late Pliocene, Ecuador)

Abstract The Canoa Basin in Manabi, Ecuador, contains a mainly marine, clastic sedimentary succession of Late Pliocene and Pleistocene Age (Canoa Formation and Tablazo Formation). A stratigraphic and sedimentologic study of the entire sedimentary succession indicated that sedimentary facies recur in consistent deepening–shallowing transgressive–regressive patterns and that on this basis it can be divided into three different informal units (Clow, Cupp, Tb). The lowest of these units (Clow) is composed of at least four depositional sequences, each bounded by a ravinement surface and containing a basal environmentally time-averaged shell bed accumulated during rapid sea-level rise under conditions of reduced terrigenous sediment supply (transgressive systems tract). Owing to intense heavy bioturbation, the internal architecture of these shell beds is not recognizable. Each of these shell beds is mantled by poorly fossiliferous sandy and silty shales accumulated during subsequent progradation (highstand and regressive systems tract). Fossil assemblages, accounting for inner/middle shelf settings, are dominated by infaunal suspension-feeding bivalves (Linga cancellaris, Chione mariae, etc.), most of which require sandy substrate or are ubiquitous. In terms of trophic/life-habit groups, the total succession shows an increase of infaunal suspension-feeder species’ richness from the lowest (C1) to the upper (C4) shell bed, whereas epifaunal taxa decrease in richness. Shell packing density also decreases from C1 to C4. The reconstruction of the short-term sedimentary dynamics based on taphonomic and paleoecologic observations indicates a slightly progradational staking pattern of depositional sequences, representing high-frequency sixth-order eustatic sea-level fluctuations within a third-order tectonically induced cycle.

Sedimentology and high-resolution sequence stratigraphy of the late middle to late Miocene Angostura Formation (western Borbón Basin, northwestern Ecuador)

An integrated analysis of facies and palaeontological content of the middle to late Miocene (c. 12.4–9.1 Ma) Angostura Formation led to the identification of facies associations indicative of shoreface and inner-shelf settings. The Angostura Formation comprises eight high-frequency sequences that are stacked to form a tectonically driven lowstand sequence set. The most complete examples of sequences are bounded by transgressively modified unconformities and exhibit a threefold subdivision: (1) a basal suite of deepening-upward shoreface sediments (transgressive systems tract), including a base-of-cycle shell concentration; (2) a mid-cycle shell bed, consisting of molluscan shells dispersed in a matrix of inner-shelf muddy fine-grained sandstones; (3) a shallowing-upward unit of inner-shelf to lower shoreface sediments almost barren of mollusc fossils (highstand systems tract). Biostratigraphic constraints allowed a reasonable correlation between sequence bounding unconformities and the late middle to late Miocene high-frequency glacio-eustatic changes derived from recent δ18O studies. This correlation has far-reaching implications and leads to the following conclusions: (1) glacio-eustasy in tune with oxygen isotope changes at fourth-order frequency (200 ka–1 Ma duration) may have been the principal factor regulating stratigraphic packaging in the Angostura Formation; (2) these sequences provide an excellent shallow-marine outcrop record of late middle to late Miocene Antarctic glaciations.

TAPHONOMIC AND PALEOECOLOGICAL ANALYSES (MOLLUSKS AND FISHES) OF THE SÚA MEMBER CONDENSED SHELLBED, UPPER ONZOLE FORMATION (EARLY PLIOCENE, ECUADOR)

Abstract The early Pliocene sediments of the Súa Member (Onzole Formation) are spectacularly exposed along coastal cliffs in the surroundings of Súa (northwestern Ecuador). The shellbed at the base of these strata contains rich mollusk and fish assemblages and provides a rare opportunity to document the virtually unknown Pliocene shallow-water faunas of Ecuador. Stratigraphic context, faunal composition (both mollusks and fishes), biofabric trend, ichnological signatures, and taphonomic features presented here, all indicate that the shellbed is a stratigraphically condensed (hiatal) skeletal concentration generated primarily by low rates of net background sedimentation. Shellbeds of this type, immediately overlying a Glossifungites-demarcated ravinement surface, are regarded as onlap shellbeds and record landward stratal convergence and attenuation during the earliest phases of marine transgression. Although composed of largely autochthonous-parautochthonous specimens, the fossil assemblage contains mollusk species from a range of water depths and both soft- and hard-bottom habitats, implying in situ time averaging and admixture of different assemblages as transgression proceeded on a sediment-starved shoreline. The mollusk assemblage is composed of 54 taxa (26 bivalves, 26 gastropods, 2 scaphopods), and the fish assemblage of 31 taxa belonging to 16 families. Both are indicative of a well-oxygenated marine biotope swept by currents and waves, and devoid of a benthic macrophyte cover. With the exception of a few fish species and a single gastropod, the largest number of the mollusk and fish taxa are presently distributed throughout the Tropical Eastern Pacific Biogeographic Region.

FIRST CETACEAN FOSSIL RECORDS FROM ECUADOR, COLLECTED FROM THE MIOCENE OF ESMERALDAS PROVINCE

Cetacean fossils from Ecuador are reported for the first time on the basis of fragmentary remains referred to odontocetes, collected during our investigations of Neogene stratigraphic sequences outcropping along the northern coast (Esmeraldas Province). One specimen was collected near Las Penas village in the Lower-Middle Miocene sediments of the Viche Formation and consists of ear bones and mandibular fragments surely belonging to Ziphiidae. It represents the oldest record of this family in the southeastern Pacific and one of the few records of this family in South America. The other fossil was collected near Rio Verde village in the Upper Miocene sediments of the Angostura Formation and consists of an isolated tooth exhibiting some ziphiid affinities. Pdf

Geomorphologic Map of Northeastern Sector of San Jorge Gulf (Chubut, Argentina)

Abstract Please click here to download the map associated with this article. This paper presents a 1:100,000 scale geomorphologic map of the Northeastern sector of San Jorge Gulf (Chubut Province) in Patagonia, Argentina, covering more than 1,000 km. Derived from remote sensing data and validated by three field surveys, it has been compiled in order to understand the past and recent evolution of the area with particular reference to sea-level oscillation studies, for which this map is the basic tool. The very low human impact and rates of dynamic landscape change allow the preservation of extensive palaeo deposits and landforms, including those indicative of sea-level variations. The relative change of sea level dominates landscape evolution, allowing the formation of widespread marine and lagoon deposits often interfingering with fluvial deposits and reworked by aeolian process in the framework of consequent beach progradation.

Warm-water mollusc assemblages from northern Chile (Mejillones Peninsula): new evidence for permanent El Niño-like conditions during Pliocene warmth?

Although results have been controversial, understanding the tropical Pacific climatic state during the Pliocene warm interval (c. 4.5–3.0 Ma) is crucial if insight is to be gained into the dynamic processes of present and future global warming. In the multi-proxy effort to reconstruct ancient climates, a critical role can be played by palaeoclimatic evidence provided by the spatial and temporal distribution of temperature-sensitive marine molluscs. Shallow-water strata of the Mejillones Peninsula, northern Chile (23°S), contain dense faunal assemblages in which molluscs exclusive to, or characteristic of, Pliocene deposits (Chlamys simpsoni, Chlamys vidali, Chorus blainvillei, Concholepas nodosa, Fusinus remondi, Herminespina mirabilis) coexist with surprisingly abundant and varied populations of extant warm-water species (Bulla punctulata, Cerithium stercusmuscarum, Olivella sp., Turbo cf. fluctuosus, Anomia peruviana, Argopecten ventricosus, Donax peruvianus, Dosinia ponderosa, Mexicardia procera, Undulostrea megodon), most of which have their current southern zoogeographical limit at 6°S. These tropical elements are reliable indicators of nearshore marine conditions and their abundant occurrence implies that sea surface temperatures (SST) along the northern Chile coast were at least 2 °C warmer in the mid-Pliocene than at present, and that these very different conditions lasted long enough to allow stable colonization of the area. Such a significantly warmer SST pattern strongly resembles general climatic conditions accompanying modern El Niño events, when warm tropical waters propagate southward along the western margin of South America; this supports the existence in this area of persistently El Niño-like conditions during the mid-Pliocene.

A new vertebrate fossiliferous site from the Late Quaternary at San José on the north coast of Ecuador: preliminary note

A new fossiliferous site is described south of Manta on the north coast of Ecuador. Estuarine sediments overlying Quaternary terraced deposits contain abundant vertebrate remains belonging to the following taxa: Eremotherium cf. laurillardi or E. rusconii, Haplomastodon chimborazi, and Geochelone s.l. On the basis of geological context and the fossil assemblage, a probable Early Holocene age is suggested, although a latest Pleistocene age cannot be ruled out. This discovery will provide crucial new information to enhance knowledge of the geologic and faunistic evolution of the area.

New insights on the Holocene marine transgression in the Bahía Camarones (Chubut, Argentina)

The stratigraphic reconstruction of the northern sector of the Bahia Camarones (Chubut, Argentina) allowed to improve our understanding of the Holocene marine transgression in the area.The first phase of the maximum of the transgression, is interpreted as dominated by the high rate of eustatic rise of sea level until ca. 6-7 ka BP possibly associated to sedimentary starvation as suggested by fossil accumulation. After this first phase, the general trend indicates a progressive fall of the relative sea level after the Middle Holocene high stand as documented in other parts of south America Atlantic coast. Our data, coupled with the robust radiocarbondata set available for the area from literature, indicate three main local steps of coastal aggradation between ca. 6600 and 5400 yr BP (ca. 7000-5600 yr cal BP), ca. 3300 and 2000 yr BP (ca. 3100-1700 yr cal BP), and ca. 1300-500 yr BP (ca. 1000-300 yr cal BP). A significant age gap in coastal aggradation is present between ca. 5300 and 4400 yr BP (ca. 5600-4500 yr cal BP), and perhaps between ca 2000 and 1300 yr BP (ca. 1700-1000 yr cal BP). These can be linked to phases of local sea level fall and/or phases of sedimentary starvation and/or changes in drift transport which can have produced local coastal cannibalization. However, no conclusive data can be advanced. Data obtained from careful measurements of sea level markers represented by the top of marsh and fluvial terraces indicate lower values for the sea level estimation compared with the data set previously proposed for the area. This stigmatizes the fact that field-oriented works are still the priority in the Patagonia coast along with accurate age measurement, especially for obtaining the fundamental information we need for predicting the environmental impact, in these coastal areas, from accelerate sea level rise as effect of global warming.

Middle- to late-Holocene relative sea-level changes at Puerto Deseado (Patagonia, Argentina)

Stratigraphic, morphologic and radiocarbon data from Puerto Deseado coastal area (Santa Cruz Province, Argentina) indicate that the Holocene coastline formed in response to the discontinuous aggradation of coarse gravely beaches since c. 6300 cal. yr BP related to a progressive falling of relative sea level. Beach ridge crests crudely approximate to the sea level showing at least three steps of aggradation and relative sea-level lowering. Two inactive abrasive notches at c. 7.9 and 3.4 m a.s.l. have recorded this sea-level trend, suggesting two important phases when sea level was stationary. This allows the estimation of a rate of relative sea-level fall in the last c. 3500 years of c. 1.8 mm/yr. Moreover, notches and morphological data indicate that the crest of the beach ridges exceeded the sea-level height by c. 2 ± 0.5 m. This value provides a reasonable regional estimate to be applied to produce comparable relative sea-level curve for Atlantic Patagonia coast.