Gian Gaspare Zuffa

Hybrid Arenites: Their Composition and Classification

ABSTRACT Misinterpretations of modes of arenites containing framework grains of both intrabasinal and extrabasinal origin lead to unreliable reconstructions of paleobasins and paleosource areas. A satisfactory scheme for the classification of such hybrid arenites is needed to provide an adequate framework for basra analysis. Criteria for the definition of the chief classes of arenites must distinguish between extrabasinal and intrabasinal detritus and minimize the dependence of arenite nomenclature on grain size. Four main groups of arenaceous grain types are recognized: 1) noncarbonate extrabasinal, 2) carbonate extrabasinal, 3) noncarbonate intrabasinal, and 4) carbonate intrabasinal. Their three dimensional configuration establishes a first-level classification o the main types of arenites. Two more groups of arenaceous modes, neovolcanic and carbonate particles of extra- or intrabasinal undetermined origin, are required for a practical scheme. The practicability of the suggested approach can be demonstrated in paleogeographic and paleotectonic reconstructions.

Turbidite Megabeds in an Oceanic Rift Valley Recording Jökulhlaups of Late Pleistocene Glacial Lakes of the Western United States

Escanaba Trough is the southernmost segment of the Gorda Ridge and is filled by sandy turbidites locally exceeding 500 m in thickness. New results from Ocean Drilling Program (ODP) Sites 1037 and 1038 that include accelerator mass spectrometry (AMS) 14C dates and revised petrographic evaluation of the sediment provenance, combined with high‐resolution seismic‐reflection profiles, provide a lithostratigraphic framework for the turbidite deposits. Three fining‐upward units of sandy turbidites from the upper 365 m at ODP Site 1037 can be correlated with sediment recovered at ODP Site 1038 and Deep Sea Drilling Program (DSDP) Site 35. Six AMS 14C ages in the upper 317 m of the sequence at Site 1037 indicate that average deposition rates exceeded 10 m/k.yr. between 32 and 11 ka, with nearly instantaneous deposition of one ∼60‐m interval of sand. Petrography of the sand beds is consistent with a Columbia River source for the entire sedimentary sequence in Escanaba Trough. High‐resolution acoustic stratigraphy shows that the turbidites in the upper 60 m at Site 1037 provide a characteristic sequence of key reflectors that occurs across the floor of the entire Escanaba Trough. Recent mapping of turbidite systems in the northeast Pacific Ocean suggests that the turbidity currents reached the Escanaba Trough along an 1100‐km‐long pathway from the Columbia River to the west flank of the Gorda Ridge. The age of the upper fining‐upward unit of sandy turbidites appears to correspond to the latest Wisconsinan outburst of glacial Lake Missoula. Many of the outbursts, or jökulhlaups, from the glacial lakes probably continued flowing as hyperpycnally generated turbidity currents on entering the sea at the mouth of the Columbia River.

Petrology and Dispersal Pattern in the Marnoso-Arenacea Formation (Miocene, Northern Apennines)

ABSTRACT Paleocurrent, gross composition, rock-fragment data, and heavy mineral data from the Miocene Marnoso-arenacea Formation of the Northern Apennines suggest six source areas. Five are primarily extrabasinal (Alpine I and II; Apenninic I, II, and III) and one is intrabasinal. The identification of characteristic multicycle materials reinforces the idea that migration of foredeeps within a plate-tectonic framework occurred in the Apennines. Integration of petrofacies, paleocurrent, and sedimentary-facies-analysis data in the study of the Marnosoarenacea depositional system has proved a very powerful method for understanding the complexity of a deep-sea fan system.

Sand Composition and Sedimentary Evolution of a Late Quaternary Depositional Sequence, Northwestern Adriatic Coast, Italy

ABSTRACT In order to detect temporal and spatial variations in detrital supply in the upper part of the last depositional sequence of late Pleistocene-Holocene age, we conducted a combined petrographic and stratigraphic study in the Romagna coastal plain, south of the Po delta, Northern Italy. We analyzed heavy minerals and the bulk composition of sands from cores and surface samples. By using petrographic and stratigraphic data on composition of modern beach and major river deposits, new aspects of detrital dispersal mechanisms and the depositional history of the study area are documented. The heavy-mineral distribution, coupled with the dolostone and volcanic rock fragment contents of bulk sands, allow identification of three different petrofacies: Petrofacies A, Petrofacies B, and Petrofacies C, which have been interpreted as of apenninic, mixed Eastern Alps/Po River basin, and Po River catchment basin provenance, respectively. Distribution of the three petrofacies changed through time in response to landward and seaward shifting of the coastline. During the late Pleistocene lowstand, a pure Apenninic provenance characterized the Romagna coastal plain (Petrofacies A). During the Holocene transgression, when the shoreline migrated tens of kilometers west of its present position, eastern Alpine sediment sources fed littoral facies (Petrofacies B), probably as a result of southward transport by littoral drift. This sediment supply continued during the early regressive phases and was cut off by a change in coastal morphology related to the development of the early Po delta. This led to the establishment of a sediment supply entirely related to the Po River catchment basin (Petrofacies C). Changes in the compositional signature of sand in the youngest beach ridges mark the abandonment of the early Po delta due to an avulsion event and testify to the establishment of a coastal system fed by rivers draining the Apennines (Petrofacies A). We emphasize sand petrography as an important tool in studying the internal architecture of sandy clastic depositional units on detailed spatial scales, and its use in deciphering the depositional history of complex sedimentary successions. FIG. 1. Location of the study area. End_Page 829------------------------

Continental collision history from arenites of episutural basins in the Northern Apennines, Italy

Integration of sandstone petrography, sedimentology, stratigraphy, and structural and paleogeographic restorations of clastic sediments deposited in episutural basins has allowed us to unravel the collisional history of the Northern Apennines from middle Eocene to Holocene time, as illustrated in seven paleogeographic maps. The episutural successions were deposited in two types of basins. 1. Late Eocene–early Oligocene basins perched on deformed Ligurian units (ophiolites and their sedimentary cover) of the southwestern side of the Alps (i.e., part of the future northern Apennine accretionary wedge). In these basins, provenance evolved from an extrawedge north-to-south supply from the Austroalpine (Adriatic) continental block to an intrawedge supply with progressive unroofing of the collisional tectonic stack composed of Ligurian units plus minor high-pressure–low-temperature metamorphic units (Pennidic units). These basins developed after the middle Eocene continental collision between the Adriatic margin and the southern European margin, a period dominated by magmatism, uplift, and block faulting of the Pennidic-Ligurian orogen. 2. Late Oligocene–Holocene basins formed on top of the migrating Apenninic orogenic wedge, which was progressively thrust onto the Adriatic margin, where thick, turbiditic successions were being deposited on the foreland. Sandstone composition is characterized by an overall increase in detritus from Pennidic units both up-section and from south to north and by detritus recycled from older sedimentary units and their Ligurian substratum. These basins developed during continental subduction of the Adriatic plate toward the west underneath the Corsica-Sardinia block coupled with extension in the future western Mediterranean area. Sedimentation patterns indicate that paleobathymetry and source rocks were markedly different north and south of the Val Secchia line, a structural lineament that does not correspond to a present-day observable structure across the Apennines.

Deep-sea sedimentary record of the late Wisconsin cataclysmic floods from the Columbia River

New results from Ocean Drilling Program Site 1037 and U.S. Geological Survey high-resolution seismic-reflection profiles confirm the great thickness, fast deposition rate, distant source, and convolute path of turbidites that fill the Escanaba Trough, the rift valley of the southernmost segment of the Gorda Ridge. Accelerator mass spectrometry 14 C measurements provide the first direct dating of the Escanaba Trough turbidites, demonstrating an average deposition rate faster than 10 m/k.y. between 32 and 11 ka and as fast as 15 m/k.y. during the oxygen isotope stage 2 lowstand. In the upper 60 m of sediment, the petrology of turbidite sand beds, which are as much as 12 m thick, show that the dominant source for the turbidites is from the Columbia River, which is more than 800 km to the north, rather than from the much closer rivers of northern California. New high-resolution seismic-reflection profiles show that, except for areas of very recent volcanism, the entire Escanaba Trough below 3200 m water depth is floored by the turbidite sequence that was cored in the upper 60 m at Site 1037B. The ages of the upper 120 m of turbidites correspond with the ages of channeled scabland deposits associated with latest Quaternary jokulhlaups from glacial Lake Missoula. The age and source characteristics suggest that these megaturbidite beds in Escanaba Trough are most likely deposits formed by hyperpycnally generated turbidity currents as the largest of the Lake Missoula floods entered the sea.

Discriminating between tectonic and sedimentary burial in a foredeep succession, Northern Apennines

An extensive apatite fission‐track survey has been carried out on the Marnoso‐arenacea foredeep succession in the Northern Apennines. The data show a general decrease of the maximum paleotemperature undergone by the sediments toward the foreland areas. The maximum burial calculated by using a geothermal gradient of 20°C km−1 spans from more than 5 km to less than 2.5 km and indicates that the previously assessed total thickness of the Marnoso‐arenacea succession is not enough to justify the determined values. It is concluded that a now eroded Ligurian wedge, up to 5 km thick, was present on top of the Marnoso‐arenacea sediments.

The Chugach Terrane, a Cretaceous trench-fill deposit, southern Alaska

Summary The Chugach terrane of southern Alaska extends for approximately 2000 km along the margin of the Gulf of Alaska. A seaward flysch facies of the terrane, the Chugach flysch terrane, represents the fill of a Late Cretaceous trench and consists of structurally deformed turbidites with some mafic volcanic rocks. It is intruded by anatectic granitic plutons of early Tertiary age. The Chugach flysch terrane in most places is bounded to the north by a landward-dipping thrust fault separating it from mélange of the Chugach terrane at least as young as Late Cretaceous. To the south the flysch terrane contains oceanic volcanic rocks and is bounded by faults that separate it from Palaeogene turbidites or upper Mesozoic metavolcanic rocks. Interpretations of folds and faults in SW Alaska suggest that the Chugach terrane was deformed during NW-directed subduction. Palaeocurrents indicate primarily westward flow along the axis of the outcrop belt, and secondary southward transport. Turbidite facies associations indicate an east-to-west progression from inner-fan to middle-fan, outer-fan, fan-fringe and basin-plain deposits down the axis of the outcrop belt, and a bounding slope facies association to the north. Rock-fragment petrography of sandstone samples from the Chugach terrane indicates derivation from a magmatic arc that was increasingly dissected eastward and from an older subduction complex. The magmatic arc and adjacent shallow-marine forearc basin deposits are located to the north in southern Alaska. Palaeomagnetic data from the Chugach terrane and adjacent deposits indicate original deposition far to the south of the present latitude of Alaska. The entire magmatic arc-forearc basin-trench complex migrated northward in early Tertiary time, when it was probably oroclinally bent and accreted as a mesoplate to Alaska prior to the middle Miocene.

Heterogeneous Distribution of Calcite Cement at the Outcrop Scale in Tertiary Sandstones, Northern Apennines, Italy

Calcite cement derived intraformationally in seven stratigraphic units of marine origin (five submarine-fan deposits and two shelf deposits) is distributed heterogeneously at the outcrop scale. Sandstone beds intercalated with calcareous shale older than Pliocene tend to be completely cemented, whereas stacked sandstone beds that lack shale interbeds have calcite cement in the form of tightly cemented concretions that make up only 10-30% of a bed. The abundance and distribution of concretions, with few exceptions, are irregular and unpredictable. Concretion shapes include spheres (<1 m diameter), oblate and prolate spheroids (<1.5 m), tabular forms (to 8 m long), and irregular forms. Patterns of concretions within beds are remarkably varied and include both random and uniform spacing; preference for either the top, middle, or bottom of beds; preference for faults that cut bedding at a high angle; and localization around shale rip-up clasts. There is no preference of concretions for shell-rich layers. Some formations have cement patterns specific to that formation, whereas other formations have different patterns at different outcrops. Most formations have more than one cement pattern in an outcrop. The lack of strong textural (grain size, graded bedding) or compositional controls on the localization of calcite cement suggests the preeminence of highly localized hydrologic factors in determining the spatial distribution of authigenic pore-filling calcite. Spherical concretions grew by diffusive supply of intraformationally derived components, whereas prolate and elongate concretions grew chiefly under the influence of advective supply. Faults apparently served as fluid conduits and were selectively cemented. In general, only sandstones intercalated with shale are totally cemented. This indicates that shales were a major source of cement components for these sandstones at least.

Detrital Mode Evolution of the Rifted Continental-Margin Longobucco Sequence (Jurassic), Calabrian Arc, Italy

ABSTRACT The Longobucco Sequence is a 1200-m-thick dominantly carbonate miogeoclinal wedge which accumulated in the Mediterranean area during Jurassic time. Sedimentary facies pass upward from continental (3% of the stratigraphic column) at the base, through shelf (6%), slope (14%), to deep-sea turbidites (77%) at the top. Sand framework grains had five sources, four are noncarbonate extrabasinal and one, the most abundant, is carbonate intrabasinal as follows: reworked arenites (quartzarenites, petrofacies A); low-rank metamorphic rocks (litharenites, petrofacies B); eolian or beach arenites (feldspathic-quartzarenites, petrofacies C); granitic and high-grade metamorphic rocks (arkoses, petrofacies D); intrabasinal shelf sediments (peloids, intraclasts, fossils and oolites). Quartzarenites, which form fluvial and shallow-marine deposits, gradually pass upward through shelf, siliciclast-rich limestone to marly slope deposits with minor interbedded litharenites. Deep-sea fan deposits are chiefly carbonate turbidites, but feldspathic quartzarenites are present in the lower part. Arkoses abruptly replace the feldspathic quartzarenites in the upper part of the section. The litharenitic source is represented throughout the stratigraphic column. Therefore, different terrigenous source areas simultaneously supplied detritus to the sedimentary basin. The evolution of detrital modes and depositional systems suggests two major tectonic events: latest Early Jurassic and probably Late Jurassic, superimposed on a progressive sinking of the basins. Both the change in depositional systems and sand composition can be related to a complex interplay of microcontinents and oceans which determined local compressive or extensional regimes during Alpine rifting in the Tethys region.