Alessandro Amorosi

Glaucony and sequence stratigraphy; a conceptual framework of distribution in siliciclastic sequences

ABSTRACT Detailed analysis of recent literature on glaucony and selected case studies (Eocene, Isle of Wight; Miocene, northern Apennines) shows that the presence of glaucony alone is not diagnostic of a specific systems tract of a depositional sequence. A reliable sequence stratigraphic interpretation of glaucony-bearing units requires additional information on glaucony, including: (1) spatial distribution, (2) maturity (distinction between nascent, slightly evolved, evolved, and highly evolved glaucony), and (3) genetic attributes (differentiation of autochtonous from allochthonous, and intrasequential from extrasequential glaucony). Autochthonous glaucony is common at various stratigraphic levels in the transgressive systems tract (TST) and the lower highstand systems tract (HST), showing an upward increase (TST) and then decrease (HST) in abundance and maturity. The condensed section can be distinguished from the overlying and underlying deposits by the higher concentration and maturity of glaucony. Allochthonous intrasequential (parautochthonous) glaucony can be present in the entire TST, HST, and lowstand systems tract (LST), generally showing lower concentration and maturity than its autochthonous counterpart. Allochthonous extrasequential (detrital) glaucony is present mainly in the LST, its concentration and composition depending on the characteristics of the source horizon. The associa ion between autochthonous and allochthonous (intrasequential and extrasequential) glaucony commonly exists in the LST and in the lower TST.

Palaeogeographic and palaeoclimatic evolution of the Po Plain from 150-ky core records

Integrated sedimentological and micropalaeontological (foraminifers, ostracods, pollen) analyses of eight continuously cored boreholes, up to 140 m deep, reveal the depositional history and the palaeoclimatic evolution of southeastern Po Plain (northern Italy) over the last 150 ky. Age assignments are supported by the chronostratigraphic sequence framework calibrated by radiocarbon and pollen data, allowing detailed correlation with the oxygen-isotope record. Facies analysis (including identification of 12 distinct microfossils associations) and detailed stratigraphic correlations across several tens of km document a cyclic sedimentation pattern (including continental, coastal and shallow-marine deposits), which defines two transgressive–regressive sequences, deposited over the last two interglacial–glacial cycles. Two prominent stratigraphic markers, corresponding to wedge-shaped coastal sand bodies, are recorded between 0–30 and 100–125 m core depths. These sedimentary bodies were deposited during the two major transgressive pulsations and subsequent sea-level highstands of the last 150 ky, assigned to the Holocene and the Tyrrhenian (oxygen-isotope Substage 5e), respectively. The stratigraphic architecture of post-Substage 5e deposits shows consistent patterns of coastal evolution with changing sea-level position. Lowering of sea level between 125 and 70 ky (onset of Substages 5d and 5b, and Stage 4) resulted in extensive and repeated basinward shifts of facies, which can be observed across closely spaced unconformity surfaces associated to alluvial plain sedimentation (falling-stage systems tract). The general phase of sea-level fall was punctuated by short transgressive phases (Substages 5c, 5a and Stage 3 interstadials), which led to widespread deposition of organic-rich (lagoonal and swamp) deposits. Upper Stage 3 and Stage 2 deposits (lowstand systems tract) are replaced across a significant part of the study area by a hiatal surface. Holocene interglacial deposits are characterized by a retrogradational stacking pattern of coastal plain and littoral facies (transgressive systems tract), reflecting invariably the landward migration of a barrier–lagoon–estuary system. Subsequent highstand deposition (highstand systems tract) was characterized by extensive progradation of wave-influenced deltas and strandplains. The two major transgressive surfaces, of Tyrrhenian and Holocene age, are considerably easier to identify than the other key surfaces for sequence stratigraphic interpretation. The lowermost transgressive deposits display a pollen signature diagnostic of warm climate (interglacial) periods, showing wide forest development and relatively high pollen concentrations. By contrast, the deposits overlying the unconformable surfaces related to successive phases of sea-level fall are fingerprinted by pollen spectra dominated by Pinus and non-arboreal pollen types, with very low pollen concentrations, reflecting the onset of a stable cold climate vegetation characteristic of stadial to fully glacial conditions. The good match between facies architecture, pollen distribution and global sea-level evolution strongly suggests that Late Quaternary sedimentation in the Po Basin developed under a predominantly glacio-eustatic control. Stratigraphic architecture of the Po Basin thus can provide a useful analog for interpretation and correlation in the stratigraphic record of very rapid glacio-eustatic (fourth-order) cycles, with frequencies of about 100 ky.

Detecting compositional, spatial, and temporal attributes of glaucony: a tool for provenance research

Abstract A reliable interpretation of glaucony-bearing units requires the characterization of compositional, spatial, and temporal attributes of glaucony. The criteria that can readily determine the differentiation of the various types of glaucony (autochthonous, parautochthonous or allochthonous intrasequential, and detrital or allochthonous extrasequential) are discussed in detail. Autochthonous vs allochthonous glaucony may be often detected on the basis of detailed sedimentological investigations, including facies interpretation of glaucony-bearing deposits and careful examination of spatial distribution of glaucony. Textural and morphological features, although generally not diagnostic on their own, provide a significant contribution to glaucony interpretation. The distinction between parautochthonous and detrital glaucony can be sometimes performed by radiometric dating; more commonly, it requires that compositional attributes of glaucony be matched against those of putative sources. Glauconies from distinct parent rocks generally have different maturity and carry unique geochemical and mineralogical fingerprints that may contribute definitive data for provenance research. Previous work on glaucony and examples from units formed in tectonically stable areas (Cretaceous/Eocene deposits of Paris Basin and Hampshire Basin) and active tectonic basin settings (Miocene strata of southern Alps and northern Apennines) provide the opportunity to discuss the validity of the proposed criteria across different structural domains and emphasize the need for an integrated approach to the study of glaucony.

Geochemical and mineralogical variations as indicators of provenance changes in Late Quaternary deposits of SE Po Plain

Combined geochemical and mineralogical characterisation of mudstone-prone successions can provide suitable indicators of sediment provenance. A case study from the Late Quaternary depositional sequence of southeastern Po Plain shows that major changes in geochemical and mineralogical composition can be ascribed to changes in sediment dispersal patterns occurring during the last 30 ka in response to fluctuating sea level. The Po River, with a mixed contribution from the Alpine and Apenninic chains and its southern tributaries flowing from the Apennines, acted as the major feeders for the study area during the Late Quaternary. Chromium (and nickel) among trace elements, and serpentine (and dolomite) among minerals appear as the major provenance indicators for the study succession, allowing distinction of an Apenninic (chromium- and serpentine-poor) province from a mixed Alpine–Apenninic, Po-related (chromium- and serpentine-rich) province. The relatively low Cr/Al2O3 and serpentine/silicate ratios recorded in sediments of late Pleistocene age (<30 ka BP), which accumulated at the last glacial maximum, suggest that during lowstand times and early stages of transgression, the southeastern Po Plain was beyond the Po River influence, being fed uniquely by rivers of Apenninic provenance. Increasing Cr/Al2O3 and serpentine/silicate ratios, recorded within back-barrier transgressive deposits (8–10 ka BP) in the Comacchio and Ravenna sectors, reflect a major change in sediment supply taking place during the Holocene, when partially barred environments largely connected with the open sea became established in the study area, and Po-derived detritus was delivered to estuaries and lagoons by littoral drift. Maximum chromium and serpentine values are recorded at peak transgression and during the following sea-level highstand (6 ka BP–Present) when sediment supply exceeded the rate at which new accommodation space was created, leading to extensive Po-delta progradation. The drop in Cr/Al2O3 and serpentine/silicate ratios recorded in some cores at top of the Holocene succession reflects the local reestablishment, behind the prograding Po-delta complexes, of an alluvial plain drained by rivers of Apenninic provenance as a result of distributary channel switching and delta lobe abandonment. Geochemical and mineralogical analyses show negligible compositional variations between Pleistocene and Holocene deposits landward of the line of maximum marine incursion especially close to the basin margin. In these instances, sample composition throughout the entire stratigraphic succession clearly suggests an ongoing supply from the adjacent Apenninic chain. Comparison with sand compositional analyses from major sand bodies of the same stratigraphic succession highlights the complementary but fundamental role of geochemical and mineralogical characterisation of clays for provenance research.

Facies Architecture and Latest Pleistocene-Holocene Depositional History of the Po Delta (Comacchio Area), Italy

Integrated sedimentological and micropaleontological study of 16 cores and 137 piezocone penetration tests, approximately 40 m deep, in the Comacchio area enables the documentation of the depositional history of southeastern Po Plain in the last 30 ka, in response to fluctuating sea level. Sedimentation within an alluvial plain was the dominant feature across the entire study area during the pronounced sea level fall that culminated in the Last Glacial Maximum. Thin lowstand fluvial sediments form the lower part of a shallow incised valley above the Last Glacial Maximum unconformity, whereas a characteristic paleosol separates the last glacial alluvial plain deposits from the overlying postglacial deposits in the interfluves. Transgressive and highstand deposits show a well‐developed stacking pattern of retrogradational (coastal plain and estuarine) and progradational (deltaic) facies. Detailed reconstruction of transgressive paleogeography shows evolutionary features that can be useful for refined interpretation of coeval and ancient analogs. At relatively early stages of transgression (10.5–9 ka B.P.), sedimentation in a coastal plain was restricted to the incised valley, whereas nondeposition and pedogenesis took place on the interfluves. With rising sea level (9–6 ka B.P.), a wave‐dominated, barred estuary developed in the former topographic low. At peak transgression, after filling up of the estuarine systems with coastal, back‐barrier sediments, wide areas outside the valleys were flooded, aggradation extended onto the interfluve unconformity, and a shallow marine depositional environment developed across most of the study area. The depositional history during the subsequent highstand phase was dominated by progradation of the early Po Delta and reflects the complex interplay between high‐frequency sea level fluctuations, climate, subsidence, and autocyclic processes.

Genetically related alluvial deposits across active fault zones: an example of alluvial fan-terrace correlation from the upper Quaternary of the southern Po Basin, Italy

Integrated studies of alluvial fan systems and related intramontane valley deposits provide potentially useful tools for understanding the character of alluvial sedimentation and its control in fault-controlled piedmont zones. An example is reported from the uppermost Quaternary alluvial deposits of the northern Apennines and the southern margin of the Po Basin. A distinctive cyclicity characterizes both the outcropping and the buried units in the study area. In intramontane valleys individual cycles consist of regular alternations of sections displaying high and low terrace preservation. In the subsurface, a rhythmic alternation of coarse-grained bodies and laterally extensive pelitic horizons occurs at various scales and represents the basic cyclic motif of the depositional system. The correlation between the terrace sequences and the depositional cycles in the subsurface allows the differentiation of distinct sequences with a characteristic internal architecture. In the lower part of sequences, poorly preserved terrace deposits are laterally correlatable with alluvial fan gravel bodies, whereas highly preserved terrace deposits are correlated laterally with predominantly floodplain sediments in the upper part of sequences. Reliable correlation lines have been constructed using 14C dating. Thrust tectonics and climate exerted a major control on sequence development. Deep valley excavation, with subsequent low terrace preservation, occurred in response to relief rejuvenation during successive thrusting events. On the other hand, decreasing erosion related to periods of minor tectonic activity led to terrace preservation in the intramontane valleys. The coeval episodes of alluvial fan growth and abandonment, recorded at the basin margin, are interpreted to reflect changes in the efficiency of the drainage system, due to variations of tectonic uplift in the adjacent areas. Alternating phases of deglaciation and renewed glaciation, however, are inferred to have played a major role in controlling the amount and type of sediment delivered to the alluvial fan complexes.

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------------------------

High-resolution sequence stratigraphy from piezocone tests: an example from the Late Quaternary deposits of the southeastern Po Plain

Abstract Cone penetration tests are traditionally regarded as a major tool for geotechnical investigations. This study, based upon interpretation of 234 cone penetration tests with pore-pressure measurements (piezocone tests or CPTU), carried out in the Late Quaternary deposits of the southeastern Po Plain, shows how CPTU tests can also be used for sedimentological purposes, including detailed facies characterization, subsurface stratigraphic correlations, and identification of the key surfaces for sequence-stratigraphic interpretation. The Late Quaternary depositional history of the southeastern Po Plain, reconstructed on the basis of data from seventeen continuously cored boreholes, includes (1) alluvial plain development during the Late Quaternary lowstand and the early stages of transgression, (2) formation of a rapidly migrating barrier–lagoon system during the late transgressive phases (8800–6000 y BP), (3) construction and progradation of a wave-dominated delta (ancient Po delta) during the following sea-level highstand (6000–800 y BP), and (4) development of the present-day alluvial plain, following the delta lobe abandonment in the 13th century A.D. Local execution of CPTU tests in coincidence with drilling sites enables the calibration of borehole data with piezocone penetration profiles. Lithofacies characterization is based upon estimation of three major parameters: corrected cone resistance ( q t ), sleeve friction ( f s ), and pore water pressure ( u ). Plotting of q t versus the ratio of cone friction to cone bearing (FR) is adopted as the major tool for sediment texture classification. Eight major facies associations for the southeastern Po Plain are identified. These are, for decreasing q t values: (1) fluvial channel sands, (2) beach-ridge sands, (3) transgressive barrier sands, (4) crevasse sands and silts, (5) levee silts and sands, (6) floodplain silts and clays, (7) prodelta clays, and (8) marsh clays and peats. CPTU profile interpretation also provides the basis for the identification of the three major key surfaces within the Late Quaternary 4th-order depositional sequence. (1) The transgressive surface (TS), marking the boundary between the locally pedogenized, stiff Pleistocene alluvial clays and the overlying Holocene transgressive paralic deposits, has a distinctive pore-pressure response, with very low u values, and is characterized by a sharp downward increase in f s , which is paralleled by a moderate increase in q t . (2) The ravinement surface (RS), corresponding to the boundary between fine-grained back-barrier deposits and the overlying transgressive barrier sands, is invariably marked by a sharp upward increase in q t (and decrease in FR). (3) The maximum flooding surface (MFS), which is not clearly recognizable on the sole basis of core data, is identified within shallow-marine (prodelta) clays in combination with minor peaks of q t , which have been interpreted to reflect a laterally extensive fossil lag, with transition in proximal areas to sand (shell-rich?) layers. Simplicity, speed, and comparatively low costs of CPTU tests imply that an extensive use of this method, when used in conjunction with core programs, can be a very attractive alternative to economically less convenient methods for the geological mapping of alluvial/coastal plain areas consisting of non-gravel deposits.

Sequence stratigraphy and the resolution of the fossil record

Sequence stratigraphy, a major theoretical achievement of earth sciences, integrates facies associations and stratal architecture within a chronological framework of the geological record. The sequence stratigraphic model implies that sediment supply to the basin, preservation potential of individual horizons, and the resolution of paleontological data co-vary with base-level fluctuations, especially for siliciclastic depositional systems. Using Holocene transgressive-regressive successions of the Po Plain (Italy), we assessed the model’s hypotheses by analyzing 249 marine mollusk shells dated individually using 14 C-calibrated amino acid racemization methods. As postulated by the model, the temporal resolution of the fossil record, frequency of depositional events, and net accumulation rates decreased upward concomitantly through the transgressive systems tract reaching minima in the condensed section (maximum flooding zone). The reverse trend, with increasingly frequent, thicker, and less time-averaged beds, was observed throughout the overlying highstand systems tract. The results quantify the postulated sequence stratigraphic asymmetry in temporal resolution of the fossil and sedimentary records.

Glaucony from the Eocene of the Isle of Wight (southern UK): implications for basin analysis and sequence-stratigraphic interpretation

The Lower-Middle Eocene sequence of the Hampshire Basin, which includes an alternation of sands, silts and clays, interpreted as fluvial to open-marine deposits, is exposed at two locations on the western (Alum Bay) and eastern (Whitecliff Bay) margin of the Isle of Wight. Glaucony is encountered throughout the study units: sand-clay alternations of lagoonal origin and estuarine channel-fill sands include less than 10% of predominantly poorly evolved, allochthonous glaucony; shelf silts and sands generally comprise more than 20% of evolved, mostly autochthonous glaucony. Vertical changes in maturity of glaucony confirm the trend predicted by recent models, indicating an overall upward increase in the transgressive systems tracts and decrease in the highstand systems tracts, with maximum values in coincidence with the condensed sections, where a remarkable peak in glaucony concentration is invariably recorded. Comparison of glaucony characteristics at different sites of depositional sequences shows a slight increase in maturity and abundance from the proximal to the distal areas. This lateral tendency is interpreted to reflect more suitable conditions for glauconitization in open-marine environments than in shallow waters. Despite documented intrasequential variability of glaucony characteristics, glaucony assemblages from distinct depositional sequences may be generally identified on the basis of more pronounced differences in maturity. Consequently, glaucony attributes can be used as an index for characterization of third-order sedimentary cycles.