Abstract The vastness of time is largely beyond human observation, but how aware are most geologists of the concept of time? Time spans of just a few thousands of years may become unfamiliar when moving from the modern, observable, and quantifiable, sedimentary processes acting on decadal to centennial time scales to the intricate series of depositional events discontinuously preserved in the rock record. Our experiential concept of geologic time built on the sequence stratigraphy of chronologically well-constrained, late Quaternary successions delineates a virtually unexplored hierarchy of hiatal surfaces (and condensed intervals) on 10 2 to 10 5 -year time scales; i.e., below the chronologic resolution of most dating techniques commonly used to interpret the ancient stratigraphic record. In continental-margin settings, the fourth-order, Late Pleistocene-Holocene depositional sequence is punctuated by sedimentary hiatuses, and highly episodic deposition appears to be the rule rather than the exception. Systems-tract and parasequence boundaries record long periods of non-deposition, erosion, and/or stratigraphic condensation, and as little as 20% of elapsed time is represented by preserved lithofacies assemblages. In the Po River basin, a significant stratigraphic break having a cumulative duration of up to 80 ky has been produced by the prolonged, stepped phase of eustatic fall and subsequent lowstand between about 120 ky and 20 ky BP. Fluvial channel-belt sand bodies developed during relatively short time periods (~10–15 ky). Early Holocene isolated transgressive sand bodies extend for tens of kilometers along dip, spanning intervals of time of just a few centuries. In coastal-plain successions in this system, up to 50% of geologic time is in the interval just below parasequence boundaries, during formation of relatively condensed peat-bearing intervals. Finally, progradational sets of highstand deltaic parasequences, up to 30-m thick, can make up to 95% of the total volume of Holocene deposits, but embrace just 10% of elapsed time. Intervals of older fluvial and shallow-marine strata having sizes and architectures similar to the Po River system, which are chronologically constrained at much lower resolutions, tend to be interpreted to have developed on larger temporal scales. In these cases, severe distortions can be generated by the over-generalized assumption that sediment packages between regional unconformities represent relatively continuous successions of strata . This assumption will result in a bias towards estimates of sedimentation rates, event frequencies or durations, and sediment fluxes that can be incorrect by orders of magnitude. In addition, stratal successions bounded by surfaces of chronostratigraphic significance may not be as closely genetically related as commonly supposed. Appreciating the highly fragmented nature of the sedimentary record can fundamentally change the interpretation of hierarchical stacking of parasequences and the time scales of formation of ancient alluvial and deltaic depositional systems. We illustrate the impact of this appreciation by comparing the late Quaternary Po Plain basin stratigraphy against older strata of the Eocene Escanilla Formation and Cretaceous Blackhawk Formation strata. It is generally accepted that geologic time can be largely unrepresented by rocks, although this concept has been poorly clarified and only roughly estimated. In this paper, we extend the uniformitarian principle that “the present is the key to the past” to encompass a broader vision in which, at least for certain periods in the Earth’s history, “the recent past is the key to the deep past.”

Abstract The bulk geochemistry of 435 near-surface sediment samples from the southern Po Plain was used to identify the major sources of sediment delivered through distinct tracts of the routing system, from the Apenninic catchments to the Po Delta and the Adriatic coast. Sediment composition from the downstream reaches of the Po River and 23 Apenninic channel-levee river systems is fingerprinted by distinctive heavy metal (chromium and nickel) concentrations, which vary primarily as a function of the local ultramafic rock contribution. For any constant provenance domain, fine-grained (floodplain) sediments are invariably enriched in trace metals relative to their coarser-sized, channel-related counterparts, thus reflecting hydraulic sorting by crevasse and overbank processes. Once the geochemical signatures of fluvial endmembers are established, the relative contribution of the individual detrital sources to the downstream segments of the system can be assessed. Using an example from a multisourced supplied system, we outline the reconstruction of source-rock lithology and sediment pathways by combined sedimentological and geochemical studies as the basis for reliable estimates of sediment budgets in a source-to-sink context.

A cyclic vertical succession of alluvial, littoral, and shallow-marine deposits is identified within two continuously cored boreholes (187-S1 and 204-S15) drilled to ∼180 m beneath the present Po coastal plain, in northern Italy. Integrated sedimentologic, micropaleontologic (benthic foraminifers and ostracods), and geochemical studies allow the reconstruction of the paleogeographic evolution of the study area during the late Quaternary, with a special emphasis on major changes in provenance and sediment dispersal patterns. Transgressive surfaces appear as the most readily identifiable stratigraphic features in the two cores, allowing identification of a series of transgressive-regressive sequences. The transgressive surfaces mark the onset of coastal to shallow-marine conditions, followed by delta and strand plain progradation and the reestablishment of continental environments. This cyclic pattern of facies is paralleled by distinctive cyclic variations in chemical composition of sediments, reflecting a systematic increase in Ni/Al within lower transgressive deposits, followed by a marked decrease in the overlying alluvial plain sediments. At relatively northern locations (core 187-S1), the maximum flooding surfaces identified within shallow-marine deposits on the basis of subtle, but consistent changes in microfaunal assemblages are characterized by anomalously high Mg/Al values. The abrupt peaks in Ni/Al recorded at the transgressive surfaces are interpreted to reflect enrichments in mafic-ultramafic detritus, probably derived from the western Alps and the northwestern Apennines and supplied by the Po River to the coastal areas. These variations took place at the onset of brackish and littoral conditions, when direct connection with the sea favored sediment dispersal from the Po River mouth to lagoonal and coastal environments via the littoral drift. High Mg/Al values within open-marine deposits at the maximum flooding surfaces likely reflect an increasing contribution from eastern Alpine (dolomite-rich) sources at time of maximum shoreline migration. The recurrent changes in geochemical composition recorded across the transgressive surfaces fully support the stratigraphic subdivision of late Quaternary deposits of the Po Basin into transgressive-regressive sequences, rather than depositional sequences. The sequence-bounding unconformities do not display distinctive geochemical signatures.

Abstract: Detailed examination of the lowermost Cenomanian, glaucony–rich deposits of Cap Blatic–Nez (Northern France) reveals a hierarchy of depositional units within a stratigraphically condensed succession, providing a fundamental tool for its sequence stratigraphic interpretation. Condensation is obvious in the highly bioturbated basal glaucony–rich unit (Strouanne Formation), where glaucony is predominantly autochtho nous, highly evolved (˂ 8% K 2 0), and generally constitutes 60–90% of the sand fraction. Glaucony concentration decreases markedly upwards (to less than 10%) in the overlying rhythmically bedded marls and chalks (Petit Blanc–Nez Formation), where the green grains are allochthonous and comparatively less evolved (6.0–6.5% K 2 0). Glaucony distribution shows a vertical cyclic pattern within the condensed interval. Maxima of glau cony concentrations are invariably recorded at intensively burrowed omission surfaces, interpreted as bases of either parasequences or parasequence sets. The characterization of glaucony attributes indicates that a prolonged break in sedimentation, on the order of 0.5–1 Ma, occurred in the study area during the Early Cenomanian. The highly condensed nature of the basal glaucony–rich deposits suggests that either one or two of the third–order Lower Cenomanian depositional sequences recognized in the coeval, less condensed successions of Western Europe are missing at Cap Blanc–Nez. Glaucony maturity in the overlying marl–chalk couplets is consistent with interpretation of these deposits as Milankovitch–scalc cycles.