Onset Of Transgression Research Articles

A new shallow-marine, high-latitude record of the trace fossil Macaronichnus in Miocene, reworked delta-front clinoforms, Punta Basílica, Tierra Del Fuego, Argentina

A new high-latitude record of the trace fossil Macaronichnus segregatis degiberti is documented from the lower Miocene Punta Basílica beds, Tierra del Fuego, which were previously considered to be deep-marine channel and levee deposits. Based on a combined ichnological and sedimentological study, however, these beds are herein re-interpreted as shallow-marine, delta-front clinoform deposits. Macaronichnus-bearing beds in the stratigraphically older clinoforms are characterized by: (1) sandstones with wave ripples and wave-ripple cross-lamination; (2) cross-stratified sandstones forming large compound dunes and tidal bars; and (3) intervening heterolithic mudstone-sandstone couplets with current and wave ripples. The sedimentary facies preserving M. s. degiberti were deposited during the onset of transgression, indicated by wave and tide reworking of underlying steeply dipping, delta-front clinoforms deposits. The master bedding surfaces of compound tidal bars are characterized by a dense, monospecific fabric of M. s. degiberti. Compared with other Macaronichnus ichnofabrics, reported elsewhere as characterizing short colonization windows on foresets of tidal dunes or bars, the Punta Basílica ichnofabric represents a longer colonization window, associated with episodic reworking and redeposition of loose, nutrient-rich sand grains just above the master bedding surfaces. The record of M. s. degiberti in Tierra del Fuego supports the previous interpretations that restrain its producer to high-latitude, shallow-marine cold waters.

Facies architecture and depositional evolution of Palaeocene–Eocene, Subathu Formation, Garhwal Himalaya, Uttarakhand, India

The Late Palaeocene–Middle Eocene marine sedimentary sequence of the Himalayan foreland basin, represented by the rocks of Subathu Formation, is enriched with the presence of foraminifera, bryozoa, corals, gastropods, bivalves and so on within both carbonate and clastic dominant lithologies. The carbonate and the shale consist mainly of larger benthic foraminifera (LBF) and the assemblage is composed of Nummulites and Assilina along with the other skeletal and non-skeletal components. The presence of a palaeosol at the base of Subathu indicates a gap in sedimentation. The lithological and biotic assemblages along with the bed form, bed geometry and primary sedimentary features helped us to establish four facies associations, A, B, C and D, and have been corroborated with the shoreline migration (transgression/regression) history. The facies association-A, representing the basal horizons of Subathu Formation, indicates the onset of transgression and deposition in lagoonal condition with carbonaceous shale and oolitic ironstone, followed by the facies association-B and -C deposited in shallow marine shoreface. The uppermost unit, i.e., the medium- to coarse-grained sandstone (facies association-D) of Subathu Formation, represents a fall in relative sea level (progradational stacking pattern), whereas the underlying contact between the facies association-B and -C is represented by an aggradational stacking pattern between the siltstone and the shale, but certainly without exposing the shelf. Petrographic studies based on characteristic features such as framework constituent, the percentage of matrix and grains, nature of cementing material, textural features and fossil content help to deduce a distinct change in depositional setting from an open marine (shelf) to shoal, a lagoon that gradually grades to foreshore/beach environment. The study reveals that the basin has gone through a transgressive (facies association-A and -B), regressive (facies association-C) as well as distinct forced regressive (facies association-D) phase of shoreline migration history.

The Maurynya section, West Siberia: a key section of the Jurassic-Cretaceous boundary deposits of shallow marine genesis

The Maurynya section exposed on the northwestern margin of West Siberia is one of few continuous sections of the Jurassic-Cretaceous boundary deposits in Boreal regions. In the meantime, it can be considered a reference section for the transitional Volgian-Ryazanian beds formed in shallow water environments of the West Siberian sea basin. This study is a synthesis of the new data with previous results of lithologic, paleontological, biostratigraphic, and (O, C, Sr) isotope studies of the Upper Volgian-lowermost Ryazanian of the Maurynya section. It has been established that the beginning of the Cretaceous (corresponding to the middle Late Volgian) is associated with a sharp increase in species number and diversity of cephalopods and bivalve mollusks on the northwestern margin of the West Siberian sea basin. This can be explained by an increase in its depths and their stabilization at a level which appeared optimal for the habitat of nekton, nektobenthos, and benthic fauna communities, superimposed on the general trend of warming. At the same time, the percentage of phytoplankton significantly increased, indicating the onset of transgression, which affected largely coastal landscapes and type of vegetation: The forests dominated by conifers were gradually succeeded by forests mostly composed of Ginkgoaceae.

Architecture and formation of transgressive–regressive cycles in marginal marine strata of the John Henry Member, Straight Cliffs Formation, Upper Cretaceous of Southern Utah, USA

AbstractMarginal marine deposits of the John Henry Member, Upper Cretaceous Straight Cliffs Formation, were deposited within a moderately high accommodation and high sediment supply setting that facilitated preservation of both transgressive and regressive marginal marine deposits. Complete transgressive–regressive cycles, comprising barrier island lagoonal transgressive deposits interfingered with regressive shoreface facies, are distinguished based on their internal facies architecture and bounding surfaces. Two main types of boundaries occur between the transgressive and regressive portions of each cycle: (i) surfaces that record the maximum regression and onset of transgression (bounding surface A); and (ii) surfaces that place deeper facies on top of shallower facies (bounding surface B). The base of a transgressive facies (bounding surface A) is defined by a process change from wave‐dominated to tide‐dominated facies, or a coaly/shelly interval indicating a shift from a regressive to a transgressive regime. The surface recording such a process change can be erosional or non‐erosive and conformable. A shift to deeper facies occurs at the base of regressive shoreface deposits along both flooding surfaces and wave ravinement surfaces (bounding surface B). These two main bounding surfaces and their subtypes generate three distinct transgressive–regressive cycle architectures: (i) tabular, shoaling‐upward marine parasequences that are bounded by flooding surfaces; (ii) transgressive and regressive unit wedges that thin basinward and landward, respectively; and (iii) tabular, transgressive lagoonal shales with intervening regressive coaly intervals. The preservation of transgressive facies under moderately high accommodation and sediment supply conditions greatly affects stratigraphic architecture of transgressive–regressive cycles. Acknowledging variation in transgressive–regressive cycles, and recognizing transgressive successions that correlate to flooding surfaces basinward, are both critical to achieving an accurate sequence stratigraphic interpretation of high‐frequency cycles.

Clastic sabkhas and diachroneity at the top of the Sherwood Sandstone Group: East Irish Sea Basin

Abstract The identification of early evaporite sulphate cements with a marine isotopic signature within the Ormskirk Sandstone Formation supports previous models of diachroneity at the contact of the Sherwood Sandstone Group and the Mercia Mudstone Group in the East Irish Sea Basin. Basin-wide correlations suggest that, at the onset of transgression, a range of sandflat and aeolian dune environments co-existed on the margins of the basin with marginal marine environments toward the basin centre. One of the major implications of this model is that the high quality aeolian sandstone reservoirs encountered near the top of the Ormskirk Sandstone in the southern parts of the East Irish Sea Basin have no reservoir correlatives in large areas of the basin but can be predicted on the margins.

Shoreline Trajectories and Sequences: Description of Variable Depositional-Dip Scenarios

ABSTRACT Shoreline migration patterns can be described in terms of the shoreline trajectory, which is the shoreline path viewed along a cross-sectional depositional-dip section. Discrete classes of shoreline trajectories can be defined: accretionary and forced regression; normal regression; and accretionary and transgression. implies that sediment supplied to the shoreline participates in determining the shoreline trajectory, whereas non-accretionary implies that existing topography dictates the trajectory. In the latter case, translation of the shoreline takes place without significant deposition. The directions of the shoreline trajectories and the above classes provide a basis for describing variable shoreline behavior for individual shorel ne excursions, for stacked shoreline migration patterns, for systems tracts, and for depositional cycles. A forced regression takes place when little or no sediment is deposited at the shoreline as sea level falls. Accretionary forced regressions occur when sea-level fall is accompanied by coastal sediment accumulation. The architecture of accretionary forced regressions is controlled by the slopes of the alluvial and marine environments, and the shoreline trajectory. A regressive surface of marine erosion is most likely to be formed when the path of the shoreline converges with the fronting depositional foundation. During normal regression, the shoreline trajectory and the depositional foundation usually diverge, with accompanying deepening of water in front of the prograding shoreline. A transgression is defined as a transgression with a shoreline trajectory coinciding with the alluvial depositional surface at the onset of transgression. Accommodation may or may not be present at the landward side of the shoreline at the onset of transgression, but will not be generated there during transgression. An accretionary transgression takes place when the shoreline trajectory diverges relative to the alluvial depositional surface at the onset of transgression. This implies that accommodation is continuously generated and filled behind the retreating shoreline. If a ravinement surface is formed during transgression, its chronostratigraphic significance is maintained only if sediments accumulating during transgression are solely preserved seaward (and not lan ward) of the transgressing shoreline. As the shoreline migrates in various directions, composite stacking patterns, systems tracts, and depositional cycles, as well as surfaces or thin intervals of slow deposition, nondeposition, or erosion are formed. These levels can be used to envelop the depositional cycles. The cycles can be subdivided into two or four systems tracts. The maximum transgressive surface is the most applicable for delineation of cycles. A ravinement surface that merges with or erodes into the subaerial unconformity, in combination with the maximum regressive surface, can also be useful for bracketing cycles.

The Triassic-Jurassic ecostratigraphic transition in the Lombardian Alps, Italy

Three paleoenvironmental phases and two declines in diversity characterize the Late Triassic to Early Jurassic history of the Lombardian Platforms. The first phase, of Late Triassic time (? Choristoceras Zone), consists of 1–5 m thick shallowing-upward subtidal cycles of molluscan, coralline, and echinoderm wackestone and packstone of the Zu Limestone. Biotic and ecostratigraphic characteristics such as typical Rhaetavicula contorta fauna and facies allow correlation to the Kössen Formation of the Northern Calcareous Alps. The second phase, of latest Triassic time (?upper Choristoceras Zone), consists of shallow restricted marine or peritidal carbonates of the Chonchodon Formation dominated by barren lime mudstone and dolostone, algal laminites, and oolitic grainstone. The Zu-Conchodon transition predates the Triassic-Jurassic boundary and represents the first, and most severe, diversity decline for the Lombardian fauna corresponding to a fall in sea-level. Where observed, the upper and lower contacts of the Conchodon Formation are conformable and do not constitute sequence boundaries as suggested by some workers. The Lower Jurassic (? Psiloceras Zone) Sedrina Limestone marks the beginning of the third phase with the onset of transgression and return of normal marine conditions. Typical microfacies include molluscan, echinoderm, and sponge wackestone and packstone with abundant anomuran microcoprolites. The second diversity decline occurred at, or just above, the Triassic-Jurassic boundary at the Conchodon-Sedrina transition, where the remaining restricted marine forms disappeared with the transgression. Anoxia was not a factor in this decline in diversity, although other mechanisms in addition to sea-level change cannot be discounted.

A Comparison of Tectonic and Eustatic Control on Maastrichtian Depositional Sequences, South-Central Wyoming: ABSTRACT

Timing and geometry of Maastrichtian third-order sequences and systems tracts of south-central Wyoming were mostly controlled by rapid basin subsidence resulting from thrust loading of foreland basin crust. The basement of the Red Desert/Washakie basin was affected by thrusting in the Sevier belt to the west and by thrusting along the edge of the Wind River Mountains and Granite Mountains. A transgressive systems tract consists of the youngest Ericson Formation above a regional unconformity, backstepping Almond Formation nearshore and nonmarine facies, and shelfal mudstones of the lower Lewis Shale. Regionally, the onset of transgression is younger from east to west, beginning at 71.5 Ma in eastern Wyoming and at 70.5-71 Ma in south-central Wyoming. The transgression culminated in a condensed shale section evident on gamma-ray logs that occurs throughout the basin. Above the gamma-ray zone, the highstand systems tract of the middle and upper Lewis Shale, Fox Hills Sandstone, and Lance Formation consists of an aggradational/progradational interval overlain by a dominantly progradational clastic unit. Depositional environments consist of delta, slope, and submarine fan in the Lewis Shale, shoreface and shallow marine in the Fox Hills Sandstone, and nonmarine facies in the Lance Formation. Tectonic control of sequences is indicated bymore » two lines of evidence. (1) The timing of the transgressive, regressive, and condensed section facies does not correlate with the inferred global eustatic curve. The Lewis Shale condensed section occurs at 70.5 Ma and is older than the closest global condensed section at 69.5 Ma. (2) Subsidence, estimated by the amount of delta aggradation of the Lewis, totals 700 m roughly 0.5 Ma and greatly exceeds the 40 m of eustatic rise hypothesized to have occurred at about the same time.« less