Coastal Onlap Research Articles

Chapter 8. Controls on North Sea Jurassic–lowermost Cretaceous sequence development

Abstract Of 40 recognized Jurassic–earliest Cretaceous sequence boundaries or surfaces, 21 are considered to have had a primary tectonic control on their generation, particularly during the Bathonian–Berriasian interval of synrift-dominated tectonism. These boundaries include the intra-J22 sequence boundary, the base J36, the base J54, the base J55, the intra-J56 transgressive surface, the base J62, the base J63, the base J64, the base J71, the base J73 and the top J76 (‘Base Cretaceous’). In the study area, these events all occurred within a marine setting and none can be unequivocally matched to times of subaerial exposure or coastal onlap. Ten Jurassic sequence surfaces appear to have had a primary eustatic control on their generation, some of which are also associated with the deposition of major marine source-rock facies, including the base J18 and the base J74.

Assessing Milankovitch forcing in disconformity‐prone cyclic shallow‐water carbonates, Upper Jurassic (Kimmeridgian), Adriatic Platform, Croatia

AbstractMost Upper Jurassic studies of astronomical forcing have focused on deeper‐water sections which are relatively continuous. An Upper Jurassic (Kimmeridgian) section on the greenhouse Adriatic Carbonate Platform, Croatia, was studied to determine if astronomical forcing can be recognized in a 5.8 ± 0.1 Myr duration, disconformity‐prone shallow platform succession. The succession consists of metre‐scale subtidal parasequences intermixed with peritidal parasequences, and intermittent subaerial breccias at sequence boundaries. Ages were constrained by biostratigraphy and δ13C chemostratigraphy, and most sequence boundaries appear to match those of the coastal onlap curve of Haq (2018). Logged sections were converted into depth–rank time series and parasequence–thickness time series. Accumulation rates were statistically evaluated for the rank series against an astronomical‐forcing model, and compared with long‐term accumulation rates (thickness divided by time). The statistical rates were used to select the ca 100 kyr eccentricity cycle to tune the series. Spectral analysis showed peaks at ca 400 kyr (superbundles) and ca 100 kyr (bundles), along with obliquity (38 kyr and 27 kyr) and precessional (18−22 kyr) cycles (parasequences). The Kimmeridgian sequences are ca 400 kyr, ca 800 kyr and ca 1.1 Myr duration. Sequence scale (0.4 to 1.2 Myr) stratigraphic completeness based on statistical accumulation rates versus long‐term rates is ca 60%. This study estimates ca 1 Myr missing time in parasequences stacked into superbundles and 1.6 Myr in four major sequence boundaries. Given that the Kimmeridgian was the hottest time of the Middle and Late Jurassic, aquifer eustasy may have influenced the timing of sequence boundaries, although documented late Kimmeridgian cooling could have triggered a glacio‐eustatic component.

Correspondences among lacustrine fluctuations, climate changes and the Milankovitch cycles in the Paleogene through tracking onlap points and correlating palaeontology in Liaozhong Depression, Bohai Bay Basin, NE China

The solar radiation or insolation at a specific locality on the Earth's surface is determined by the distance of this point from the sun and by the angle with which the radiation hits the earth's surface. The theory that variations in the earth's orbit determine the amount of radiation and therefore the climate which is the Milankovitch Cycle. In non‐marine lacustrine rift basin, the changes of climate can be recorded by the palaeontology, and it also plays an important role in controlling lacustrine fluctuations. The lacustrine fluctuations are indicated by coastal onlap, which is the landward or lake‐ward onlap of non‐marine coastal deposits. In this paper, based on borehole data, wireline logs, seismic datasets, biostratigraphic data and palaeontology, the lacustrine fluctuation curve can be reconstructed from the onlap of lacustrine deposits in non‐marine sequences. The durations and magnitudes of these changes can be used to construct charts showing cycles of the relative rising and falling of lake level. In addition, the changes of lake level can also be reflected by the palaeontology such as pollen. The living conditions and distributions of pollen can be a good indicator for climate changes and lacustrine fluctuations in an a non‐marine setting. The calculation of the Milankovitch Cycle can be determined by the GR log in the study area which keeps a record of cycle changes. All climatic changes and lacustrine fluctuations are mainly resulted from the Milankovitch Cycles, and therefore, through the reconstruction of lacustrine fluctuations, climate changes and the Milankovitch Cycle, the correspondences among them can be clearly addressed. All the results can contribute a lot in theory research in the study area and lead to a new focus in practical hydrocarbon exploration and exploitation.

A geomorphic enthalpy method: Description and application to the evolution of fluvial-deltas under sea-level cycles

Fluvial deltas are composites of two primary sedimentary environments: a depositional fluvial region and an offshore region. The fluvial region is defined by two geomorphic moving boundaries: an alluvial-bedrock transition (ABT), which separates the sediment prism from the non-erodible bedrock basement, and the shoreline (SH), where the delta meets the ocean. The trajectories of these boundaries in time and space define the evolution of the shape of the sedimentary prism, and are often used as stratigraphic indicators, particularly in seismic studies, of changes in relative sea level and the identification of stratigraphic sequences. In order to better understand the relative role of sea-level variations, sediment supply, and basin geometry on the evolution of the ABT and SH, we develop a forward stratigraphic model that accounts for curvature changes of the fluvial surface and treats the SH and ABT as moving boundaries (i.e., internal boundaries that are not known a priori and their location must be calculated as part of the solution to the overall problem). This forward model extends a numerical technique from heat transfer (i.e., enthalpy method), previously applied to the evolution of sedimentary basins, to account for sea-level variations, including eustatic sea-level cycles. In general, model results demonstrate the importance of the dynamics of the fluvial surface on the system response under a large range of input parameter values. Specifically, model results suggest that time lags in the ABT response during sea-level cycles can result in geologically long-lived river incision in the upper and mid portions of the fluvial surface during sea-level rise. These results suggest that the relationship between the coastal onlap configuration of strata and relative changes in sea level is complex, and therefore not necessarily a good indicator of contemporaneous sea-level changes.

Palaeoclimatic Evidences from the Quaternary Coastal Deposits, Southwestern, Nigeria

The studies on some samples retrieved from the coastal sediment deposit have been carried out. The palynological and geochemical indices were evaluated with the view of determining their chronology, palaeoclimatic conditions that prevailed during the time of their emplacement and also to re-asses the palaeoenvironment of the Lagos coastal deposit in Dahomey Basin. The identification of diagnostic species age entails the palynological analysis while the geochemical analysis determines the provenance of these Quaternary sediments. The occurrences of Laevigatosporites sp., Zonocostites ramonae, Acrostichum aureum in abundance along with few long ranging forms suggest that the vegetation development was under a humid climate and that the sediments were deposited during cooler and wetter conditions. The presence and high abundance of Crassoretitriletes vanraadshooveni, Zonocostites ramonae, Canthiumidites sp., Sapotaceoidaepollenites sp. and Pachydermitesdiederixi palynomorphs indicate an age range of late Pliocene (Gelasian) to early Pleistocene (Calabrian) (2.588 - 1.806 Ma). This age range is known to correspond to the 3.7 - 3.8 depositional cycles of relative change of coastal On-lap. The geochemical appraisal showed that the ratios of organic carbon-nitrogen (C/N) indicate that the sediments were sourced from aquatic, protein-rich and cellulose-poor milieu. The wetter climatic period has enhanced algae productivity as a consequence of greater wash-in of soil nutrients, and these periods are recorded as increased rate of organic carbon mass accumulation. Conversely, the dominance of a mangrove habitat, Zonocostites ramonae suggests a mangrove swamp environment which was the most prevalent environment of the Lagos lagoon in the Pleistocene.

The transgressive-regressive cycle of the Romualdo Formation (Araripe Basin): Sedimentary archive of the Early Cretaceous marine ingression in the interior of Northeast Brazil

Geologic events related to the opening of the South Atlantic Ocean deeply influenced the sedimentary record of the Araripe Basin. As consequence, upper stratigraphic units of the basin record a marine ingression in northeastern Brazil during the late Aptian. The timing and stratigraphic architecture of these units are crucial to understand the paleogeography of Gondwana and how the proto-Atlantic Ocean reached interior NE Brazil during the early Cretaceous. This marine ingression is recorded in the Araripe Basin as the Romualdo Formation, characterized by a transgressive-regressive cycle bounded by two regional unconformities. In the eastern part of the basin, the Romualdo depositional sequence comprises coastal alluvial and tide-dominated deposits followed by marine transgressive facies characterized by two fossil-rich intervals: a lower interval of black shales with fossil-rich carbonate concretions (Konservat-Lagerstätten) and an upper level with mollusk-dominated shell beds and shelly limestones. Following the marine ingression, an incomplete regressive succession of marginal-marine facies records the return of continental environments to the basin. The stratigraphic framework based on the correlation of several sections defines a transgressive-regressive cycle with depositional dip towards southeast, decreasing in thickness towards northwest, and with source areas located at the northern side of the basin. The facies-cycle wedge-geometry, together with paleocurrent data, indicates a coastal onlap towards NNW. Therefore, contrary to several paleogeographic scenarios previously proposed, the marine ingression would have reached the western parts of the Araripe Basin from the SSE.

Cambrian trilobite biostratigraphy and its role in developing an integrated history of the Earth system

One of the principal biostratigraphical tools used in the lower Palaeozoic, and especially the Cambrian System, is trilobites. Historically, both polymerids and agnostoids have commonly been included as 'trilobite' groups, although currently the question of whether they comprise a monophyletic group or are polyphyletic is unsettled. Beginning in the late 1800s, the base of the Cambrian System was marked by the lowest occurrence of olenelline trilobites. Discovery of a rich pre-trilobite metazoan record in the mid-1900s led to significant modification of that concept, but polymerids and especially agnostoids still rank among the leading biostratigraphical and chronostratigraphical guides through much of the Cambrian. Chronostratigraphical sub-division of the trilobite-bearing part of the Cambrian System derives largely from biostratigraphical, ecological and evolutionary concepts about agnostoids and polymerids introduced in the 1940s to 1970s by A.H. Westergard, A.R. Palmer and R.A. Robison. Westergard introduced a zonation for Scandinavia that was largely based on agnostoids. Palmer explained the distribution of Cambrian trilobite faunas in terms of restricted and unrestricted access to open oceans. Together, these ideas coalesced in Robison's recognition of separate zonation schemes for restricted-shelf polymerids, open-shelf polymerids and open-shelf (cosmopolitan) agnostoids. Palmer also introduced the concept of biomeres, which placed sharp limits on biostratigraphical intervals recognizable from trilobites. Global correlation in the upper half of the Cambrian today depends to a large extent on the ranges of agnostoids and some polymerids characteristic of open-shelf to slope areas, ones that facilitate identification of precise intercontinental tie points. Agnostoid and polymerid biostratigraphy is now being integrated with information about coastal onlap and eustatic sea-level history, geochemical cycling and other data to provide a more complete understanding of the early Palaeozoic biosphere and its complex physico-chemical context.

Synsedimentary ash rains and paleoenvironmental conditions during the deposition of the Chachil Formation (Pliensbachian) at its type locality, Neuquén Basin, Argentina

A detailed sedimentological analysis of the so called “Chachil Limestones” at its type locality around the Mirador del Chachil area, southwestern Neuquén province, Argentina, is presented in this paper for the first time. It is based on a macro/microfacial analysis and their environmental interpretation by means on texture, fabric, bioclasts, intrabasinal and extrabasinal grain amounts, sedimentary structures, bioturbations and hydro-dynamism. Because of the recognition of different facies associations, but no pure limestones, it is more suitable to refer these sediments as the Chachil Formation. The depositional environment of this unit is interpreted to correspond to an internal platform dominated by tides, with carbonate sedimentation disturbed by repeated explosive volcanic episodes, which reduced the sedimentation space, causing retrogradation of the sedimentary system and coastal onlap. In addition, a new recalibration of the U-Pb zircon dating used for the geochronological analysis reveals a small change with regard to previous information that has been used to recalculate the data, is presented in this paper.

Accommodation succession (δ A /δ S ) sequence stratigraphy: observational method, utility and insights into sequence boundary formation

The future of sequence stratigraphy depends on stratigraphers making observations with a common method so that physical frameworks can be clearly separated from interpretations of driving mechanisms. Depositional sequence boundary selection is a well-known controversy that could be resolved with objective recognition criteria. Accommodation succession sequence stratigraphy refines traditional methods, using sedimentary facies, facies associations, vertical stacking, stratal geometries and stratal terminations as the objective record of competing rates of accommodation change and sediment fill through time. Observations are placed in context of lateral (transgression and regression) and vertical (aggradation and degradation) movement of shoreline through time, across multiple timescales in hierarchal stacks. The repeating motif consists of a subaerial unconformity and its correlative subaqueous surface overlain in coastal settings by a basinward shift in coastal onlap and strata with progradational to aggradation stacking, then retrogradation and aggradation–progradation–degradation stacking. These stacking patterns are bounded by key surfaces, recognized by stratal terminations and characteristic vertical successions of facies. This pattern is independent of time duration or position on a sea-level curve, but incorporates data resolution, regional extent and hierarchal stacking. Examples from multiple datasets show the utility and objectivity of the method and provide insights into sequence boundary formation.

ALLOPOLYMORPHINA N. GEN. (POLYMORPHINIDAE) FROM THE EARLIEST PLIOCENE OF FLINDERS ISLAND, TASMANIA

A new polymorphinid genus and species Allopolymorphina flindersi Quilty and Whitehead is defined from earliest Pliocene shallow-water, fully marine, calcareous sediments from eastern Flinders Island, northeast Tasmania, Tasman Sea. It is a large, elongate form up to 2.5 mm in length (holotype 1.85 mm), initially polymorphinid but changing early to uniserial frondicularine in later chambers. Specimens occur in most samples of this study, but not in others collected previously from the island. The thin source sequence comes from a previously unrecognized depositional event in outcrop in the western margin of the hydrocarbon-bearing Gippsland Basin of southeastern Australia. Deposition occurred during an interval of high sea level and major coastal onlap.

Testing the controls on the seismic sequence stratigraphy of the Eocene–Oligocene boundary in Southern Iran with a Wheeler diagram derived from outcrops, seismic and well logs data

In this study of Southern Iran the timing of the boundary between the Eocene to Oligo–Miocene sections was determined along with a better understanding of the accumulation of the Paleocene to Eocene sediments. This was established by generating Wheeler diagrams from local seismic, well log data and surface data. This boundary was found to be mainly erosional and the time gap between Eocene to Oligo–Miocene displayed by the Wheeler diagram suggests a “degradational vacuity” formed. Relative sea level changes were found to be responsible for the seaward progradational character of the Jahrum Formation sediments. Red sediments and an intraformational conglomerate overlie this erosional boundary between the Paleocene to the Eocene Jahrum Formation and the Oligo–Miocene Asmari Formation.Long-term lower frequency trends in both regional tectonic and global sea-level curves determined from the Paleocene–Eocene sediments of south Iran, when compared to the coastal plain sediments of US New Jersey and the global coastal onlap chart, suggest that contemporaneous eustastic signals in lower Eocene time produced matching sedimentary patterns.The results of the study recorded in this paper are intended to be used as the foundation of the study of petroleum related facies and petroleum system components (source, reservoir and seal rocks) in the Tertiary portion of sedimentary section.

Stratal slice: The next generation

Vail and his colleagues (1977a, b) laid a foundation for seismic chronostratigraphy. By analyzing the relative rise of sea level indicated by coastal onlap, these workers invented a novel way of converting a stratigraphic cross section in the depth or traveltime domain to a chronostratigraphic chart in the Wheeler domain (Figure 1a and Figure 1b). With the widespread use of seismic data in the industry, such conversion can now be achieved conveniently with the proper software at the basinal, regional, or prospect scale (Figure 1c and Figure 1d).

Palynofacies and sedimentology-based high-resolution sequence stratigraphy of the lignite-bearing muddy coastal deposits (early Eocene) in the Vastan Lignite Mine, Gulf of Cambay, India

Geological records of early Paleogene warming are rare in low latitudinal regions. The Indian subcontinent preserves records of this global event on western and eastern margins. We attempt to decipher paleoenvironmental setup and facies architecture of the paleo-equatorial early Eocene succession at the Vastan Lignite Mine, Gulf of Cambay, western India. The Vastan lignite succession was deposited in a low-energy coastal marsh-bay complex receiving only fine-grained muddy sediments from the weathered Deccan Traps. The lower part of the Vastan lignite deposit, designated as “Vastan Succession A”, comprises four depositional facies representing distinct environments (open bay, restricted bay, creek and channel, and coastal marsh) and one diagenetic facies. Palynofacies analysis, backed by precise sedimentological framework, records changes in terrestrial supply and fluctuating marine characters of bay and marshes. Eleven Palyno-Units are identified in distinct lithofacies sequences stacked in shallowing-upward cycles representing five parasequences that constitute a Transgressive Systems Tract (TST) deposit. Each parasequence starts with a transgressive sheet deposit, followed by shallowing-upward bay fill-marsh deposits. In the vertical succession, each parasequence acquires increasing marine character, culminating in a maximum flooding surface (shell carbonate) that represents large-scale coastal onlap during early Ypresian time. The TST is followed by a Highstand Systems Tract deposit, which shows an erosional surface at the top of the upper lignite indicating Lowstand Systems Tract and a sequence boundary at ~52 Ma. The Vastan Succession A represents TST (3rd-order cycle) deposits with parasequences and hemicycles representing 4th- and 5th-order cycles. The study demonstrates sea level rise along the Indian western coastal margin in response to early Eocene warming between ~55 and ~52 Ma with maximum transgression at 53.7 Ma.

Middle-Upper Miocene paleogeography of southern Turkey: insights from stratigraphy and calcareous nannofossil biochronology of the Olukpınar and Başyayla sections (Mut-Ermenek Basin)

The age of the marine succession capping the basement rocks of the central Taurides in the Mut-Ermenek Basin is constrained using calcareous nannofossil biochronology. The Olukpinar section, which correlates with the biozones between MNN5a and MNN6b (late Langhian-late Serravallian), represents a deeper marine environment developed lateral to the Ermenek platform. The long-term transgressive-regressive cycle of the Olukpinar section, which corresponds to the middle-upper part of the TB2 supercycle, can be subdivided into 2 large-scale (106 years) cycles (TB2.4 and TB2.5) based on the occurrence of 2 major debris flow deposits linked to sea-level drop. The younger late Tortonian Basyayla section represents a transgressive-regressive sedimentary cycle (TB3.2 cycle) responsible for the major transgressive event that occurred at the southern margin of the Central Anatolian Plateau during the Late Miocene. This major transgressive event is responsible for the migration of the coastal onlap of the Mediterranean Sea towards the interior of the Anatolian Peninsula, almost 100 km inland from the present Eastern Mediterranean coast line. The ~8 Ma age of the younger marine sediments deposited during the TB3.2 cycle, which onlap the basement rocks of the central Taurides at an elevation of ~2 km, represents a maximum age for the start of surface uplift of the Central Anatolian Plateau's southern margin.

Stratigraphic Hierarchy and Shelf-to-Basin Architecture of Aptian–Albian Mural Shelf, Cerro Caloso Range, Sonora, Mexico

The Mural-Caloso Shelf in Sonora, Mexico is one of only a limited number of outcrops that provide continuous shelf-to-basin profiles of reef-rimmed carbonate platforms with well-defined lowstand deposits and documentable downward shifts in coastal onlap. This paper examines the shelf-to-basin transition across three thirdorder sequences at the Cerro Caloso outcrop. Four parameters: reef framestone lateral and vertical dimensions, fore-reef slope lateral and vertical dimensions, evolution of platform interior facies, and presence or absence of lowstand systems tract siliciclastic wedges with or without mixed oolitic grainstone were used as the basis for the characterization of the shelf to basin profile evolution. The lower two sequences (S3 and S4) are mostly aggradational with broad coral-rudist reef rimmed margins and thick platform interiors dominated by carbonate facies. The uppermost sequence (S5) is progradational and downstepping with a narrow coralrudist reef rimmed margin and a thin siliciclastic-rich platform interior. A lowstand wedge of fine-medium grained sandstone and cross-bedded oolitic grainstones that onlaps the S4 coralrudist margin documents downstepping and a minimum of 20 m of downward shift in coastal onlap. The lower sequence wedge is relatively thin compared to the thickness of the S5 wedge. The long-term progradation and increased clastic component observed in the latter stages of shelf evolution indicate a large second order relative sea level fall during the deposition of the Lower Albian age Mural-Caloso Shelf. The continuous outcrop provides a good reference model for facies patterns and stratigraphic architecture for the time equivalent Cretaceous platforms that develop similar geometries such as the Shuiba shelf in the Middle East.

Sedimentary evolution of an inner foreland basin margin: Paleogene Promina Beds of the type area, Mt. Promina (Dinarides, Croatia)

Mt. Promina became famous for its coal mines, vertebrate remains and fossil flora, and for the use of its name in denoting sediments covering a wide area of the Dinaric coastal range. Up to 1 km thick Promina Beds of the type area, Mt. Promina, evolved in the context of propagating Dinaric deformation towards the foreland. They consist of eight sedimentary units differing in facies, i.e. depositional environments and systems, which include alluvial, lacustrine, deltaic, lagoonal and shelf deposits. The sedimentary units constitute three depositional cycles. Cycle I is represented by a late lowstand alluvium, while Cycle II and III consist of transgressive and highstand (regressive) systems tracts. Cycle III also includes a coastal onlap of limestones and an alluvial onlap. Transgressive tracts are specific in being represented by lacustrine deposits and having non-erosive bases. Falling-stage sediments have not been identified. The Promina Beds of Mt. Promina represent an example of sedimentary and sequence evolution at a foreland basin margin. The evolution of this margin is characterised by structural and stratigraphic complexities including syndepositional folding, growth strata, angular unconformities, intraformational faulting, alluvial and marine onlaps onto deformed basement, and tectonically induced, proximal-distal variations in facies and sediment thickness. The origin of lacustrine transgressive tracts may have been related to backtilting in the proximal part of the basin. High sediment supply and subsidence pulses are proposed to have been the two main factors which shaped depositional styles, sequence development and distribution of sediments. The dominant role of tectonism in the sedimentary evolution was related to the position of the area within deforming, marginal parts of the foreland basin, situated in the close vicinity to the main, active Dinaric thrust units.

Presenting seismic stratigraphy and attribute analysis as pioneer techniques for delineation of reservoir quality sand bodies in the Indus Offshore Basin, southwest Pakistan

It is generally believed that various exploration failures in the offshore areas of Pakistan are due to the absence of a mature petroleum system. However, a few recent gas shows and shales with 0.55–3.24% TOC suggest that a Middle-Upper Miocene lowstand systems tract (LST) exists that possesses organic, paleo-environmental, and sequence stratigraphic characteristics of a potential source rock. The objective of this work was to highlight and delineate the slope and basin facies through seismic sequence stratigraphy and seismic attribute analysis as a new approach to the interpretation of the southwestern Lower Indus Basin of Pakistan. Working within a sequence stratigraphic framework, four sequences have been delineated on the basis of reflection termination patterns. A sea-level curve was constructed from the reflection patterns representing coastal onlap. This sea-level curve was compared to a hypothetical sea-level curve generated by a spectral analysis Fourier transform function on the density logs.

Neoproterozoic basin evolution in Fennoscandia, East Greenland and Svalbard

Neoproterozoic successions of Fennoscandia, East Greenland and Svalbard are related to crustal extension and formation of sedimentary basins along the margins of northern Baltica (Fennoscandia) and eastern Laurentia (East Greenland and Svalbard), preceding final break-up of Rodinia. The early rift stage (late Tonian-Cryogenian) is characterized by up to 16 km thick sedimentary successions of deep-marine sandstones and conglomerates linked to rift and strike-slip basins. Pericratonic basins expanded during Cryogenian-Cambrian coastal onlap. Cryogenian tropical climate is reflected by carbonate and evaporitic formations, most of which predate Cryogenian-Ediacaran glaciations. Glacial units, collectively referred to the Varanger Ice Age, may be equivalent to the Marinoan (c. 630 Ma) and the Gaskiers (c. 580 Ma) glacial periods. The final stage in break-up of Rodinia commenced with the emplacement of dolerite dyke swarms along the Baltoscandian margin at c. 600 Ma and the opening of the Iapetus Ocean and other sea ways. No such dyke swarms have been recorded along the East Greenland segment of the Laurentian margin. Several Tonian-Cambrian tectonic and magmatic events recorded within the Kalak Nappe Complex in northern Finnmark make this unit an exotic terrane relative to the autochthonous Baltoscandian platform.

Shallow-marine carbonates of the tropical-temperature transition zone: effects of hinterland climate and basin physiography (late Miocene, Crete, Greece)

Abstract In modern oceans, the transition zone between the tropical and temperature carbonate province is gradual and covers a wide latitudinal belt. Little knowledge exists regarding the geological signatures of this zone. This paper describes a late Miocene (early Tortonian-early Messinian) transitional carbonate system that combines elements of the tropical and cool-water carbonate systems (Iraklion Basin, island of Crete, Greece). As documented in stratal geometries, the submarine topography of the basin was controlled by tilting blocks. Coral reefs formed by Porites and occurred in a narrow clastic coastal belt along a central Cretan landmass and steep escarpments formed by faulting. On the gentle dip-slope ramps of those blocks having the widest geographical distribution within the basin, extensive covers of level-bottom communities existed in a low-energy environment. Isolated colonial corals were present in the shallow segments of the ramps. Consistent patterns of landward and basinward shift of coastal onlap in all outcrop studies reveal an overriding control of third- and fourth-order sea-level changes on sediment dynamics and facies distributions over block movements. An increasingly dry climate and the complex submarine topography of the fault-block mosaic kept sediment and nutrient discharge from a central Cretan landmass at a minimum. The skeletal limestone facies therefore reflects oligotrophic conditions and sea surface temperatures near the lower threshold temperature of coral reefs in a climatic position transitional between the tropical coral reef belt and the temperate zone. It is suggested that the recognition of an overall late Miocene aridification trend helps to explain the Mediterranean-wide distribution of shallow-marine carbonates, both cool-water and warm-water, in settings adjacent to uplifting mountain ranges (intramontane basins).

Fluviodeltaic response to sea level perturbations: Amplitude and timing of shoreline translation and coastal onlap

Alluvial river systems respond to changing sea level by adjusting channel length through shoreline translation and coastal onlap/offlap at the alluvial basement transition. The magnitude and timing of shoreline translation and coastal onlap/offlap relative to the driving sea level signal depend on the dynamics of the fluvial system and have important implications for interpreting ancient fluviodeltaic deposits. I obtain analytical solutions to a diffusive geometric model of fluviodeltaic response to sea level perturbations that treats the shoreline and alluvial basement transition as moving boundaries. Fluviodeltaic behavior is most sensitive to the ratio of the period of eustatic cycling to the fluvial response time, which varies with the length and water discharge of the fluvial system. The square root of this ratio provides a first‐order estimate of the fraction of the fluvial long profile affected by eustatic forcing. The fluviodeltaic system behaves as a low‐pass filter with respect to eustatic cycling: The amplitude of both shoreline translation and coastal onlap/offlap increase with increasing cycle period within a relatively narrow bandwidth in which the fluvial response time is comparable to the cycle period. Only small time lags develop between shoreline response and eustatic forcing. The time lag between maximum coastal onlap and maximum sea level increases without bound as the forcing period decreases relative to the fluvial response time. Model predictions agree with experimental data and differ considerably from the model predictions of Pitman (1978) and Angevine (1989), which suggest that fluviodeltaic systems are strong high‐pass filters to sea level cycling.