Proglacial Sediment—Landform Associations of a Polythermal Glacier: Storglaciären, Northern Sweden

Mapping and laboratory analysis of the sediment—landform associations in the proglacial area of polythermal Storglaciären, Tarfala, northern Sweden, reveal six distinct lithofacies. Sandy gravel, silty gravel, massive sand and silty sand are interpreted as glaciofluvial in origin. A variable, pervasively deformed to massive clast‐rich sandy diamicton is interpreted as the product of an actively deforming subglacial till layer. Massive block gravels, comprising two distinctive moraine ridges, reflect supraglacial sedimentation and ice‐marginal and subglacial reworking of heterogeneous proglacial sediments during the Little Ice Age and an earlier more extensive advance. Visual estimation of the relative abundance of these lithofacies suggests that the sandy gravel lithofacies is of the most volumetric importance, followed by the diamicton and block gravels. Sedimentological analysis suggests that the role of a deforming basal till layer has been the dominant factor controlling glacier flow throughout the Little Ice Age, punctuated by shorter (warmer and wetter climatic) periods where high water pressures may have played a more important role. These results contribute to the database that facilitates discrimination of past glacier thermal regimes and dynamics in areas that are no longer glacierized, as well as older glaciations in the geological record.     

ZONATION OF GLACIOMARGINAL ENVIRONMENT INFERRED FROM PLEISTOCENE DEPOSITS OF MYSLIBORZ LAKELAND, NW POLAND

The study area in Mysliborz Lakeland (Western Pomerania), NW Poland, is located in the glaciomarginal zone of the Pomeranian Phase (Weichselian glaciation). Three subenvironments of deposits are exposed in the six analysed pits: glacial ice-contact subenvironment, transitional subenvironment and glaciofluvial subenvironment. The glacial ice-contact subenvironment is dominated by coarse-grained, massive and thick beds derived from debris flows and also horizontally laminated sand and gravel deposited by sheet flows. The transitional subenvironment represents deposits between end moraine and proximal outwash plain (sandur). The transitional subenvironment is dominated by coarse-grained diamicton derived from debris flows, horizontally laminated beds deposited by sheet flows, and additionally cross-stratified sediments from channel flows and massive sand from hyper-concentrated flows. The glaciofluvial subenvironment corresponds with the distal part of the glaciomarginal zone. The processes of sediment deposition in the glaciofluvial subenvironment are derived from shallow sheet flows and channelized flows. Architectural element analysis highlights the change in lithofacies associations from glacial ice-contact, through transitional to glaciofluvial subenvironments. This sedimentological zonation of the glaciomarginal zone may have been developed in other glaciomarginal belts and remains to be recognized.     

SPECIFIC EROSIONAL AND DEPOSITIONAL PROCESSES IN A PLEISTOCENE SUBGLACIAL TUNNEL IN THE WIELKOPOLSKA REGION,POLAND

The Pleistocene Cie?le succession accumulated in a subglacial tunnel and shows three sedimentological units: (1) trough cross‐stratified sand with granules deposited in deep channels up to 5.4 m, (2) trough stratified and massive gravels deposited in a very deep channel up to 6.2 m eroded by a catastrophic hyperconcentrated flow, and (3) a massive diamicton, interpreted as a basal till of melt‐out type. We focus on angular and deformed sandy clasts that occur in the second unit. It appears that thermal erosion, short transport in a sediment‐laden current and sudden sedimentation were responsible for the oversized sandy clasts that occur in the gravel glaciofluvial deposits. The deposits are characterized by large‐scale erosional scours, massive structure, and fluid‐escape deformations. This combination of features can be used as a key tool for the interpretation of hyperconcentrated‐flow conditions beneath Pleistocene ice sheets.     

Trends in Year-To-Year Fluctuations in Surface Air Temperature,Canadian Arctic

The northwest portion of the Tekonsha Moraine has traditionally been considered to be a product of, and composed of sediments from, the Lake Michigan Lobe. A recent study has also correlated till from the moraine with an exposure (Ganges till) along the shore of Lake Michigan. The results of this paper contradict those interpretations. Multiple lines of evidence—till fabrics, 7-Å/10-Å peak height ratios of clay minerals, indicator stones, morphology, and elevation relationships—indicate a Saginaw Lobe origin for the till and glaciofluvial sediments. Moreover, aerial photographs reveal the presence of ice-stagnation topography. The moraine is herein reinterpreted as a hummocky and pitted (collapsed) outwash plain/kame moraine. Given that this feature has long been considered a moraine, similar landforms in Michigan and elsewhere may be more common than heretofore recognized. [Key words: glacial geomorphology, Michigan, Quaternary, Tekonsha Moraine.]     

Sedimentary Record of the Younger Saalian Ice Margin Stagnation in Eastern Poland: Development of a Regular Pattern of Glaciolacustrine Kames

Glaciolacustrine kames in the Bielsk Podlaski area (eastern Poland) exhibit a unique regular pattern. Three representative morphological kame types were chosen for detailed sedimentological analyses, specifically: isolated, isometric mounds; isolated, elongated hills; and branching ridges. All types comprised fine‐grained sandy and sandy/silty deposits. Lithofacies analysis resulted in the distinction of several lithofacies associations. Associations dominated by medium‐ or large‐scale, massive or horizontally laminated sands are interpreted as proximal subaqueous fans; associations dominated by medium‐ or small‐scale lithofacies of ripple‐drift cross‐laminated sand are interpreted as distal subaqueous fans; and those dominated by sandy/silty, silty or silty/clayey lithofacies with horizontal lamination are interpreted as lake bottomsets. Rates of sediment accumulation appear to have been fast, resulting in syndepositional and metadepositional deformation structures of two types: water‐escape structures, and slumps on subaqueous slopes. After the ice‐walled lake basins filled with sediment, glaciofluvial erosion and deposition alternated, resulting in erosional channels of up to 1 m deep, later filled with gravel or gravely sand. The results indicate that kames developed in a supraglacial environment within a topography of ice‐cored moraines containing ice‐walled lakes that persisted due to the presence of permafrost. Pauses during retreat of the ice walls resulted in ice‐contact deformations at the edges of the kames. Kame formation is therefore consistent with a continental climate and this may explain the increased abundance of this type of kame system in Eastern Europe.     

Late glacial and Holocene marine records from the Independence Fjord and Wandel Sea regions, North Greenland

The first marine sediment cores from the unexplored Independence Fjord system and the Wandel Sea, North Greenland, have been investigated to reveal the glacial marine history of the region. Two key sites in the Independence Fjord system, and an earlier analysed site from the Wandel Sea continental slope, off the mouth of Independence Fjord, are presented. The Independence Fjord sites reveal an early Holocene record (10.0–8.9 Kya) of fine-grained reddish muds with calcareous microfossils, dominated by the benthic foraminifera Cassidulina neoteretis . We suggest that a semi-permanent fast ice cover characterized the region in the early Holocene, and that the deeper troughs in the mouth region of the Independence Fjord system were intruded by subsurface Atlantic water. A stiff diamicton, at least 1.3 m thick, with coal and sandstone clasts of mainly local origin, and a 0.5-m-thick Holocene cover, are found in one of the sites. The diamicton is assumed to represent a subglacial till predating the early Holocene sediments (>10 Kya). Shallow seismic records off the mouth of Independence Fjord reveal kilometre-sized troughs with signs of glacial erosion, till deposition and a Holocene glaciomarine deposition. These features could indicate that glacial ice debouching from the Independence Fjord system at some time during the last glacial period extended to the mid-outer Wandel Sea shelf. Data from a high-resolution sediment core previously retrieved from the adjacent Wandel Sea slope indicate that the maximum ice sheet advance in this area culminated about 25–20 Kya.     

Evidence of subglacial karstification in Glomdal,Svartisen, Norway

The central Glomdal caves, Storbekkgrotta and Kalkrastgrotta, are found within a 30 m (approximately) thick band of white marble. The karst system is allogenic and the cave morphology and surface topography provide evidence of several phases of development:

1. Phreatic conduit development.

2. Drainage and vadose incision.

3. Partial or complete infill with clastic (glaciogenic) sediments.

4. Glacial truncation of the conduits.

5. Reversed, phreatic flow in the up-hill direction, producing paragenetic halftubes and rock pendants.

6. Partial outwash of the previous infilling sediment.

7. Holocene vadose invasion.

Since the paragenetic features are superimposed onto vadose features, the reversed flow must have occurred after base-level (i.e. bedrock) lowering. Together with the topographical situation of the karst, phase 5 (paragenesis under reversed flow) may be explained by subglacial water flow. This implies a temperate glacier sole at a time when the icefield could over-ride the local valley topography and provide an up-hill directed englacial hydraulic gradient.

The results suggest that sub-glacial water flow may utilize pre-existing karst conduits (which are extremely stable Nye-channels in the glaciohydrologic context) and enlarge them somewhat. It is still not proven that entire cave passages with radii of 1 to 2 m may form under such sub-glacial conditions, as suggested by Horn in 1947. The chemistry and conduit hydrology of deep sub-glacial waters are crucial parameters in this regard.     

Downstream Fining and Sorting of Gravel Clasts in the Braided Rivers of mid-Canterbury, New Zealand

Gravel clast size dimensions have been determined in the Rakaia, Ashburton, and Rangitata rivers by measuring 100 clasts at representative sample locations along each river. In all rivers, gravel size decreases and sorting improves downstream for mean, D₅₀, and D₉₀ fractions of the bed material. Clast size entering the sea is similar in all rivers (30–40 mm b‐axis dimension), despite large variations in transport distance, input size of clasts at their gorges, and discharge. The greatest size reduction occurs in the Rangitata River which has the shortest transport distance and steepest gradient. Rates of downstream clast size reduction are greater than would be assumed from Sternberg's Law, suggesting that additional factors, other than physical abrasion, such as sorting and selective entrainment operate.     

The distribution of silty soils in the Grayling Fingers region of Michigan: Evidence for loess deposition onto frozen ground

This paper presents textural, geochemical, mineralogical, soils, and geomorphic data on the sediments of the Grayling Fingers region of northern Lower Michigan. The Fingers are mainly comprised of glaciofluvial sediment, capped by sandy till. The focus of this research is a thin silty cap that overlies the till and outwash; data presented here suggest that it is local-source loess, derived from the Port Huron outwash plain and its down-river extension, the Mainstee River valley. The silt is geochemically and texturally unlike the glacial sediments that underlie it and is located only on the flattest parts of the Finger uplands and in the bottoms of upland, dry kettles. On sloping sites, the silty cap is absent. The silt was probably deposited on the Fingers during the Port Huron meltwater event; a loess deposit roughly 90 km down the Manistee River valley has a comparable origin. Data suggest that the loess was only able to persist on upland surfaces that were either closed depressions (currently, dry kettles) or flat because of erosion during and after loess deposition. Deep, low-order tributary gullies (almost ubiquitous on Finger sideslopes) could only have formed by runoff, and soil data from them confirm that the end of gully formation (and hence, the end of runoff) was contemporaneous with the stabilization of the outwash surfaces in the lowlands. Therefore, runoff from the Finger uplands during the loess depositional event is the likely reason for the absence of loess at sites in the Fingers. Because of the sandy nature and high permeability of the Fingers' sediments, runoff on this scale could only have occurred under frozen ground conditions. Frozen ground and windy conditions in the Fingers at the time of the Port Huron advance is likely because the area would have been surrounded by ice on roughly three sides. This research (1) shows that outwash plains and meltwater streams of only medium size can be significant loess sources and (2) is the first to present evidence for frozen ground conditions in this part of the upper Midwest.     

The sedimentary record of the Issyk Kul basin,Kyrgyzstan: climatic and tectonic inferences

A broad array of new provenance and stable isotope data are presented from two magnetostratigraphically dated sections in the south‐eastern Issyk Kul basin of the Central Kyrgyz Tien Shan. The results presented here are discussed and interpreted for two plausible magnetostratigraphic age models. A combination of zircon U‐Pb provenance, paleocurrent and conglomerate clast count analyses is used to determine sediment provenance. This analysis reveals that the first coarse‐grained, syn‐tectonic sediments (Dzhety Oguz formation) were sourced from the nearby Terskey Range, supporting previous thermochronology‐based estimates of a ca. 25–20 Ma onset of deformation in the range. Climate variations are inferred using carbonate stable isotope (δ¹⁸O and δ¹³C) data from 53 samples collected in the two sections and are compared with the oxygen isotope compositions of modern water from 128 samples. Two key features are identified in the stable isotope data set derived from the sediments: (1) isotope values, in particular δ¹³C, decrease between ca. 26.0 and 23.6 or 25.6 and 21.0 Ma, and (2) the scatter of δ¹⁸O values increased significantly after ca. 22.6 or 16.9 Ma. The first feature is interpreted to reflect progressively wetter conditions. Because this feature slightly post‐dates the onset of deformation in the Terskey Range, we suggest that it has been caused by orographically enhanced precipitation, implying that surface uplift accompanied late Cenozoic deformation and rock uplift in the Terskey Range. The increased scatter could reflect variable moisture source or availability caused by global climate change following the onset of Miocene glaciations at ca. 22.6 Ma, or enhanced evaporation during the Mid‐Miocene climatic optimum at ca. 17–15 Ma.     

Tracking Holocene environmental changes in an alpine lake sediment core: application of regional diatom calibration, geochemistry, and pollen

Diatom abundances in the surface sediment samples of 41 mountain lakes in the central Austrian Alps (Niedere Tauern) were related to environmental variables using multi-variate techniques. Canonical correspondence analysis (CCA) revealed that the pH, date of autumn mixing (A _mix), mean August water temperature (T _Aug), dissolved organic carbon (DOC), and relative water depth (Z _rel) made significant contributions to explain the diatom assemblage variation in the lakes of the training set. A weighted averaging partial least square regression and calibration model was used to establish Di-pH (R ² _boot= 0.72, RMSEP_boot= 0.131), and a thermistor measurements-based PLS model for A _mix (R ² _boot= 0.71, RMSEP_boot= 0.006 log₁₀ Julian days). The latter showed a better prediction than T _Aug, and was used in terms of climate change. These transfer functions, together with analyses of loss on ignition (LOI), the total carbon/nitrogen (C/N)-ratios, and selected pollen, were applied to an early to mid-Holocene (11.5–4 cal. ky BP) sediment core section from an Austrian Alpine treeline lake on crystalline bedrock. Additionally, passive sample scores in the CCA of the diatom training set were used to show trends in the variables DOC and Z _rel. During the early Holocene, diatoms indicative of increased pH, extended warm summers, and low water levels dominated. Between 10.2 and 7.6 cal. ky BP it was followed by diatom assemblages that indicated an increase in lake water depth and an earlier A _mix. The multi-proxy data suggest that the A _mix decline is the result of a series of snow-rich summer cool and wet climate fluctuations, which were divided by climate warming at ∼9 cal. ky BP. Increased A _mix, LOI and DOC, and the correspondent decline in the C/N-ratios, show subsequent climate warming between 7.3 and 6 cal. ky BP. The long-term trend in Di-pH indicates the impact of catchment-related processes during the early-Holocene, that were superimposed by climate.     

The Pliocene‐Quaternary wedge‐top basins of southern Italy: an expression of propagating lateral slab tear beneath the Apennines

In recent years, contrasting seismic tomographic images have given rise to an extensive debate about the occurrence and implications of migrating slab detachment beneath southern Italy. One of the most pertinent aspects of this process is the concentration of the slab pull force, and particularly its surface expression in terms of vertical motions and related basin subsidence/uplift. In this study we focused on shallow‐water to continental, Pliocene‐Quaternary basins that formed on top of the Apennine allochthonous wedge after its emplacement onto a large foreland carbonate platform domain (Apulian Platform). Due to the thick‐skinned style of deformation controlling the Pliocene‐Pleistocene stages of continental shortening, a high degree of coupling with the downgoing plate appears to characterize the late tectonic evolution of the southern Apennines. Therefore, the wedge‐top basins analysed in this study, although occurring on the deformed edge of the overriding plate, are capable of recording deep geodynamic processes affecting the slab. Detailed stratigraphic work on these wedge‐top basins points to a progressive SE‐ward migration of basin subsidence from c. 4 to c. 2.8 Ma over a distance of about 140 km along the strike of the Apennine belt. Such a migration is consistent with a redistribution of slab‐pull forces associated with the progressive lateral migration at a mean rate in the range of 12–14 cm y^–1 of a slab tear within the down‐going Adriatic lithosphere. These results yield fundamental information on the rates of first‐order geodynamic processes affecting the slab, and on related surface response.     

Unroofing the core of the central Andean fold‐thrust belt during focused late Miocene exhumation: evidence from the Tipuani‐Mapiri wedge‐top basin,Bolivia

As the highest part of the central Andean fold‐thrust belt, the Eastern Cordillera defines an orographic barrier dividing the Altiplano hinterland from the South American foreland. Although the Eastern Cordillera influences the climatic and geomorphic evolution of the central Andes, the interplay among tectonics, climate and erosion remains unclear. We investigate these relationships through analyses of the depositional systems, sediment provenance and ⁴⁰Ar/³⁹Ar geochronology of the upper Miocene Cangalli Formation exposed in the Tipuani‐Mapiri basin (15–16°S) along the boundary of the Eastern Cordillera and Interandean Zone in Bolivia. Results indicate that coarse‐grained nonmarine sediments accumulated in a wedge‐top basin upon a palaeotopographic surface deeply incised into deformed Palaeozoic rocks. Seven lithofacies and three lithofacies associations reflect deposition by high‐energy braided river systems, with stratigraphic relationships revealing significant (～500 m) palaeorelief. Palaeocurrents and compositional provenance data link sediment accumulation to pronounced late Miocene erosion of the deepest levels of the Eastern Cordillera. ⁴⁰Ar/³⁹Ar ages of interbedded tuffs suggest that sedimentation along the Eastern Cordillera–Interandean Zone boundary was ongoing by 9.2 Ma and continued until at least ～7.4 Ma. Limited deformation of subhorizontal basin fill, in comparison with folded and faulted rocks of the unconformably underlying Palaeozoic section, implies that the thrust front had advanced into the Subandean Zone by the 11–9 Ma onset of basin filling. Documented rapid exhumation of the Eastern Cordillera from ～11 Ma onward was decoupled from upper‐crustal shortening and coeval with sedimentation in the Tipuani‐Mapiri basin, suggesting climate change (enhanced precipitation) or lower crustal and mantle processes (stacking of basement thrust sheets or removal of mantle lithosphere) as possible controls on late Cenozoic erosion and wedge‐top accumulation. Regardless of the precise trigger, we propose that an abruptly increased supply of wedge‐top sediment produced an additional sedimentary load that helped promote late Miocene advance of the central Andean thrust front in the Subandean Zone.     

A complete classification of spatial relations using the Voronoi-based nine-intersection model

In this article we show that the Voronoi-based nine-intersection (V9I) model proposed by Chen et al. (2001, A Voronoi-based 9-intersection model for spatial relations. International Journal of Geographical Information Science, 15 (3), 201–220) is more expressive than what has been believed before. Given any two spatial entities A and B, the V9I relation between A and B is represented as a 3 × 3 Boolean matrix. For each pair of types of spatial entities that is, points, lines, and regions, we first show that most Boolean matrices do not represent a V9I relation by using topological constraints and the definition of Voronoi regions. Then, we provide illustrations for all the remaining matrices. This guarantees that our method is sound and complete. In particular, we show that there are 18 V9I relations between two areas with connected interior, while there are only nine four-intersection relations. Our investigations also show that, unlike many other spatial relation models, V9I relations are context or shape sensitive. That is, the existence of other entities or the shape of the entities may affect the validity of certain relations.     

The Origin and Significance of Debris-charged Ridges at the Surface of Storglaciären, Northern Sweden

Storglaciären is a 3.2 km long polythermal valley glacier in northern Sweden. Since 1994 a number of small (1–2 m high) transverse debris‐charged ridges have emerged at the ice surface in the terminal zone of the glacier. This paper presents the results of a combined structural glaciological, isotopic, sedimentological and ground‐penetrating radar (GPR) study of the terminal area of the glacier with the aim of understanding the evolution of these debris‐charged ridges, features which are typical of many polythermal glaciers. The ridges originate from steeply dipping (50–70°) curvilinear fractures on the glacier surface. Here, the fractures contain bands of sediment‐rich ice between 0.2 and 0.4 m thick composed of sandy gravel and diamicton, interpreted as glaciofluvial and basal glacial material, respectively. Structural mapping of the glacier from aerial photography demonstrates that the curvilinear fractures cannot be traced up‐glacier into pre‐existing structures visible at the glacier surface such as crevasses or crevasse traces. These curvilinear fractures are therefore interpreted as new features formed near the glacier snout. Ice adjacent to these fractures shows complex folding, partly defined by variations in ice facies, and partly by disseminated sediment. The isotopic composition (δ¹⁸O) of both coarse‐clear and coarse‐bubbly glacier ice facies is similar to the isotopic composition of the interstitial ice in debris layers that forms the debris‐charged ridges, implying that none of these facies have undergone any significant isotopic fractionation by the incomplete freezing of available water. The GPR survey shows strong internal reflections within the ice beneath the debris‐charged ridges, interpreted as debris layers within the glacier. Overall, the morphology and distribution of the fractures indicate an origin by compressional glaciotectonics near the snout, either at the thermal boundary, where active temperate glacier ice is being thrust over cold stagnant ice near the snout, or as a result of large‐scale recumbent folding in the glacier. Further work is required to elucidate the precise role of each of these mechanisms in elevating the basal glacial and glaciofluvial material to the ice surface.     

Changing sedimentation patterns due to historical land-use change in Frenchman’s Bay,Pickering, Canada: evidence from high-resolution textural analysis

This study examines the anthropogenic alteration of sedimentation in Frenchman’s Bay in Lake Ontario, using high-resolution particle size analysis in two 200 cm cores. Lithofacies were determined using the particle size data of both the terrigenous sediment and terrigenous sediment + diatom fractions. Terrigenous particle size data from the centre of the lagoon provided the most representative record of anthropogenic impacts. Three distinctive lithofacies were recognized: (1) a Natural Wetland (NW) lithofacies (106–200 cm) had an average mean particle size of 49.4 μm, a mode of 29.4 μm and an average standard deviation of 119.1 μm; (2) an Agricultural and Deforestation (AD) lithofacies (40–105 cm) had a statistically significant lower average mean (30.8 μm), mode (13.5 μm), and standard deviation (48.5) μm; (3) an Urbanized (U) lithofacies (0–40 cm) showed a continued trend towards smaller particle sizes with an average mean of 21.2 μm, a mode of 9.4 μm, and an average standard deviation of 32.7 μm. The lithofacies correlated with previously identified trends in thecamoebian biofacies and magnetic susceptibility data showing post-colonial lagoon eutrophication and increased overland soil erosion. The up-core trend towards finer and less variable particle sizes is attributed to erosion of fine-grained watershed sediments (glacial Lake Iroquois silts and clay) during land-clearance and modification of natural drainage patterns. The influx of silts and clays into the lagoon is also recorded by increased sediment accumulation rates and a reduction in seasonal sediment variability in the wetland. Based on the ²¹⁰Pb dates, sedimentation rates increased at 1850 ±56 AD (AD lithofacies) and suggest an exponentially increasing trend in accumulation rates. Increasing sedimentation rates can be attributed to the progressive loss of native vegetation and intensified erosion of Lake Iroquois deposits via stream and hillslope erosion. Ecologically, the increased input of fine-grained sediments into the wetland has resulted in reduced water clarity and has altered the wetland substrate contributing to wetland loss in Frenchman’s Bay.     

The grade index model as a rationale for autogenic nonequilibrium responses of deltaic clinoform to relative sea-level rise

Grade index (G_index) is a dimensionless number given as the volume-in-unit-time ratio of subaerial allocation to both subaerial and subaqueous allocations of sediment supplied to a delta from upstream. It was originally proposed for understanding the effect of basin water depth on the morphodynamics of delta distributary channels under stationary relative sea level. We here examine how rising relative sea level modulates the G_index, using geometrical reasoning and numerical simulations. We find that the grade index model can account for autoretreat of the deltaic shoreline, autodrowning of the whole system, and autobreak of the deltaic sedimentation, all of which are the consequences of autogenic nonequilibrium responses to steadily rising relative sea level. The regressive-to-transgressive threshold (i.e. the onset of autoretreat) is crossed when the delta plain's dimensionless basal area (A_t*) encounters a critical value that is expressed in terms of G_index: regression and transgression are sustained when A_t* is below and above the threshold, respectively. The mode of transgression depends on the slope conditions. If the hinterland slope (γ) is steeper than the foreset slope (β), both A_t* and G_index decrease as the relative sea-level rises. Eventually, the depositional system experiences autodrowning when A_t* = G_index = 0. If γ < β; on the other hand, both A_t* and G_index increase. This latter slope condition eventually causes autobreak of the deltaic sedimentation, afterward of which A_t* = G_index = 1. The grade index model is useful for interpreting and predicting the stratigraphic responses of natural deltaic clinoforms in conditions of rising relative sea level.