Mound Morphology Research Articles

Supply of Allochthonous Sediment and Its Effects on Development of Carbonate Mud Mounds, Mississippian Lake Valley Formation, Sacramento Mountains, South-Central New Mexico, U.S.A.

ABSTRACT Carbonate mud mounds in the Mississippian Lake Valley Formation, New Mexico, can be classified according to their external morphology, internal growth phases, and paleogeographic location along the Lake Valley ramp. Most of the Lake Valley mounds grew during a complete third-order accommodation cycle (Alamogordo-Nunn-Tierra Blanca interval; 3-6 Myr duration), which directly influenced mound biota, facies types, and the physical interactions between the mounds and sediment gravity flows that transported skeletal sediment around and over the mounds during the regressive phase of the cycle. Lenticular mounds are found in updip parts of the preserved Lake Valley ramp, which largely consists of outer ramp facies. Most lenticular mounds grew exclusively during deposition of the transgressive Alamogordo Member, when outer parts of the ramp were somewhat sediment starved. Coarse-grained carbonate gravity-flow deposits (Nunn and Tierra Blanca members) typically onlap lenticular mounds and only locally interfinger with mound-core facies. These relationships indicate lenticular mounds were buried by carbonate gravity-flow deposits, which accumulated much faster than mound cores could aggrade. The sediment gravity flows constructed shingled lobes, sand sheets, and oblique clinoforms and were erosive at times, which also probably helped to shape lenticular mounds. Transitional mounds are typically located downdip from the lenticular mounds and had an initial lenticular growth phase, which was followed by aggradational growth of mound cores. Aggradational mound cores were locally modified by submarine erosion or failure, which created sharp stratal truncation surfaces that were later onlapped by gravity-flow deposits. After the aggradational phase and stratal onlap, mounds with remnant seafloor relief served as substrates for continued growth during deposition of the Tierra Blanca Member. This final growth phase in transitional mounds records minor aggradation and significant, typically asymmetric, lateral accretion, which suggests that: (1) the mounds had grown to some accommodation limit (possibly storm wave base), and (2) the complex dispersal and accumulation patterns for the gravity flows also influenced how mound cores accreted laterally. For example, the upcurrent sides of some transitional mounds were buried preferentially by onlapping Tierra Blanca gravity-flow deposits, whereas their downcurrent sides locally accreted for significantly greater distances and with very different stratal geometries. Hemispherical mounds are located farthest downdip on the Lake Valley ramp. They are the largest mounds and contain all three (lenticular, aggradational, and accretionary) growth phases. Later growth phases of hemispherical mounds have steeply dipping mound core-to-flank relationships with abrupt facies transitions. Because of their distal position along the Lake Valley ramp and greater distance from the progradational sand bodies, the hemispherical mounds were the dominant sources for coarse-grained carbonate sediment to adjacent non-mound environments during Nunn and Tierra Blanca time. Thus, the distance of lateral accretion and overall morphology of hemispherical mounds were not affected greatly by allochthonous sediment from updip parts of the ramp.

Epitaxial mounding in limited-mobility models of surface growth

We study, through large-scale stochastic simulations using the noise reduction technique, surface growth via vapor deposition, e.g., molecular beam epitaxy (MBE), for simple nonequilibrium limited mobility solid-on-solid growth models, such as the Family model, the Das Sarma--Tamborenea model, the Wolf-Villain (WV) model, the larger-curvature (LC) model, and other related models. We find that $(d=2+1)$-dimensional surface growth in several noise reduced models (most notably the WV and LC models) exhibits spectacular quasiregular mound formation with slope selection in their dynamical surface morphology in contrast to the standard statistically scale-invariant kinetically rough surface growth expected (and earlier reported in the literature) for such growth models. The mounding instability in these epitaxial growth models does not involve the Ehrlich-Schwoebel step-edge diffusion barrier. The mounded morphology in these growth models arises from the interplay between the line tension along step edges in the plane parallel to the average surface and the suppression of noise and island nucleation. The line tension tends to stabilize some of the step orientations that coincide with in-plane high-symmetry crystalline directions, and thus the mounds that are formed assume quasiregular structures. The noise reduction technique developed originally for Eden-type models can be used to control the stochastic noise and enhance diffusion along the step edge, which ultimately leads to the formation of quasiregular mounds during growth. We show that by increasing the diffusion surface length together with supression of nucleation and deposition noise, one can obtain a self-organization of the pyramids in quasiregular patterns. The mounding instability in these simple epitaxial growth models is closely related to the cluster-edge diffusion (as opposed to step-edge barrier) driven mounding in MBE growth, which has been recently discussed in the literature. The epitaxial mound formation studied here is a kinetic-topological instability [which can happen only in $(d=2+1)$-dimensional, or higher dimensional, growth, but not in $(d=1+1)$-dimensional growth because no cluster diffusion around a closed surface loop is possible in ``one-dimensional'' surfaces], which is likely to be quite generic in real MBE-type surface growth. Our extensive numerical simulations produce mounded (and slope-selected) surface growth morphologies which are strikingly visually similar to many recently reported experimental MBE growth morphologies.

Complex caddisfly-dominated bioherms from the Eocene Green River Formation

Complex, caddisfly-dominated (Insecta: Trichoptera) carbonate mounds up to 9 m tall and 40 m in diameter formed in the nearshore environment of Eocene Lake Gosiute. The mounds outcrop for 70 km in reef-like geometries along the northern margin of Lake Gosiute in Wyoming. The relationships among the caddisfly larvae, the benthic microbial mat and physicochemical nearshore processes of Eocene Lake Gosiute resulted in unique external and internal carbonate mound morphology. Externally, the large carbonate mounds are formed by the lateral and vertical coalescence of several layers of smaller columns. The smaller columns are generally 1–2 m tall and are 0.5–1 m in diameter. Each layer or generation of smaller columns tends to have a unique external morphology. This suggests that variable paleoenvironmental conditions produced subtle differences in tufa and stromatolite morphology. Internally, each of the smaller columns is composed of a core of caddisfly larval cases surrounded by layers of tufa and stromatolites. The smaller column cores are characterized by centimeter thick microbial-caddisfly couplets in which layers or packets of calcified caddisfly larval cases are covered by microbial mat-mediated, microlaminated carbonate. The microbial-caddisfly couplets suggest that both metazoans and microbes were responsible for column height and shape. In this paper, we propose a mechanism for the growth of these caddisfly-dominated mounds. The base of the Laney Member of the Green River Formation records a freshwater lacustrine transgression over the surrounding floodplains and mudflats of the Cathedral Bluffs Tongue of the Wasatch Formation. In nearshore areas of the lake's northern margin, carbonate hardgrounds developed in some areas of the softer, carbonate-rich, bottom muds. These hardgrounds provided nucleation sites for the carbonate mounds and columns by providing a stable substrate for the benthic microbial mat and for caddisfly larval case attachment during pupation. The larval cases became calcified, and became a new stable substrate for the benthic microbial mat during, or shortly after, pupation. The microbial-caddisfly couplets may record a yearly cycle in which caddisfly pupation and aggregation behavior regularly interrupted the microbial mat-mediated carbonate buildups in these unique carbonate mounds.

Non-universal mound formation in non-equilibrium surface growth

We demonstrate, using well-established non-equilibrium limited-mobility solid-on-solid growth models, that mound formation in the dynamical surface growth morphology does not necessarily imply the existence of a surface edge diffusion bias (‘the Schwoebel barrier’). We find mounded morphologies in several non-equilibrium growth models that incorporate no Schwoebel barrier. Our numerical results indicate that mounded morphologies in non-equilibrium surface growth may arise from a number of distinct physical mechanisms, with the Schwoebel instability being one of them.

Mechanisms of mound coarsening in unstable epitaxial growth

Corner diffusion is shown to play a crucial role in determining the asymptotic mound coarsening exponent n in the case of unstable epitaxial growth on $(001)$ and $(111)$ surfaces. For the case of island-relaxation without corner diffusion the asymptotic exponent is found to satisfy $n\ensuremath{\simeq}1/4.$ However, when rapid corner-diffusion is allowed, the coarsening exponent is found to approach $1/3.$ An explanation for these results is presented in terms of the effects of corner diffusion on the surface current and mound morphology.

Nonuniversality in mound formation during semiconductor growth

The growth of epitaxial GaAs(100) and InP(100) via molecular beam epitaxy has been characterized in vacuo using scanning tunneling microscopy and a height-height correlation analysis of the resulting images. The GaAs growth characteristics can be changed from a layered to a mounded morphology via a change in the As:Ga flux ratio. For InP growth, mounding is observed for growth on a nominally flat substrate, but layered growth occurs on a vicinal substrate. For both the GaAs and InP cases, the measured growth exponents are different for layered vs mounded growth, and do not conform to the predictions of existing dynamical scaling theories.

Evolution of the mound morphology in (111) oriented polycrystalline Pd films and Co/Pd multilayered films with Pd underlayers

Pd thin films and Co/Pd multilayered films with progressively thicker Pd underlayers are prepared by physical vapor depositions. Their growth behaviors are investigated using atomic force microscopy and transmission electron microscopy. We observed that a mound occurs on top of each crystallite of (111) oriented polycrystalline Pd films and that the average mound size increases according to the capillary-induced coalescence mechanism. We attribute this observed growth instability to the step barrier which resists step-down diffusion of deposited atoms. We also observed that the mound slopes of Co/Pd multilayers are smaller than those of their Pd underlayers. We suggest that this results from a downhill current driven by the interface energy between the Co and Pd layers.

A discrete model for non-equilibrium growth under surface diffusion bias

A limited mobility non-equilibrium solid-on-solid dynamical model for kinetic surface growth is introduced as a simple description for the morphological evolution of a growing interface under random vapor deposition and surface diffusion bias conditions. Simulations using a local coordination dependent instantaneous relaxation of the deposited atoms produce complex surface mound morphologies whose dynamical evolution is inconsistent with all the proposed continuum surface growth equations. For any finite bias, mound coarsening is found to be only an initial transient that vanishes asymptotically, with the asymptotic growth exponent being 0.5 in both 1+1 and 2+1 dimensions. Possible experimental implications of the proposed limited mobility non-equilibrium model for real interface growth under a surface diffusion bias are critically discussed.

Pattern-forming instabilities in homoepitaxial crystal growth

The theory of morphological instabilities caused by reduced interlayer transport in homoepitaxial crystal growth is briefly reviewed. The discussion is based on a large scale phenomenological equation of motion for the growing surface, which is motivated by the notion of growth-induced mass currents. Many (though not all) aspects of the coarsening mound morphology can be understood by analogy with phase ordering kinetics. For step flow growth on vicinal surfaces effectively one-dimensional growth equations can be derived from microscopic step dynamics.

Mechanisms of Mound Coarsening in Unstable Epitaxial Growth

ABSTRACTThe effects of various island and mound relaxation mechanisms on the mound morphology and asymptotic mound coarsening exponent n are investigated for growth on fcc/bcc(001) and fcc (111) metal surfaces. While the strength of the diffusion bias (due for example to an Ehrlich-Schwoebel step barrier) affects the crossover time, the presence or absence of corner diffusion is found to play a crucial role in determining the asymptotic value of the coarsening exponent. In the absence of island-relaxation or in the case of island-relaxation without rapid corner diffusion the asymptotic exponent is found to satisfy n ≃ 1/4. However, when rapid corner-diffusion is allowed, the coarsening exponent is found to approach 1/3. For the case of reversible island growth a strong step-barrier leads to an effective corner diffusion so that a growth exponent of either 1/4 or 1/3 may be observed depending on the strength of the step-barrier. These results appear to account for recent experimental observations of a large coarsening exponent (n ≃ 0.33) in epitaxial growth of Rh/Rh(111) at high temperature as well as of a smaller coarsening exponent (n ≃ 1/4) observed in epitaxial growth of Fe/Fe(001) and Cu/Cu(001). An explanation for these results is also presented in terms of the effects of corner diffusion on the surface current and mound morphology.

Burner Rig Hot Corrosion of a Single Crystal Ni-48Al-Ti-Hf-Ga Alloy

ABSTRACTThe hot corrosion resistance of a single crystal Ni-48Al-1Ti-0.5Hf-0.2Ga‡ alloy was examined in a Mach 0.3 burner rig at 900C for 300 hours. The combustion chamber was doped with 2 ppmw synthetic sea salt. The hot corrosion attack produced a random mound morphology on the surface. Microstructurally, the hot corrosion attack appeared to initiate with oxide-filled pits which were often broad and shallow. At an intermediate stage, the pits increased in size to incorporate unoxidized Ni islands in the corrosion product. The rampant attack stage, which was observed only at sharp sample corners, was characterized by rapid inward growth of alumina in finger-like protrusions incorporating significant amounts of Al-depleted Ni islands. Aluminum consumption in the oxide fingers resulted in the growth of a γ' layer ahead of the advancing oxide fingers.

Multibeam bathymetry of glaciated terrain off southwest Newfoundland

Inner St. George's Bay, Newfoundland, was mapped in 1988 using a suite of conventional geophysical and sampling tools. The Quaternary deposits, sea-level history, and Holocene littoral sediments were described in a series of papers. In 1995, part of the previously mapped area was imaged using a multibeam mapping system. The resulting high-resolution images of bathymetry and backscatter are interpreted with reference to the previously collected data. The multibeam imagery shows a Late Wisconsinan morainal bank that has been modified by marine processes during and following a t2¯5 m postglacial sea-level lowstand. Two types of terrain occur on the morainal bank. In the extreme west, where morainal till is close to the seabed, the image shows areas with high reflectivity and rough texture that consist of bouldery gravel. Farther east, the flanks of the moraine are buried by a prograded wedge of sediment eroded from the bank during the Holocene transgression and transported towards the adjacent basin by waves and currents. Here, areas of relatively smooth seabed on the multibeam bathymetry image consist of either mobile, rippled, poorly sorted gravel (high reflectivity), or sand (lower reflectivity). The image shows a submarine valley which, according to the previous data, was glacially overdeepened and subsequently partly filled by glacial-marine and postglacial sediments. Shallow areas with high reflectivity on either side of the valley are seafloor exposures of a draped, glacial-marine unit that was deposited when the former ice margin was located close to the present coast (between 13.3 and 10.8 ka). An area of mounded morphology and high backscatter, located below the level of the postglacial sea-level lowstand, is interpreted as till that has not been modified by wave processes. The multibeam bathymetry and backscatter images reveal the inadequacies of the previous map that was compiled using data from conventional survey techniques.

Computational modeling of mound development in Dictyostelium

One of the structures seen during the course of Dictyostelium discoideum development is the loose aggregate or early mound structure. In this work, we attempt to simulate the formation of this three-dimensional mound by a model which incorporates cAMP chemical signaling and resultant cell motions due to chemotaxis. Simulation results are compared to measurements of the mound morphology as determined by confocal fluorescence microscopy. Good agreement is obtained only if the cells exhibit an adaptive response to high cell density and change their basic motion patterns. We argue that in the actual biological system, this occurs because the high resting value of cAMP in the mound can effectively saturate the high-affinity receptors responsible for aggregation stage dynamics. Tests of our ideas using mutant cell lines are briefly discussed.

Evolution of Mound Morphology in Reversible Homoepitaxy on Cu(100)

Evolution of mound morphology in reversible homoepitaxy on Cu(100) was studied via spot-profile-analysis (SPA) LEED and scanning tunneling microscopy. The mound separation shows coarsening vs growth time with $L(t)\ensuremath{\sim}{t}^{1/4}$, in support of theory based on capillarity between mounds. The growth ultimately reaches a steady state characterized by a selected mound angle of $\ensuremath{\sim}5.6\ifmmode^\circ\else\textdegree\fi{}$. We suggest that this results from a downhill current driven by step edge line tension in balance with an uphill current due to the Schwoebel barrier effect. Also, we have clarified the interpretation for the evolution of the SPA-LEED profile from a ring structure to a single time-invariant peak.

Coarsening of surface structures in unstable epitaxial growth

We study unstable epitaxy on singular surfaces using continuum equations with a prescribed slope-dependent surface current. We derive scaling relations for the late stage of growth, where power law coarsening of the mound morphology is observed. For the lateral size of mounds we obtain $\xi \sim t^{1/z}$ with $z \geq 4$. An analytic treatment within a self-consistent mean-field approximation predicts multiscaling of the height-height correlation function, while the direct numerical solution of the continuum equation shows conventional scaling with z=4, independent of the shape of the surface current.

Unstable epitaxy on vicinal surfaces

Epitaxial growth on a vicinal surface in the step flow regime, where the diffusion length exceeds the step spacing, is studied by simulation of a continuum equation and a solid-on-solid model. Such a surface is known to undergo a meandering instability if step edge barriers suppress downward interlayer transport. We show that the resulting ripple pattern is itself unstable, and evolves at long times into an essentially isotropic mound morphology which is qualitatively and quantitatively indistinguishable from that obtained on singular surfaces.

Effects of crystalline microstructure on epitaxial growth.

The results of kinetic Monte Carlo simulations of epitaxial growth on fcc(100) and bcc(100) surfaces in which the correct crystal geometry is taken into account are reported. The existence of downward funneling to fourfold hollow sites leads to a downward current for large angles and to angle selection as observed in a variety of experiments. We have used our model to simulate Fe/Fe(100) deposition at room temperature and have compared our results with recent experiments. Excellent agreement is found for the selected angle, mound coarsening exponent n, and kinetic roughening exponent \ensuremath{\beta} as well as for the mound morphology. A theoretical analysis also leads to an accurate prediction of the observed mound angle for Fe/Fe(100) deposition at room temperature. The general dependence of the surface skewness, mound angle, and coarsening kinetics on temperature, deposition rate, and strength of the step barrier to interlayer diffusion is also studied and compared with recent experiments. While for a moderate step barrier we find an effective coarsening exponent n\ensuremath{\simeq}0.16--0.25, for the case of a very large step barrier we find n\ensuremath{\simeq}1/3, which is significantly larger than found in previous models but in agreement with recent experiments on Rh/Rh(111). \textcopyright{} 1996 The American Physical Society.

Instability of Kinetic Roughening in Sputter-Deposition Growth of Pt on Glass

Using scanning tunneling microscopy, we have studied the kinetic roughening in the growth of Pt sputter deposited on glass at room temperature for a film thickness range of 15\char21{}140 nm. The growth exhibits an irregularly growing mound morphology and shows an instability with anomalous scaling behavior characterized by the $\sqrt{\mathrm{ln}(t)}$ dependence of the local slope, where $t$ is the growth time, and also by the roughness exponent $\ensuremath{\alpha}\ensuremath{\simeq}0.9$ and interface growth exponent $\ensuremath{\beta}\ensuremath{\simeq}0.26$. These characteristics clearly indicate that the growth is consistent with a statistical model of linear diffusion dynamics.

Geological Mounds and Their Seismic Expression

Mound geometry (convex upward structure developed above a subhorizontal surface) is common in many geological environments and frequently observed in 2-dimensions on seismic sections. Seismic mounds are typically associated with deep-water clastic sediments, e.g. submarine fans and slumps, and with a variety of carbonate depositional settings, e.g., reefs and banks, but also exist in other depositional settings. Recognition will be dependent on mound dimension, velocity contrast, amplitude strength, and the resolution of the seismic data. Since mounds can represent an important exploration target and recognition of porous, hydrocarbon-bearing section is all-important, careful restitution of the original depositional morphology from the seismic data is required. Details of present velocity distribution are critical, along with a realistic concept of any post-depositional modification, such as compaction, which may have taken place during burial. Where differential compaction is taking place, for example between sand and shale, seismic expression of morphology will be continually modified during progressive burial. Analysis of structure at the top and base of the mound can provide support for lithological interpretation based on other criteria, such as seismic facies analysis based on internal and external reflections. Modeling, using parameters from mounds in a variety of known depositional settings, illustrates many ofmore » the interpretational problems associated with seismic mounds and provides some objective criteria for analysis of mound morphology. Comparison is made with real data, principally from northwest Europe and North America.« less

Frost Mounds Formed by Degradation at Slope Mountain, Alaska, U.S.A.

Stable, ice-cored frost mounds can form by differential ablation of seasonally developed massive ground ice. Although degradation frost mounds have no diagnostic mound morphology, these features can be identified by observing formative processes in the field and by examination of the mound core. Several field techniques were employed on massive ice-cored mounds which developed in a marsh near Slope Mountain, Alaska. Stratigraphic relationships within the mound core, ice petrofabrics, and ionic concentration patterns of ice crystals as they vary with core depth were correlated to the mound-forming processes, and indicate that these landforms are stable remnants of a sill-like massive ground ice plateau which has been differentially eroded by stream action. In addition, the core data and field observation can be used to infer the processes associated with massive ground-ice formation. It appears that a 1-m-thick, massive ice plateau develops in the marsh each winter due to the concordant injection and freez...