Paleoflow Indicators Research Articles

Hybrid turbidite-drift channel complexes: An integrated multiscale model

Abstract The interaction of deep-marine bottom currents with episodic, unsteady sediment gravity flows affects global sediment transport, forms climate archives, and controls the evolution of continental slopes. Despite their importance, contradictory hypotheses for reconstructing past flow regimes have arisen from a paucity of studies and the lack of direct monitoring of such hybrid systems. Here, we address this controversy by analyzing deposits, high-resolution seafloor data, and near-bed current measurements from two sites where eastward-flowing gravity flows interact(ed) with northward-flowing bottom currents. Extensive seismic and core data from offshore Tanzania reveal a 1650-m-thick asymmetric hybrid channel levee-drift system, deposited over a period of ∼20 m.y. (Upper Cretaceous to Paleocene). High-resolution modern seafloor data from offshore Mozambique reveal similar asymmetric channel geometries, which are related to northward-flowing near-bed currents with measured velocities of up to 1.4 m/s. Higher sediment accumulation occurs on the downstream flank of channel margins (with respect to bottom currents), with inhibited deposition or scouring on the upstream flank (where velocities are highest). Toes of the drift deposits, consisting of thick laminated muddy siltstone, which progressively step back into the channel axis over time, result in an interfingering relationship with the sandstone-dominated channel fill. Bottom-current flow directions contrast with those of previous models, which lacked direct current measurements or paleoflow indicators. We finally show how large-scale depositional architecture is built through the temporally variable coupling of these two globally important sediment transport processes. Our findings enable more-robust reconstructions of past oceanic circulation and diagnosis of ancient hybrid turbidite-drift systems.

Tectonic and geomorphic controls on the lacustrine deposits of the Neogene Vinchina basin, northwestern Argentina

ABSTRACTTwo episodes of lacustrine sedimentation, separated by an erosional surface and fluvial sedimentation, took place in the southern part of the broken foreland Vinchina basin (NW Argentina) between 11 and 5 Ma. The lacustrine deposits, 768 and 740 meters thick, are recorded in the upper part of the Vinchina Formation (“Vinchina lake”) and the lower part of the Toro Formation (“Toro Negro lake”) respectively. According to sedimentological features, four sedimentary facies associations (FAs) are recognized in the lacustrine deposits: 1) thinly laminated mudstones facies association (FA 1), 2) coarsening- and thickening-upward muddy to sandy cycles (FA 2), 3) medium- to coarse-grained sandstones (FA 3), and 4) mudstones, sandstones, and oolitic limestones (FA 4). Altogether, these facies correspond to ephemeral, shallow, lacustrine systems including saline mudflats. The total thickness of each lacustrine interval, the thickness of the individual cycles and their lithology, and the overall aggradational facies arrangement suggest that both lakes developed during underfilled stages of the basin. The coarsening-upward cycles can be regarded as lacustrine parasequences representing cyclic episodes of expansion and contraction of the lake, but unlike marine parasequences these cycles do not correlate to water depth. The development of lacustrine conditions and continuous base-level rise, together with the coeval southward-directed paleoflow indicators, suggest axial drainages and that the basin was externally closed (endorheic) at that time. The large thicknesses of each lacustrine interval also points to high accommodation in the southern part of the Vinchina basin during these times. Lake filling cycles are one order of magnitude thicker than lake depth, so we postulate that subsidence (tectonic) and rise of the spill point (geomorphology) increased accommodation but not water depth. Thus, unlike marine parasequences, the analyzed coarsening-upward cycles do not correlate to water depth, but rather they are controlled by more complex basinal accommodation processes. We hypothesize that the coeval uplift of the Umango and Espinal basement block to the south, coupled with the initial doming of the Sierra de Los Colorados to the east, may have generated the damming of the southward-directed drainage and a zone of maximum accommodation, then controlling the location of the two lakes and the preservation of their thick sedimentary records. Therefore, localized accommodation was enhanced by a combination of tectonic subsidence and topographic growth. The two lacustrine intervals and the intervening fluvial deposits record changing contributions from axial to transverse drainages and different cycles of closed and open conditions in the basin. A low-frequency, closed to open and back to closed (axial to transverse and return to axial drainage) basin evolution, is envisaged by the development of the two lakes (closed stages) and the erosional surface followed by the interval of fluvial sedimentation that separates them (open stage). In addition, several high-frequency lake fluctuations (expansion–contraction) are represented by the coarsening-upward cycles within each lacustrine interval. The thick lacustrine intervals and their intermediate incision surfaces record cyclic filling and re-excavation stages and localized episodes of increased subsidence in the Vinchina basin, which seem to be a common feature of tectonically active broken foreland basins.

Topographic Controls On the Development of Contemporaneous but Contrasting Basin-Floor Depositional Architectures

Research Article| October 16, 2018 Topographic Controls On the Development of Contemporaneous but Contrasting Basin-Floor Depositional Architectures Daniel Bell; Daniel Bell 1SedRESQ, School of Earth and Environmental Sciences, University of Manchester, Manchester M13 9PL, U.K. Search for other works by this author on: GSW Google Scholar Christopher J. Stevenson; Christopher J. Stevenson 2School of Environmental Sciences, University of Liverpool, Liverpool L69 3GP, U.K. Search for other works by this author on: GSW Google Scholar Ian A. Kane; Ian A. Kane 1SedRESQ, School of Earth and Environmental Sciences, University of Manchester, Manchester M13 9PL, U.K. Search for other works by this author on: GSW Google Scholar David M. Hodgson; David M. Hodgson 3The Stratigraphy Group, School of Earth and Environment, University of Leeds, Leeds LS2 9JT, U.K. Search for other works by this author on: GSW Google Scholar Miquel Poyatos-Moré Miquel Poyatos-Moré 4Department of Geosciences, University of Oslo, 0371 Oslo, Norway Search for other works by this author on: GSW Google Scholar Journal of Sedimentary Research (2018) 88 (10): 1166–1189. https://doi.org/10.2110/jsr.2018.58 Article history first online: 16 Oct 2018 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Tools Icon Tools Get Permissions Search Site Citation Daniel Bell, Christopher J. Stevenson, Ian A. Kane, David M. Hodgson, Miquel Poyatos-Moré; Topographic Controls On the Development of Contemporaneous but Contrasting Basin-Floor Depositional Architectures. Journal of Sedimentary Research 2018;; 88 (10): 1166–1189. doi: https://doi.org/10.2110/jsr.2018.58 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search nav search search input Search input auto suggest search filter All ContentBy SocietyJournal of Sedimentary Research Search Advanced Search Abstract Sediment-laden gravity-driven-flow deposits on the basin floor are typically considered to form either discrete lobes that stack compensationally, or packages of laterally extensive beds, commonly termed “sheets.” These end-member stacking patterns are documented in several basinfills. However, whether they can coexist in a single basin, or there are intermediate or transitional stacking patterns, is poorly understood. An analysis of depositional architecture and stacking patterns along a 70 km dip-oriented transect in the Upper Broto Turbidite System (Jaca Basin, south-central Pyrenees, Spain), which displays disparate stacking patterns in contemporaneous strata, is presented. Proximal and medial deposits are characterized by discrete packages of clean sandstones with sharp bed tops which exhibit predictable lateral and longitudinal facies changes, and are interpreted as lobes. Distal deposits comprise both relatively clean sandstones and hybrid beds that do not stack to form lobes. Instead, localized relatively thick hybrid beds are inferred to have inhibited the development of lobes. Hybrid beds developed under flows which were deflected and entrained carbonate mud substrate off a carbonate slope that bounded the basin to the south; evidence for this interpretation includes: 1) divergent paleoflow indicators and hummock-like features in individual beds; 2) a decrease in hybrid-bed thickness and abundance away from the lateral confining slope; 3) a carbonate-rich upper division, not seen in more proximal turbidites. The study demonstrates the co-occurrence of different styles of basin-floor stacking patterns in the same stratigraphic interval, and suggests that characterization of deep-water systems as either lobes or sheets is a false dichotomy. You do not currently have access to this article.

Stratal Architecture and Morphodynamics of Downstream-Migrating Fluvial Point Bars (Jurassic Scalby Formation, U.K.)

Abstract Assessments of depositional architecture and paleodrainage in planform are increasingly recognized as critical for the reconstruction of ancient fluvial morphodynamics; however, planview exposures of ancient, undeformed meandering-fluvial deposits are scattered, and exceptional occurrences such as the exhumed meander plain of the Scalby Formation (Yorkshire, UK) deserve particular attention. Here we specifically focus on the planview style and morphodynamics of downstream-migrating point bars, providing parallels with modern meandering rivers sharing scale and processes. The Middle Jurassic Scalby Formation lies in the Cleveland Basin of Yorkshire (UK), and accumulated in incised valleys eroded onto shallow-marine strata. Five well preserved downstream-migrating point bars are exposed along a modern tidal platform. Through 1:2000-scale mapping, point bars are analyzed for sedimentology, stratal architecture, vectors of paleoflow and bar migration, and nature of adjacent and underlying deposits. Results indicate that downstream migration was facilitated by—but not strictly related to—the occurrence of resistant outer banks. Meander-bend migration promoted cannibalization of upstream-bar portions, subordinate preservation of central-bar portions, and widespread development of downstream-bar scrolls. The most reliable indicators of regional transport are upstream-bar paleoflow indicators and the dip direction of downstream-bar inclined beds. The generated sectional architecture overlaps in part with that of other fluvial depositional models, such as those for braid bars, indicating that lack of planform exposures may introduce bias when reconstructing ancient fluvial morphodynamics.

Small-Scale Spatial Variability in Turbidity-Current Flow Controlled by Roughness Resulting from Substrate Erosion: Field Evidence for a Feedback Mechanism

Abstract Analysis of turbidity-flow processes from turbidite outcrops remains difficult. In this paper the distribution of orientations of paleoflow indicators around scours a few centimeters to a few decimeters deep is analyzed. Two examples are presented; one from the Gres de Champsaur Formation in southeastern France and one from the Macigno Formation in northern Italy. The orientations of paleoflow data sets on either side of the scours are analyzed and differ 10–15° over horizontal distances of only a few meters. This mismatch is argued to represent a rotation of the flow whilst it flowed over the scour. The reconstructed rotation is a new example of a feedback mechanism involving interaction between the current and its substrate. Paleoflow indicators only record flow direction at the base of the flow. However, the flow–substrate interaction can be expected to affect the complete turbulent velocity structure of the flow, so that erosion by frontal parts of the current influences the suspended-sediment transport processes of hindward regions of the current.

Influence of Host-Rock Texture on the Morphology of Carbonate Concretions in a Meteoric Diagenetic Environment

Abstract The integrated stratigraphic-petrographic–geochemical study of Quaternary marine terraces that have undergone well-constrained sea-level and water-table oscillation(s) over geologically short time periods can provide important clues on diagenetic processes at the marine–continental interface. The Punta di i Cani marine terrace (Quaternary; Saint-Florent, Corsica) unconformably overlies the well-cemented, low-permeability arenites of the Miocene Monte S. Angelo Formation and is erosively overlain by unlithified, high-permeability Holocene continental sediments. The terrace is composed of a retrogradational, fining-upward succession, from fluvial pebbly sandstone to beach and shoreface deposits. The entire thickness of the terrace hosts concretions made of low-Mg calcite. Concretion geometry is linked to the sedimentological facies of the host sediment: (i) massive fluvial pebbly sandstone at the base of the terrace contains knobby, subhorizontal concretions somewhat resembling turbidite flute casts; (ii) very low-angle cross-bedded, well sorted beach deposits contain pipy, subhorizontal concretions; (iii) trough-crossbedded upper-shoreface deposits in the upper part of the terrace contain a subtle diagenetic lineation on the laminae, locally obliterated by a patchy, isotropic cementation. Overall, the degree of cementation decreases from the bottom to the top of the terrace. Cement chemistry and morphology, as well as stable carbon and oxygen isotopic signatures, indicate that the concretions formed by precipitation of meteoric phreatic water flowing toward the coast. Elongate concretions are invariably parallel to the topographic gradient, consistent with the notion that the growth of concretions was controlled by advection. Smectitic vadose cement is locally present; its crosscutting relationships with calcite cement point to periodic oscillations of the water table. The results of this study (i) show that elongate concretions can be used as interstitial paleoflow indicators and (ii) suggest that concretion geometry in an advection-dominated diagenetic environment can depend on the small-scale permeability distribution, as suggested by the fact that each type of concretion is invariably associated with a specific sedimentary facies.

Sedimentary record for exhumation of ultrahigh pressure (UHP) rocks in the northern Cordillera, British Columbia, Canada

We use field relationships, paleoflow indicators, petrography, and major and trace element mineral chemistry to examine the protolith and provenance of detrital mantle-derived ultrahigh pressure (UHP—2.8 GPa) minerals in immature clastic sediments of an Early Jurassic basin (Laberge Group) in northwestern British Columbia, Canada. Our results show that fresh mantle detritus in the Laberge Group was derived from mantle lithosphere that equilibrated at >2.8 GPa and temperatures of 850–1100 °C, exhumed in orogenic massifs and quickly deposited over a restricted time interval. Two models are proposed for the exhumation and denudation of the UHP rocks in either (1) an arc-continent collision between the Stikinia and Yukon Tanana terranes, or (2) an exposed forearc and accretionary melange in a convergent margin in the Cache Creek terrane. A collision between the Stikinia and Yukon Tanana terranes has not previously been documented, but this scenario is the most consistent with widespread evidence for rapid exposure of deep-seated rocks at the required time period for deposition of UHP detritus ca. 185 Ma.

River Warren boulders, Minnesota, USA: catastrophic paleoflow indicators in the southern spillway of glacial Lake Agassiz

River Warren boulders, Minnesota, USA: catastrophic paleoflow indicators in the southern spillway of glacial Lake Agassiz

River Warren boulders, Minnesota, USA: catastrophic paleoflow indicators in the southern spillway of glacial Lake Agassiz

Boulders resting on meltwater‐sculpted and striated‐granite bedrock near the head of the southern outlet spillway of glacial Lake Agassiz are used to generate paleodischarge calculations. The rounded nature of many boulders suggests fluvial transport and a corestone origin. The distribution of boulders in clusters and linear trains records the interaction of clasts during transport and deposition. The geomorphology of the spillway with streamlined erosional remnant hills is characteristic of other large flood spillways. Using the Manning equation and a variety of empirical equations to determine paleovelocity, preferred discharges between 0.364 and 0.102 Sv are calculated. These discharges agree well with flood discharges using modeling methodologies, and most likely represent ephemeral and catastrophic flood events linked to either episodic incision at the outlet or the result of rapid inputs of meltwater to Lake Agassiz.

Mid‐Cretaceous transtension in the Canadian Cordillera: Evidence from the Rocky Ridge volcanics of the Skeena Group

The age relations, geochemistry, and sedimentology of the Rocky Ridge Formation of the Skeena Group are used to test competing tectonic reconstructions for the mid‐Cretaceous Canadian Cordillera as well as the timing and location of the accretion of the Insular Superterrane. Pollen and macrofossil assemblages indicate that these intrabasinal basalts were erupted along the southern margin of the Bowser basin in the Early Albian to Early Cenomanian. Single‐crystal fusion and step‐heating 40Ar/39Ar dating of hornblendes in one basalt flow from the uppermost part of the formation yielded Middle Cenomanian ages of 94.3 ± 0.4, 95.6 ± 1.6, and 95.0 ± 1.6 Ma. Vesicular basalt flows interbedded with crystal‐rich tuff breccias contain evidence for hot emplacement as pyroclastic flows. Individual eruptive centers are identified by their proximal facies, paleoflow indicators within the lava flows, paleoflow indicators within interbedded volcaniclastic fluvial deposits, geochemical differences, and geographic isolation of volcanic deposits. Major and trace‐element geochemistry from 20 sampled lava flows indicates an alkali basalt composition for the volcanics. The basalts of the northern Rocky Ridge volcanic center show enrichment of light rare earth and large ion lithophile elements with strong negative Nb‐Ta anomalies whereas the basalts of the southern Tahtsa Lake volcanic center show depletion to slight enrichment of light rare earth elements, slight enrichment of large ion lithophile elements with minimal negative Nb‐Ta anomalies. The geochemistry combined with paleogeographic and regional tectonic reconstruction suggests a continental arc setting with intraarc extension. The presence of deeper marine facies to the west and the lack of a western sediment source in the Skeena Group indicate that the technically active Insular Superterrane was not west of the study area during mid‐Cretaceous time. Thus we reconsider the Omineca Belt as the main axis of a mid‐Cretaceous continental arc, placing the Intermontane Superterrane in the intraarc to forearc position with the Rocky Ridge volcanics erupted along the forearc side of the Omineca arc. Coeval regional strike‐slip faulting and reconstructed oblique plate convergence suggest a transtensional setting for Rocky Ridge intraarc extension. An electronic supplement of Tables A1–A2 may be obtained on a diskette or Anonymous FTP from KOSMOS.AGU.ORG (LOGIN to AGU's FTP account using ANONYMOUS as the username and GUEST as the password. Go to the right directory by typing CD APEND. Type LS to see what files are available. Type GET and the name of the file to get it. Finally, type EXIT to leave the system.) (Paper 95TC03496, Mid‐Cretaceous transtension in the Canadian Cordillera: Evidence from the Rocky Ridge volcanics of the Skeena Group, Kari N. Bassett and Karen L. Kleinspehn). Diskette may be ordered from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009; $15.00. Payment must accompany order.

Evidence of Cordilleran Late Wisconsinan glaciers in the ‘ice-free corridor’

Glaciers flowing down the Athabasca River valley during the last glacial period were fed both by local montane glaciers and by Cordilleran ice moving easterly across the Continental Divide from British Columbia, resulting in a major eastward surge into the ‘ice- free corridor’. The timing of this event has been disputed for many years and has important implications for the study of paleoenvironmental conditions and the north-south migration of biota in the corridor. Chronologic and lithostratigraphic data from the Jasper region suggest that this event took place in the Late Wisconsinan. Subtill radiocarbon dates on wood in the Athabasca River valley indicate that non-glacial alluvial sediments began accumulating there sometime before 48 ka. A finite date of 29,100 ± 560 BP (GSC 3792) on wood under till near Jasper provides a maximum age for the onset of the last glaciation in the region, although ice from west of the Rocky Mountain Trench probably did not cross the Continental Divide until after 21.5 ka. The occurrence of supraglacial metamorphic erratics as well as the distribution and nature of mapped glacial deposits and the trend of subglacial paleoflow indicators in the region east of the Rockies suggests that Late Wisconsinan glaciers flowing from the mountains coalesced with Laurentide glaciers and were deflected southeasterly along the mountain front. An even more extensive glacial event may have effected the region in pre-Late Wisconsinan times. The model of Late Wisconsinan glaciation presented here is in agreement with all known radiocarbon data from the area and is especially attractive when supratill dates on non-woody lacustrine organics are viewed in light of possible contamination by old carbon.

Grain Fabric in Hummocky Cross-Stratified Storm Beds: Genetic Implications

ABSTRACT A detailed analysis of grain fabric in samples from hummocky cross-stratified (HCS) storm beds suggests that these shallow marine sandstones were deposited from waning combined (oscillatory/unidirectional) flows. Particle a-axis orientations in the deposits, in plan view parallel to visible lamination, vary widely but have a mean orientation approximately normal to unidirectional paleoflow indicators (sole marks) and subparallel to regional shoreline. This alignment of a-axes is attributed to deposition under combined flows with a multidirectional oscillatory component produced by wind-induced surface gravity waves with a modal crest orientation approximately parallel to the shoreline. Imbrication of particles varies in both the horizontally-laminated and HCS intervals of the HCS stor beds, due to waxing and waning of boundary shear stress caused by the oscillatory component of the flow. In the basal, horizontally-laminated interval of an HCS storm bed, grain imbrication angles vary about a mean of approximately 13° from the horizontal, dipping into the paleoflow inferred from sole marks. This preferred imbrication direction is attributed to boundary shear stress influenced by the unidirectional component of the combined flow. However, quasi-cyclic variation in imbrication angle over a broad range and periodic reversal of imbrication direction suggest that the oscillatory component was also significant during deposition of the horizontally-laminated interval. In HCS intervals, imbrication angles vary about a mean that is parallel to visible laminae, suggesting th t oscillatory flow had become predominant. The sequence of structures and changing patterns of fabric through these HCS storm beds is consistent with deposition under waning combined flows. Erosional scour, forming the sharp base and unidirectional sole marks, occurred in response to combined flows with an effective unidirectional component. The unidirectional component continued to influence the direction of boundary shear stress during deposition of the horizontally-laminated sands but waned and became negligible relative to a strong oscillatory component during formation of HCS.

Shelf sandstones and hummocky cross-stratification: New insights on a stormy debate

Research Article| June 01, 1991 Shelf sandstones and hummocky cross-stratification: New insights on a stormy debate William L. Duke; William L. Duke 1Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802 Search for other works by this author on: GSW Google Scholar R.W.C. Arnott; R.W.C. Arnott 2ESSO Resources Canada Ltd., 237 4th Avenue S.W., Calgary, Alberta T2P 0H6, Canada Search for other works by this author on: GSW Google Scholar Richard J. Cheel Richard J. Cheel 3Department of Geological Sciences, Brock University, St. Catharines, Ontario L2S 3A1, Canada Search for other works by this author on: GSW Google Scholar Geology (1991) 19 (6): 625–628. https://doi.org/10.1130/0091-7613(1991)019<0625:SSAHCS>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation William L. Duke, R.W.C. Arnott, Richard J. Cheel; Shelf sandstones and hummocky cross-stratification: New insights on a stormy debate. Geology 1991;; 19 (6): 625–628. doi: https://doi.org/10.1130/0091-7613(1991)019<0625:SSAHCS>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract A new depositional model for hummocky cross-stratified sandstones is based on the following observations. (1) Storms transport coastal sand to the inner shelf under oscillatory-dominant combined flows exerting peak instantaneous bed shear stresses roughly normal to shore. (2) Isotropic hummocky cross-stratification is formed experimentally by large, three-dimensional, symmetrical bed forms generated by long-period, purely oscillatory flow and very strongly oscillatory-dominant combined flow. Anisotropic hummocky cross-stratification is formed by strongly oscillatory-dominant flow. (3) Grain fabric in hummocky sandstones indicates rapid reversals of bed shear stress consistent with deposition under a shore-normal oscillatory flow, initially superimposed on a relatively weak bottom current with a seaward-directed component of motion. Thus, shore-normal transport of coarse bedload on the inner shelf during storms (inferred from studies of ancient units) is caused by the interaction of high-speed oscillatory bottom motions under long-period shoaling waves and a relatively slow shore-oblique bottom current driven by geostrophically balanced coastal downwelling. Turbidity currents are not required to form shore-normal paleoflow indicators from hummocky beds. Large three-dimensional wave ripples generated by waning-storm or swell waves are responsible for much of the hummocky cross-stratification in the stratigraphic record. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Geostrophic circulation or shallow marine turbidity currents? The dilemma of paleoflow patterns in strom-influenced prograding shoreline systems

In ancient storm-influenced prograding shoreline sequences, sole marks (mainly tool marks) from hummocky crossstratified storm deposits are commonly oriented normal to paleoshoreline and the trend of paleobathymetric contours in the basin. Asymmetrical tool marks typically indicate flows directed offshore. Several workers have attributed their formation to storm-generated, shallow marine turbidity currents. This interpretation conflicts with observations from modern shelves, where storm-driven circulation generally is geostrophically balanced, and time-averaged bottom currents approximately parallel bathymetric contours and the local shoreline. The resolution of these apparently conflicting observations may lie in the realization that tool marks (and many other small paleoflow indicators) form almost instantly as the result of instantaneous flow conditions very near the bed. Beneath storm-generated flows in the shallow ocean, instantaneous and time-averaged characteristics of the bottom boundary layer generally exhibit little similarity. Storm-generated tool marks are formed by the movement of large tools within the thin (less than 1 m) inner boundary layer resulting from the superimposition of waves and currents. The orientation of the peak instantaneous shear stress moving large tools under such combined flows mainly reflects wave-orbital motions, which typically are normal to shore. The magnitude of stress is greatly increased in the offshore direction (and decreased in the onshore direction) by superimposition of a steady current with an offshore component of flow, but the direction of stress is only slightly affected. In ancient storm-influenced sequences, therefore, shore-normal tool marks generally were not formed by turbidity currents; rather, their orientation is best attributed to shoaling waves approaching the coast at a very high angle. Asymmetrical tool marks are directed offshore due to enhanced shear stress on the offshore stroke of waves superimposed on a geostrophic current with an offshore flow component. Tool marks do not reflect the time-averaged bottom-flow direction; in fact, they provide almost no information concerning steady bottom currents. In contrast, high-angle cross-beds (formed in coarser sediment by the migration of dunes and sandwaves), although relatively rare in offshore storm deposits, generally reveal approximately shore-parallel flows in ancient systems. Cross-beds closely reflect the time-averaged flow direction in the outer boundary layer of a geostrophic current, for three reasons: 1) the net transport direction for sand moving as bed load beneath a combined flow lies between the directions of peak instantaneous shear stress and time-averaged shear stress; 2) large ripples disrupt the thin inner boundary layer; and 3) long time intervals (relative to wave-induced velocity oscillations) are required to form large ripples.