Sedimentary processes and structures

The Paleozoic epicontinental seas of ancestral North America were often characterized by the in situ production, distribution, and deposition of biologically produced carbonate grains. The Upper Ordovician Trenton Limestones are no exception to this observation. As a rule the production, transport, and deposition of carbonate sediments occurs within the same basin in which they originate; thus they are referred to as intrabasinal or autocthonous. Given the wide variety of carbonate rock types observed in the Trenton Limestones, it is no surprise that the deposition of these rock units was impacted by a variety of sedimentary processes; these include sediment production processes, transport processes, as well as depositional and erosional processes. The following discussion considers some of the implications of these processes in the context of the Trenton Limestone. 

Carbonate production

It is necessary to briefly consider the range of carbonate sediment types found in the Trenton and their formational history in order to understand the processes through which they are ultimately transported and deposited.

As mentioned in the discussion of Trenton "Lithologies," two main carbonate materials are represented in the Trenton Limestone: fine-grained carbonate mud (the most dominant component of the limestones), and coarser-grained skeletal debris. Ultimately, the physical breakdown of macrofaunal/floral, and microfaunal/floral skeletal particles of varying compositions is the primary source for carbonate sedimentary materials. Hence most of the Trenton limestones are likely derived from the physical breakdown of a variety of skeletal elements. >>Back to Top

• Carbonate mud production in the Trenton

The following figure originally from Scholle (1978) introduces the major carbonate sediment producers from the fossil record, and the primary mineralogy of each taxonomic group. Highlighted in darker purple are the taxa that are most commonly found in the Trenton. Notice from the diagram that taxa present in the Trenton have a range of mineralogic compositions. Despite the fact that some of the major sediment producers in modern environments build aragonitic skeletons, like the calcareous green algae, the most dominant skeletal constituents from the Trenton (brachiopods, crinoids, and bryozoans) tend to be calcite producers.

Compositionally, although both aragonite and calcite have the same chemical makeup, calcite carbonate is much more stable than aragonite in both depositional and diagenetic environments. In addition to their differential chemical stabilities, aragonitic skeletal fragments tend to disaggregate rapidly to fine-grained mud sized particles, while calcite fragments, especially those of echinoderms, brachiopods, and bryozoans, are more robust and tend to break down only to the fine sand to coarse silt size.

The Trenton Limestone as a whole is dominated by carbonate mud, thus it is not unreasonable to assume that the majority of these carbonates were produced originally by aragonitic source materials. Given the figure above showing all the major carbonate sediment producers, and assuming that the production of carbonate mud was not significantly different than today, it is likely that the majority of these fine-grained carbonate sediments were derived from aragonitic calcareous red and green algae.

Having established that the majority of Trenton units were deposited in shallow wave-dominated depositional environments through offshore deep water ramp environments, the large volume of carbonate mud in the Trenton is slightly problematic. In the modern Bahamian platform region, the production of most fine-grained carbonate mud is in the shallow lagoonal depositional setting. The Tucker and Wright (1990) model discussed previously would place the lagoonal environment as part of the "back ramp" setting. The propensity of these shallow water environments to foster the growth and disaggregation of calcareous green algae, coupled with higher rates of bioerosion (micritization) results in a very high proportion of lime mud in modern Bahamian lagoons. The diagram below, from Tucker and Wright (1990) represents a typical lime mud budget for the Bight of Abaco region in the Bahamas. Notice that although the greatest amount of modern lime mud budget resides on the floor of lagoons, a certain percentage of these muds are transported offshore in suspension, are dissolved into solution, are formed into pellets, or are transported onshore into tidal flat environments.

Of the four main lime mud sinks, that is, mechanisms responsible for reduction in the amount of mud in the lagoons, only three have implications for the deposition of the Trenton Group. Moreover, unless the Ordovician epeiric seas experienced large-scale abiotic chemical precipitation of carbonates via "whitings", it is more than likely that offshore transport of carbonate muds via pelletization and suspension processes were responsible for the introduction of lagoonally or "back ramp" produced lime muds into the offshore depositional environments typical of the Trenton.

Given these assumptions, and the rather limited proximity of shallow lagoonal environments on the Trenton Shelf, especially during the deposition of the middle Trenton, the source of these lime muds is still at issue. Reflecting on the paleogeographic, paleoceanographic and paleoclimatic circulation models for the Trenton Shelf region, it is plausible that the fine-grained carbonates that dominate the Trenton were produced, and moved offshore from shallower regions at some distance from the Trenton Shelf. In fact, it is likely that much of the carbonate mud deposited on the Trenton sea-bottom was derived from more equatorward (and more tropical) shallow shelf regions to the north and west of the Trenton Shelf including southern Ontario and Quebec. >>Back to Top

• Skeletal carbonate production in the Trenton

The second most important component of many of the Trenton Limestones are the larger skeletal fragments which range in size from fairly large entire skeletal elements down to fragmented and abraded grains in the fine sand to coarse silt size fraction (down to 1/16 mm). Some of the Trenton Limestones exhibit wackestone, packstone or grainstone lithofacies, so the production and concentration of these coarser-grained skeletal fragments is important to unraveling the depositional processes responsible for their final accumulation.

The most predominant carbonate constituents in most Trenton units are the macrofaunal calcite producers brachiopods, crinoids, and bryozoans. It is suspect that these organisms have not been fully fragmented or otherwise abraded prior to final deposition as is the case with many of the poorly-preserved aragonitic forms. Based on the preservation of many of these fossil taxa from the Trenton, it is readily apparent that many of these specimens are fairly well-preserved if not fully articulated. Thus in contrast to the fine-grained lime mud component of these limestones, most skeletal concentrations in the Trenton are generally deposited in situ or nearly in situ as documented by the taphonomic signatures of these beds. The general observation of well preserved specimens thus suggests that these specimens were not transported great distances, and were often buried catastrophically.

As discussed previously, the majority of the Trenton Limestone has been interpreted to have been deposited in a range of offshore deep water settings. These environments are characterized by the dominance of lower energy conditions below fairweather wave base, yet are periodically influenced by higher energy storm events. As they are still shallow enough that they lie in the photic zone and are rather well oxygenated, these environments are exceptionally well situated for the growth of carbonate producing benthic fauna. The diagram below modified from Einsele (1998), illustrates the relationships between water depth and the lateral distribution of different types of shell bed concentrations.

One can see on the figure that the primary production of biogenically produced hardparts occurs with the greatest rates in the region between fairweather wave base (FWB) and storm wave base (SWB). In both upslope and downslope positions, the rate of primary production of shelly faunas decreases substantially as indicated by the length of the hachure marks. Although this figure emphasizes shelly faunas, the same patterns are arguably true for crinoids and bryozoans as well.

Collectively then, the production of biogenic hardparts in the intermediate to deep ramp depositional settings is consistent with the observation of such shell bed accumulations in the Trenton Limestones. Moreover, taphonomic clues from the Trenton faunas within the various Trenton units help to constrain the relative depositional processes impacting the production, transport, and final deposition of these beds, and can ultimately be used to interpret the relative depositional settings of these limestones. >>Back to Top

Carbonate sediment suspension, erosion & transport processes

The ultimate deposition of the Trenton Limestone was impacted by a variety of processes including those that transport sedimentary materials from one position to another. In the diagram above by Einsele (1998), section "C" illustrates the conceptual mechanisms for shell bed accumulations via sediment erosion and transport. Once carbonate sediments are produced, either abiotically or biotically, prior to final deposition these materials are acted upon by a variety of processes that disarticulate, abrade or otherwise break them down into smaller pieces. Once worked upon via these various fragmentation processes, sediments can be acted upon (eroded) and relocated (transported) to other environments by suspension or bottom-hugging flows, based on their physical textural properties. The movement or transport of carbonate sediments is therefore based on energistic constraints and on the physical size and shape of the materials themselves.

The most prevalent transport processes impacting the movement of carbonate sediments are tides, waves, storms, gravity flows, and currents. In most cases, storms and current transport are involved in the direct down slope transport of sedimentary materials, while both tidal and wave transport processes impact the winnowing, sorting and suspension of fine grained materials into the open water column. The following discussion focuses on those transport mechanisms that were most likely involved in the deposition of the Trenton Limestones. 

Tides: In shallow water settings, and most commonly along the coasts of large landmasses, daily, monthly and yearly fluctuations in sea surface elevation impact the transport and deposition of sediments both into and out of these nearshore settings. The transport of sediments in peritidal regions is normally only very local, with most carbonates remaining in this depositional setting or directed onshore. However during periods of time with exceptionally strong tidal influences some fine carbonate sediments (usually mud) can be picked up and entrained into the moving water mass as tides subside. In these cases, tidally driven currents can transport sediments offshore through tidal channels where they can either immediately settle out or remain in suspension until later processes allow their deposition.

It is not fully clear as to the source of carbonate mud on the Trenton Shelf, or if the region even experienced substantial tides. However, it is at least likely that minor amounts of carbonate mud found in the Trenton Limestones were indeed derived from tidally influenced transport processes, albeit not originating locally on the Trenton Shelf. 

Waves: Next to tides, the most active process in the shallow shelf region impacting the production, erosion, suspension and transport of sediment grains is wave action. The interaction of winds with the surface of open water bodies generate surface waves which can impinge on the sea bottom in shallow water settings. Once the base of a wave touches the sea-bottom, the top of the wave rises up and due to gravitational pull the top or crest of the wave crashes back down creating a high-energy "surf" zone. Because of this wave-crashing phenomenon, shallow open-shelf environments tend to be characterized by rather distinctive coarse-grained facies deposited in beach to shoal environments.

Under these high-energy conditions, carbonate sediments are highly mobile; individual clasts are lifted up and bashed against one another so that over time they are fragmented, abraded, and physically rounded. As with tidal-influenced transport, most wave-dominated environments act to transport materials from offshore to onshore environments through the construction of beaches, barrier bars, and wave influenced shoals. Moreover, given the high-energy regimes, these environments tend to be characterized by winnowing processes. During the oscillation of waves, bottom sediments are often stirred and shifted so that fine-grained materials can be entrained into the water column, with coarser and heavier materials left behind in the swash zone. When this occurs, the majority of fine-grained materials are carried in suspension into areas where energy levels are lower and settling processes allow the fine-grained materials to be deposited.

Although these wave-dominated environmental settings are widespread in shallow portions of most basins, the development of these high-energy depositional settings on the Trenton Shelf are limited to a few horizons near the base of the Trenton and again at the top. During the majority of Trenton deposition, the Trenton Shelf region was in effect well below normal or fair weather wave base. Thus the daily winnowing of sediments by oscillating waves was important only during the deposition of the lower Kings Falls Formation and then again during the deposition of the Steuben Formation at the top of the Trenton. 

Storms: Probably one of the most important mechanisms for sediment transport on the Ordovician Trenton Shelf is that originating from storm events. Both in the modern and in the geologic record, most shallow shelf to shallow ramp areas are characterized by relatively flat quiet water regions bordered by high-energy zones. During normal weather conditions these regions (including back ramp areas, lagoons, strandplains etc.) accumulate large quantities of both fine-grained and somewhat coarser-grained skeletal debris, especially in the vicinity of high-energy zones. However, as these regions are characterized generally by sediment accumulation (especially during sea-level rise), the change from fair weather conditions to stormy conditions changes the dynamics of sedimentation in these environments.

Meterologically, tropical to subtropical regions are well-known for their seasonal patterns of stormy weather both in terms of periods of prolonged high-energy gales (monsoonal or cyclonic weather patterns) or from short-term hurricane and typhoon events. It is not uncommon for hurricane/typhoon events to be preceded by the development of storm surges which push or draw excess volumes of water up onto the confined nearshore setting.

When these storm surges make contact with steeply oriented surface terrains (beaches, bars, islands, dunes, etc.), these waters tend to rise rapidly and because they are confined in the landward direction are forced back to sea. When storm surge waters pile-up in the near-shore environments, gravitational forces draw the waters back out to sea through narrow channels, troughs or other depressions on the seafloor. The excess water is forced below the surface creating riptides and undercurrents that hug the sea-bottom as they move back out to sea. Due to the increased wave activity associated with higher wind velocities, and because of the development of riptides, previously deposited sediments are commonly entrained into the water column and transported away from shore into deeper shelf/ramp environments.

Once these sediment laden water masses reach open-water settings, they begin to slow down and coarser materials settle out of suspension. The development of storm surges and return bottom flows can last for hours to days. When the storm event has passed and turbulent storm energy subsides, the finer materials still held in suspension can begin to settle out. Thus a common signature of a storm-influenced deposit, otherwise termed a tempestite, is the development of a normally graded bed. That is, a bed which displays the coarsest materials at the base and fining upward to an upper surface.

Most storm-influenced deposition occurs in regions just below normal wave base. These events, if large enough (i.e. mega hurricanes) and provided the trajectory of the storm is right and the amount of sediment removed from the shelf is large, can influence deposition of materials well beyond proximal shelf regions. If enough sediment is entrained into the water column in the storm surge return flows, once the water mass has moved deep enough and out of range of the storm influenced energy, additional processes can take effect and move the entrained materials to still further depths. (See figure below)

Gravity flows: Various types of flows impact the deepest portions of most deep slope to deep ramp carbonate depositional environments. Gravity flows can usually be divided into two main categories: turbidity flows and debris flows, although a third type referred to as a pycnoclinal flow is possible. In any case, given a steep enough gradient in these deep water settings, sediments can be moved rapidly along the bottom until they reach a more stable energy situation and settle out. >>Back to Top

Turbidity flows: As mentioned, during storm events large quantities of sediment are entrained into the water column and carried offshore. Once offshore into regions where storm energy has subsided, it is typical for entrained sediments to settle out of the water mass. On some occasions these highly turbid water masses move directly offshore to a steepened slope or ramp area where, because of the greater density of the sediment-laden water relative to surrounding water masses, the mass of suspended sediments can flow downslope. This type of downslope movement is classified as a gravity flow because of the direct influence of gravity. Because turbidity flows tend to be extremely chaotic or turbulent and move downslope rather rapidly, they can entrain additional sedimentary particles as they travel. This may produce scour features after loose sediments are plucked and eroded from the sea bottom.

Debris flows: The second type of gravity flow is referred to as a debris flow. In the event that sediments are rapidly deposited in upslope regions by storm events, it is possible that the sediments can pileup in great thicknesses. Commonly these thick sediment masses are water-rich; that is, they are not well consolidated and have large volumes of pore water, and are extremely unstable especially if unsupported laterally. Sediment masses that are in close proximity to oversteepened slopes are prone to gravity-induced movement. In these situations, the weight of sediments on the unstable sea-bottom can result in movement of the mass rapidly downslope. Again, like turbidity flows, debris flows tend to move very rapidly and can entrain materials into their mass. In addition to moving great quantities of water and sediment, these masses can carry very large segments of previously consolidated rocks either as blocks or as sheets. Debris flows are common in regions with steep slopes and high sedimentation rates, and in regions where earthquakes are common.

Pycnoclinal flow: In the event that a turbidity flow or a debris flow induces movement of dense, sediment laden water down a slope it is sometime possible for less dense portions of these gravity flows (those portions with finer-grained materials) to move to the top of the mass. Where ocean waters are stratified, either by salinity or temperature differences, it is common that a boundary layer is established between these water masses. The boundary layer is often very sharp and can develop strong density gradients across the boundary. In the event that a gravity-induced flow moves across the density gradient, referred to as a pycnocline, the less-dense portions of the gravity flow may separate from the main sediment mass and travel along the boundary layer. Once in the boundary layer, these sediment masses can travel great distances as sheets until chemical flocculation, biological pelletization, or simple settling processes allow the sediments to sink. Due to the lag time in settling of pycnoclinal flows, these density/gravity flows can potentially be recognized in the sedimentary record (Kohrs, 2003). >>Back to Top

Currents: The final mechanism for sediment transport are currents. Although tides and storm flows technically produce currents, those processes are relatively narrowly focused; they impact more constrained regions and interact in much more substantial ways and so are considered separately. However, in the open ocean the presence of multiple independently derived water masses, such as surface waters, intermediate waters, bottom waters, saline waters, cold waters, etc., often interact in ways that move these water masses or otherwise circulate them throughout the world's oceans. Any variety of density-induced circulation movements, along with the earth's rotation and Coriolis effects, can produce currents within open, and even within restricted water masses.

Ocean currents tend to be very sluggish and impart little influence on the tranport of sediments over great distances. However in the case of long-shore currents, and contour currents, there is a potential for the transport of sediments. In most cases, whether the shallower long-shore or deeper water variety called contour currents , these currents can move sediments in directions perpendicular to topographic gradients. It is possible then to move sediments from onshore to offshore, and it is also possible to move sediments parallel to shore. Although such currents tend to be gentle and sluggish, some long-shore or contour currents may move materials rapidly enough for erosion and transport of sediments to occur. In those instances where strong currents prevail, the result is often a current-swept seafloor which accumulates very little if any sediment. These regions are characterized by hardgrounds and pre-cemented sea bottom areas. >>Back to Top

Carbonate sediment suspension, erosion & transport processes on the Trenton Shelf?

As previously discussed, the sedimentologic record from the Trenton Limestones provides insights into the depositional history of the Trenton Shelf. It has already been established that the deposition of these fossiliferous carbonates occurred in relatively deep water environments at some distance from shore. Moreover, although it is well supported that the diverse shelly faunas found in the Trenton were likely derived locally, the high proportion of carbonate mud making up the limestones was probably transported from some distance.

In an effort to illustrate the variety of sediment transport processes, as discussed above, the following figure has been modified from Baird and others (1992). The individual stratigraphic divisions are not the focus here; however, the diagram illustrates a potential cross-section down slope from Trenton Falls to the region of Middleville, New York during the deposition of the Middle Trenton Denley Formation. Note that the post-compactional vertical scale is exaggerated 1000 to 1.

Of the variety of transport mechanisms discussed, sedimentologic evidence from the Trenton Limestone suggests that a variety of storm-influenced, gravity influenced, and suspension settling transport mechanisms were active in the accumulation of these carbonates. 

Mud transport in the trenton: According to studies by Titus (1974) and Mehrtens (1988, 1992), aside from purely micritic beds with nearly 100% micritic mud, nearly all coarser-grained limestone beds in the Trenton are composed of between 10 and 50% micritic mud. Some of this micritic mud was potentially produced in the shallowest settings locally on the Trenton Shelf, but it is unlikely that the entire micritic mud budget is locally derived. Moreover, based on petrographic studies of these carbonates, most of the micritic muds appear to have been deposited both as flocculated and/or pelleted aggregates of mud particles, both in association with event bed deposits and in overlying beds representing background sedimentation.

Pelletal and flocculated muds were both transported as components of down-slope transport processes, either storm-generated or gravity generated, as well as through suspension settling processes as evidence by beds deposited during background sedimentation. In most cases where the mud-aggregates are associated with event bed deposits, it is possible to ascertain whether these particles were transported down-slope via turbidity currents, storm currents, or potentially as part of debris flows. In the case of background sedimentation events, it is often difficult to differentiate whether the gentle suspension settling accumulations are related to storm-induced winnowing, normal wave-induced winnowing, tidal winnowing, pycnoclinal settling, etc. >>Back to Top

Allochem transport in the trenton: Aside from the extreme dominance of micritic mud within the Trenton limestone, the majority of carbonate beds are composed of variable amounts of skeletal fragments; there are lesser amounts of void space filling spar, microspar, and intraclast components. Based on the measurements of Titus (1974), for basal Trenton carbonates, most of the coarser-grained facies (including calcisilts, calcarenites, and coquinal calcarenites) contain between 14 and 25% recognizable skeletal fragments with up to 62 % additional spar and microspar. Given the high values of skeletal and spar components in these samples with simultaneous low mud content, it is clear that many of the carbonate beds in basal Trenton units, including the Kings Falls Formation, show significant evidence for winnowing and removal of finer-grained materials. Moreover, although much of the lower Kings Falls shows significant evidence for normal wave base influenced deposition, the upper Kings Falls shows evidence for development of storm winnowing and tempestite deposition. No individual studies have been performed on the Steuben Formation (also dominated by well-winnowed skeletal, sparry grainstones), but it is likely that like the former as well as the latter formations were influenced by highly-energetic transport and depositional regimes that favored the fragmentation, and abrasion of skeletal elements and removal of fine-grained sediments.

In middle to upper Trenton deposits, Mehrtens (1988, 1992) recognized somewhat lower percentages of allochems and spar components when investigating turbidite facies interbedded with shales. In the fossiliferous components of turbidite facies, she recognized that depending on the layer, skeletal compositions can range from approximately 70% (in basal turbidite beds) down to 10 to 15 %. She reported that spar components were significantly less than those in the lower Trenton. In the Denley and Rust formations for instance, spar composition percentages range (based on turbidite layer) from 15 to 30 %. Clearly the middle to upper Trenton facies contain more recognizable fossil fragments (are better preserved) and are significantly more muddy than the lower Trenton. Based on these observations: that is the well-preserved, high skeletal allochem composition (with many whole fossil specimens), collectively with lower spar content, and relatively greater percentage of micritic mud, it is strongly supported that the middle Trenton units were deposited in significantly less energetic, quiet water settings. Based on sedimentologic evidence, Mehrtens classified many of these carbonate horizons as representing turbidite horizons that by definition suggest down-slope gravity-driven deposition from shallower water environs. If this transport mechanism was indeed responsible for deposition of many of these limestones, the lower void space or porosity filling spar compositions suggest that these beds were transported and deposited as a thick slurry of materials dominated by sediments.

In addition to petrographic evidence for bottom transport, the occurrence of both the "lower" and "upper disturbed zones," quite obviously represent gravity-induced transport of both unconsolidated and semi-consolidated sediments down-slope. It is not clear whether the formation of the two disturbed horizons in the Trenton were triggered by simple over-steepening processes associated with high-sedimentation rates and slope-failure, or by seismic induced liquefaction. However,it is clear that these deposits were moved at least some distance along the sea-bottom before coming to rest in their final position. 

Sedimentary structures: As labeled on the image of the Trenton ramp and discussed above, a variety of transport and depositional mechanisms were involved in the accumulation of these limestones. Although tectonic induced changes in shelf/ramp morphology and sea-level rise were generally responsible for the change to these deeper-water depositional processes, a variety of wave, storm, flow, and current-generated transport mechanisms were directly involved in the deposition of the Trenton Limestones. The following discussion helps to highlight the signatures of key processes responsible for deposition in deeper-water settings, shelf and ramp settings. As depositional processes impacting these deeper water settings often produce very similar sedimentary structures, it is a focus of this discussion to provide some criteria used to distinguish these features.

Most sedimentary structures have physical textural properties that help to distinguish aspects of the flows from which they were formed or deposited. Such physical and textural properties include information regarding stratification, bedforms, bedding-plane markings, soul features, and biological structures, all of which are are important in the interpretation of depositional structures and hence the processes responsible for depositing them.

Most sedimentary structures are grouped into those textural and physical properties exhibited in cross-sectional view, in bedding-plane view, or those structures that are simply biogenically constructed. Within these main categories, energistic patterns including energy level, flow turbulence, and flow interactions all interact to produce characteristic bedding/stratification patterns. Shown in the figure to the right are some examples of sedimentary structures and bedding styles that result from a variety of depositional processes. Although this figure is not intended to provide an exhaustive list of potential sedimentary structures, those features that are relatively common in the Trenton or in equivalent down-slope facies of the Dolgeville are shown.

The main categories of depositional structures include stratification/bedforms, bedding-plane markings, and biogenic structures. Within the context of cross-sectional stratification/bedforms, the most common properties include: development of bedding (massive, thick, medium, thin, and laminated which refer to the relative thickness of any bed); development of graded bedding (either normally-graded with a fining-upward pattern, or reverse-graded with a coarsening upward pattern); development of ripples and cross-bedding (including oscillation ripples, interference ripples, unidirectional current ripples, climbing ripple structures, and their associated cross-sectional patterns); development of hummocky, swaley, or festoon cross-stratification (typical of large-amplitude wave influenced turbulent flows); and any other irregular stratification. Generally, the more complex the bedding structures, the more complex the process that formed it. For example, simple laminated bedding patterns suggest rather simple uniform laminar flows alternating with more quiet water deposition. The development of normally graded bedding requires transport of multiple sediment sizes both in suspension, and as bottom or traction loads, which subsequent to flow energy subsidence, settle out in order of weight and density.

In addition to stratification/bedform characteristics, the assessment of bedding-plane features and markings help to diagnose pre-depositional processes, usually in the form of erosional structures, and post-depositional processes. These types of bedding-plane features are classified as: tool-marks (those structures formed by the bouncing, prodding, scrapping, or rolling of loose sedimentary particles into semi-firm substrates as they are moved along in bottom currents); scour marks (those structures formed by the formation of small turbulent eddies, localized spiral flows, and laminar sheet flows along the sea-bottom); and load casts (structures produced by the foundering or subsidence of rapidly deposited sediments down into a soupy, unconsolidated underlying sediment body). Most often these structures are found as casts on the bottom of beds and represent, as in the case of flute marks shown to the right, pre-depositional flow erosion followed by deposition and sediment filling of eroded negative areas on top of underlying bed.

Stacked sedimentary structures: Clues to depositional processes: Aside from the recognition of very specific individual depositional structures, it is very common to find a number of these depositional structures in close juxtaposition to one another in a single unit. Most often the association of these closely juxtaposed structures is ascribed to key depositional processes that impact a range of depositional environments from the shallow shelf through deeper-water slope environments.

As described previously, the development of storm currents, as well as a variety of gravity induced flows, tend to be quite complex and highly variable in flow speed, flow turbulence, and sediment load. It is possible to produce a succession of internal beds that reflect the evolution of various event beds. Most often considered in this framework are tempestite deposits and turbidite deposits, both of which originate in relatively shallow or proximal settings, and fade out into distal or deeper water settings as both energy and sediment content decrease. Historically the recognition of tempestite versus turbidite deposits has been based on the establishment of a number of key criteria resulting from the ideal or conceptualized models of how deposition changes throughout the course of either of these event bed types. Shown below to the left are two comparative depositional bed sets that illustrate the pattern of succession expected in proximal tempestites and proximal turbidites. >>Back to Top

Tempestites: Within the context of a tempestite deposit, proximal settings should record the following: 1) pre-depositional high-energy event in the form of a scour or erosional surface down into pre-existing shelf muds, 2) a massive normally graded, fining upward skeletal shell-bed representing initial deposition from traction bed loads followed by less-dense sediments from a quieting turbulent flow regime, 3) a coarse-to-fine grained, planar laminated horizon deposited during the initial change to laminar, non-turbulent lower energy flows, 4) hummocky cross-stratified horizons resulting from shutdown in laminar, unidirectional flows and residual interactions between storm waves and bottom currents, 5) another layer of planar laminated fine-sands deposited as a result of rapid deposition during storm waning period, and 6) wave-rippled intervals reflecting last storm energy dissipation.

As tempestite induced bottom flows migrate down-slope, they often pick-up additional unconsolidated materials during their movement. Then, as energy subsides in distal regions, individual components of the tempestite succession begin to drop off the bottom and the top, so that in the most distal settings, only a few repetitive bedding signatures remain. In the image to the right below, modified from Einsele (1998), a distal tempestite succession is drawn showing the relationship between background sedimentation (dark grey) and distal tempestite deposition. In this case, distal tempestites continue to show a sharp, potentially erosional base, that is overlain by a laminated, cross-bedded sand unit followed by a laminated suspensional settling mud layer. Given the dynamics and timing of storm influenced currents, and the areal extent of deposits, it is sometimes easy to recognize storm-dominated deposits based on these outlined criteria.

Turbidites: Gravity-induced flow processes including turbidity flows also can produce characterisitc successions of beds, often referred to as Bouma Sequences. These "sequences" are modeled below for an ideal proximal tempestite to the left, and a proximal turbidite to the right. In the succession, two of the major differences between turbidites and tempestites lie in the nature of the basal and capping intervals of the successions. In the proximal carbonate turbidite diagram, basal beds can transition from normally graded beds to highly chaotic or reverse grading and then back to normal grading, all within a basal massive bed. The development of this succession of beds is related to the relatively dense and cohesive nature of turbidite flows relative to storm flows. At the cap of the tempestite-turbidite succession, the lack of wave-rippled cross-stratified sands and presence of vertically burrowed laminated/graded mud deposits in the upper portion of the turbidite succession help to differentiate it from a tempestite. Due to a variety of reasons, turbidites can also show development of chert bands in the uppermost beds.

In the distal turbidite, two main lithologies remain in most settings. That is the background depositional layer, and the turbidite layer that ranges from a rather homogeneous thinly-bedded, laminated to normally graded barren mud horizons. Most lithologies in the Trenton do not show this most distal facies as they usually contain somewhat more fossiliferous conditions. The basal Napanee, the lower to middle Sugar River, and some intervals in the overlying Denley have some structures characteristic of distal turbidites; however, in most cases, the bioturbation typical at the top-most contact of the bed usually penetrate and homogenize most bedding planes.

Most studies of the Trenton at Trenton Falls have focused primarily on the stratigraphy, aspects of the paleontology, or aspects of foreland basin dynamics of the Trenton Limestone. Very few sedimentologic investigations have been published regarding the Trenton. In the case of those that are published, these research reports generally look at narrow intervals of the Trenton and not the unit as a whole. However, the rather distinctive facies patterns of the Trenton Limestone have been investigated in other areas of the Trenton Shelf, including those areas immediately north and west of New York State. Studies by Brett and Brookfield (1984) and Brookfield and Brett (1988) helped to substantiate the lithologic criteria for subdividing individual lithofacies within the Trenton and establishing relative depositional histories for each of their delineated lithofacies. Some of the details of this report are discussed in the section on "Depositional Environments". In any case, having established that the Trenton overall represents a range of quiet-water, sub wave-base to sub-storm wave base depositional environments, the question still remains as to what mechanisms were responsible for the final deposition of these interbedded shale and carbonate beds.

Having already documented the rather distinctive set of debris flow/slump flow beds in the form of the two disturbed zones, the rest of Trenton lithofacies remain enigmatic in their depositional history. The controversy lies in the distinction between tempestite and turbidite deposition. 

Tempestite influenced sedimentation in the Trenton?

In their 1988 paper, Brookfield and Brett made a pretty strong stand for storm influenced deposition along the shoal-basin transect from Ontario to New York State. The observation of thin dark shales complexly interbedded with a variety of micrite-rich skeletal wackestones, packstones and grainstones lithologies combined with a variety of graded bedding and cross-lamination features helped these authors to establish their interpretation of storm influenced sedimentation. Moreover, given the oscillatory or repetitive nature of these interbeds, the periodic patterning suggests that whatever the process forming the interbeds was, it was related to periodic high-energy events that resulted in the rapid deposition of some layers followed by longer periods of low-energy deposition. As Brookfield and Brett were able to differentiate patterns of deposition similar to those discussed previously for tempestite successions (shown also on the figures above), they were able to confidently establish this process as an important one in the deposition of the Trenton Limestone. 

Turbidite influenced sedimentation in the Trenton?

Brookfield and Brett (1988) and Titus, (1974) advocated for storm-influenced depositional processes for the Trenton, but studies of foreland basin sedimentation of the Trenton Group by Mehrtens (1984, 1988, 1992), have advocated an alternative depositional mechanism for some individual layers if not the majority of deposits within the Trenton. The following figure is adapted from Mehrtens (1992), to illustrate a composite stratigraphic section for the Trenton Falls area. In the diagram Mehrtens has highlighted the importance of turbiditic flows as a major depositional process in the Trenton Limestones, especially in the middle Trenton. The figure shows the relative stratigraphic position of each individual turbidite horizon studied, as well as the positions of the "lower" and "upper disturbed zones." Within the Trenton the occurrence of turbidites in the upper part of the Russia Member, as well as in the Middle and Upper part of the Rust Formation is clearly noted. Of some 34 different turbidite horizons studied, the average thickness of the horizons was approximately 6.5 cm.

In order to establish that these individual beds were indeed formed by turbidity flows, Mehrtens argued that the pattern of graded bedding collectively with the pattern of scour and interbedded shales (between turbidites) favored turbidity rather than tempestite induced deposition.

The next two diagramas modified from the three publications by Mehrtens (1984, 1988, and 1992) show, again, the ideal pattern of development of a given turbidite succession. Color coded in the diagrams are those individual horizons within which a variety of flow regimes impart a different depositional effect in the development of an individual turbidite. For ease of comparisons to the Trenton turbidite horizons she studied, Mehrtens used the lettering scheme after Walker (1965).

The following figure represents the general results from Mehrtens' study of turbidites both in the Trenton Denley and in the down-ramp equivalent facies of the Dolgeville.

In her study, Mehrtens differentiated individual turbidite horizons and their internal layering structures. From point count studies, Mehrtens was able to plot the relative composition of the variously developed carbonate layers. Clearly illustrated in the diagram is the overall pattern of fining-upward (coarsest materials in the "Turbidite A" layer and finest micritic materials in the "Turbidite D' " layer. Interestingly, when compared to the Dolgeville facies, the Denley Limestone turbidites show increased allochem and spar values which suggest a more proximal pattern, thus relegating the Dolgeville facies to much more distal facies than that of the Denley.

Collectively then, given the paucity of arguments for tempestite and turbidite deposition, it is likely that the deposition of the Trenton Limestone experienced both phenomena in alternation with background sedimentation processes. The relative dominance of either one of these processes was likely influenced by the relative water depth during sea-level rise and fall events, as well as by tectonic induced steepening of the distal ramp region during the onset of Taconic Orogenesis.