Paralic (marginal marine) environmentsGEOL 342 Sedimentation and StratigraphyLecture 11: Eolian, glacial, and paralic environments14 March 2005Assoc. Prof. A. Jay KaufmanEolian environmentsEolian sand dunes are among the best-known depositionalenvironments because of their spectacular cross beds and their use aspaleoenvironmental indicators. High angle (20 to 35o) cross beds; individualbeds up to 35 meters (there is no depth limitation to the air) with generallylonger asymptotic bottom sets than marine dunes. Most beds are tabular-planar, or trough or wedge-shaped. Wind ripples are typically lower inamplitude and more assymetrical than water ripples. Sand dunes are best known from desert environments. A desert is anarid region, which generally lacks vegetation and cannot support a largepopulation. Many deserts occur where dry air descends along the 30oN and Slatitudes in areas of persistant high pressure, or behind mountain ranges thathave a rain shadow. Deserts can also occur in coastal areas, arctic regions,and far inland in continental interiors..Wind has much less competence to entrain particles than does water(it is some 700x less viscous) so eolian processes are very different thansubaqueous processes. 1How then might we differentiate between a beach sand dune and aneolian sand dune?Wind has much less competence than water to pick up particles, so thecoarsest material is often left behind as a deflation lag, or desert pavement.Deposits are very well sorted, well rounded, and surfaces of quartz(predominantly, but not always) are pitted and frosted.Glacial environmentsOnly about 10% of the Earth’s surface is currently covered bypermanent glacial ice and associated glacial deposits, but these arenonetheless important indicators of cold climate and have been intensivelystudied. During the Pleistocene ice age as much as 30% of the surface wasglaciated, but in older events the coverage has been considered by some tobe complete.2During a so called “Snowball Earth” event, which may have lasted formillions of years, it is suggested that there would have been a nearlycomplete shutdown of surface processes due to extreme albedo and cold,including oceanic photosynthesis, the hydrologic cycle, as well as weathering(more on this latter).How would the Earth ever have gotten out of this mess?Glaciations occurred in several discrete intervals of Earth history,including the Paleoproterozoic, Neoproterozoic, Ordovician, Carboniferous,and, of course, the Pleistocene, which we know the most about.Although glacial environments have been primarily studied bygeomorphologists and Quaternary geologists, they still deserve attentionfrom sedimentologists and economic geologists as climate indicators. Theprimary glacial deposits are often massive, but poorly sorted due to theinability of ice to sort materials. Glacial deposits (tills and diamictites) mostoften lack internal stratification, but these should not be immediatelyinterpreted as glacial in origin as other types of mass wasting, includingdebris flows and mud flows. Indicators that a diamictite is glacial include thepresence of dropstones, as well as striations on clasts or on bedrockpavements. In Proterozoic examples, the glacial deposits are often overlainby uniquely textured and isotopically anomalous carbonates, called “capcarbonates”, which were formed during post-glacial sea level transgression.3Most glacial deposits have a lodgement till at the base, which may beoverlain by braided gravel bars or cross-bedded sands and gravels fromoutwash streams, or possibly by varved lacustrine mudstones, which maycontain dropstones.Paralic (marginal marine) environmentsThe vast majority of sedimentary record accumulated in marine settings on continentalshelves. As such, most land aquifers & hydrocarbon reservoirs are associated with theselocations. In addition, coastal land uses are critically linked to coastal processes, which can bestudied in modern and ancient successions. Recognition:Paralic setting are diverse in occurrence, and a number of schemes have been used tobreak out these specific systems. All schemes differentiate based on the primary sedimentarytransport mechanism, tied to hydraulic energy. They also look for biological signatures as well tohelp identify specific environments.Classification:Paralic systems fall into three broad categories:Name Main sedimenttransport agentMain sedimentarystructuresBioloigalsignatures(trace fossils)Map patternDeltaicRiver transport,turbidity currentsUnidirectional current Low diversity,low abundancePaddle-shapedShorefaceWave action Oscillation ripples anddunes (hummocks) High diversity,high abundanceLong and linearTidalTide action Unidirectional, butorganized by regularLow diversity,moderateNarrow andentrenched4energy variation(Sigmoidal, flaser)abundance Deltas: Rivers bring sediment to the coast. When they do, the flow of the river expands anddecelerates. As it does, the flow quickly drops its sedimentary load. This means that sedimentaccumulates rapidly at the river front, forming a delta.Deltaic sub-environments (from distal to proximal), include the following:1) Prodelta – fine grained distal mudstones2) Delta front – thin bedded turbidites, increasing invertical amalgamation upwards. This is the steeply-dipping part of the delta.3) Mouth bars – channelized sands that accumulate atthe mouth of distributary channels. They fail, makingthe turbidity currents that accumulate on the deltafront. These are characterized by massive bedding,trough cross-bedding, and climbing current ripplesand dunes.Due do the high volume of sediment coming into the system and the rapidly changingsalinity, deltas are stressed environments and are commonly poor in organisms. Shorefaces/Beaches: One the sediment accumulates in deltas, wave action moves the sedimentalong the coast by long-shore drift to form beaches. The wave action sorts the sand by energylevel, creating a beach profile. The different zones in a beach profile are a functin of regular andstorm wave energy.1)