Presentation on theme: "Harry Williams, Historical Geology1 HISTORICAL GEOLOGY LECTURE 2. SEDIMENTARY ENVIRONMENTS AND PALEOGEOGRAPHY Sedimentary rocks, much more so than igneous."— Presentation transcript:
Harry Williams, Historical Geology1 HISTORICAL GEOLOGY LECTURE 2. SEDIMENTARY ENVIRONMENTS AND PALEOGEOGRAPHY Sedimentary rocks, much more so than igneous or metamorphic, provide a geologic history of a region. ANCIENT ENVIRONMENTAL CONDITIONS LITHOLOGIC AND BIOLOGIC CHARACTERISTICS OF SEDIMENTARY ROCK INTERPRETATION OF ENVIRONMENT OF DEPOSITION Examples….?
Harry Williams, Historical Geology2 Environmental Conditions: Two major components = tectonic setting and environment of deposition. TECTONIC SETTING - loosely defined, refers to presence/absence of tectonic activity. Some regions, usually younger, are tectonically active - these are OROGENIC BELTS: Volcanism and vertical crustal movements result in mountains, which are high energy environments and undergo rapid erosion – they are therefore sediment sources; adjacent lowlands and oceans become sites of sediment accumulation.
Harry Williams, Historical Geology3 Other regions, usually older, are tectonically stable and more subdued geologically. These are CRATONS - the ancient, eroded interior "core" of continents; usually consisting of deformed igneous and metamorphic rocks. Shield areas are exposed areas of cratons; platforms are parts of the craton covered by a relatively undeformed blanket of sedimentary rocks. These areas are much less active tectonically. The tectonic setting influences: a) the type of sedimentary rock e.g. rugged mountains -> high energy -> conglomerates and sandstones. Subdued hills, plains -> lower energy -> shales. b) thickness of rock i.e. high rate of sediment supply -> thick accumulation of sediment.
Harry Williams, Historical Geology4 ENVIRONMENTS OF DEPOSITION: Environmental conditions influence many aspects of sedimentary deposits such as texture, fossils and primary sedimentary structures.
Harry Williams, Historical Geology5 TEXTURE (SIZE). Particle size in clastic sedimentary rocks reflects the ENERGY of the depositional environment. E.g. (above) Nearshore - waves crashing on beaches - > fairly high energy -> coarse textured deposits (pebbles/sand); offshore -> progressively lower energy environments -> progressively finer textured deposits - medium sand - fine sand - silt/mud - clay - carbonates (beyond land- derived sedimentation in shallow tropical oceans).
Harry Williams, Historical Geology6 Other environments are also reflected by texture e.g. Fine-grained sandstone from desert sand dunes. Coarse-grained conglomerate from debris flows, pebbly beach, mountain stream.
Harry Williams, Historical Geology7 PRIMARY SEDIMENTARY STRUCTURES: These are structures within the deposit that form during deposition e.g. mud cracks -> swamps, tidal marshes (places that dry out – not marine)
Harry Williams, Historical Geology8 Stratification or layering indicates deposition in water e.g. rivers, glacial meltwater, lakes, oceans.
Harry Williams, Historical Geology9 ripple marks -> dunes, tidal flats, river beds
Harry Williams, Historical Geology10 cross bedding -> deltas, sand dunes, river deposits Planar cross beds, Woodbine sandstone, Dallas.
Harry Williams, Historical Geology11 Planar cross beds in modern and ancient sand dunes.
Harry Williams, Historical Geology12 EXAMPLES OF SEDIMENTARY ROCKS RELATED TO DEPOSITIONAL ENVIRONMENTS. 1. SANDSTONES: Sandstones vary in quartz content, grain rounding & matrix percentage.
Harry Williams, Historical Geology13 a) Quartz sandstone - predominantly quartz grains ("clean sandstone"). Long transportation (quartz survives long transportation because it is relatively hard). Distant from mountainous regions, tectonically stable. Often form at coastlines, in deserts, on higher energy coastal plains and river floodplains (e.g. Padre Island). Quartz grains make up 90%+ of rock and the grains are well rounded. Cross beds and ripples are common.
Harry Williams, Historical Geology14 Clean quartz sandstone.
Harry Williams, Historical Geology15 b) Arkose - terrestrial; derived from granitic highlands, contain > 25% feldspar grains (implies fairly short transportation, because feldspar is relatively soft and erodes over long distances). Commonly pink-red color.
Harry Williams, Historical Geology16 c) Graywacke – mixture of sand, clay and rock fragments ("dirty sandstone"). Indicates tectonic activity, rapid erosion/sediment accumulation, short transportation. Often deposited as turbidites (submarine landslide deposits). Matrix is usually 30%. Beds are often graded (sorted by size - coarse at the base, finer at the top).
Harry Williams, Historical Geology17 Hand specimen and thin section of graywacke.
Harry Williams, Historical Geology18 d) lithic sandstone - typical of deltaic deposits e.g. Mississippi delta. Matrix < 15%. Transitional between quartz sandstones and graywackes.
Harry Williams, Historical Geology19 SHALES: Form in similar environments to sandstones, only deposited under lower energy conditions (i.e. "quieter" locations) -> finer particles (clay, silt). Shallow marine, marshes, lakes, lower energy coastal plains and floodplains. Finely layered, often fissile. Common fossils.
Harry Williams, Historical Geology20 CARBONATES: Most common = limestone (calcium carbonate). Formed by abundant marine organisms and the precipitation of calcium carbonate from sea water. Warm, clear, shallow tropical oceans - particularly common in platform areas.
Harry Williams, Historical Geology21 NAMING ROCKS: Because sedimentary rocks are so useful in deciphering the past, there must be a way of precisely referring to them to avoid confusion or errors. The fundamental rock unit is the FORMATION - "a lithologically distinct body of rock”. A formation normally has 2 names; the first its geographic locality, the second its lithology eg. DENTON SANDSTONE. However, a formation is not necessarily of a uniform lithology; for example, a thick sandstone with interbedded shale layers could be a formation: in these cases the locality name is simply followed by "formation" i.e. DENTON FORMATION. The lithologically distinct subdivisions of the formation are referred to as MEMBERS. Several adjacent formations may also be combined into a GROUP. MEMBERS, FORMATIONS and GROUPS all constitute ROCK UNITS.
Harry Williams, Historical Geology22 Time unit Rock units
Harry Williams, Historical Geology23 The major inadequacy in this approach is that no regard is made to TIME BOUNDARIES and, consequently, formations may be DIACHRONOUS: This sandstone layer may be a lot older to the right than to the left.
Harry Williams, Historical Geology24 CHRONOSTRATIGRAPHIC UNITS (or Time-Rock Units). To overcome this major shortcoming of rock units; the chronostratigraphic unit was introduced; this refers to sedimentary rocks deposited during the same time period. Unlike rock units, lithology can vary greatly within chronostratigraphic units, and the upper and lower boundaries consist of time-planes. The fundamental chronostratigraphic unit is the SYSTEM, which corresponds to a PERIOD of geologic time. The importance of the chronostratigraphic unit is that it enables PALEOGEOGRAPHIC MAPS to be constructed (i.e. maps showing the geography of a region at a given instant in the geologic past). A good example would be to divide a chronostratigraphic unit into continental (terrestrial) and marine deposits and thereby map the location of former shorelines.
Harry Williams, Historical Geology25
Harry Williams, Historical Geology26 FACIES. The example shows how time-rock units can be used to map paleogeography - but an important question is.."how is the distinction between terrestrial and marine deposits made?" The answer obviously involves PALEOENVIRONMENTAL INTERPRETATION and introduces another concept in historical geology - THE FACIES CONCEPT. A FACIES is a description of a sedimentary rock which is intended to aid in the interpretation of paleoenvironments. Examples: 1. Trough cross-bedded, clean well-rounded, quartz sandstone. 2. Reef-coral limestone. 3. Organic-rich, shales, containing abundant clam fossils and bulrush pollen. So a paleogeographic map is also a FACIES MAP. A series of successive facies maps -> shifting of facies through time -> changing environmental conditions.
Harry Williams, Historical Geology27 SEA-LEVEL CHANGES: Examples of major changes in environmental conditions are sea- level changes, which have occurred frequently in the geologic past. Global, or EUSTATIC, sea-level changes have resulted in inland seas, referred to as EPEIRIC SEAS, covering as much as 2/3 of the North American continent. Much of the sedimentary rock record of the Paleozoic and Mesozoic Eras were deposited during these periods of marine inundation. Characteristic vertical facies sequences are created by transgressions and regressions. Coastal facies
Harry Williams, Historical Geology28 In a marine regression (falling sea-level) nearshore facies migrate out over offshore facies, resulting in a coarsening-up stratigraphic sequence.
Harry Williams, Historical Geology29 The opposite occurs in a marine transgression, resulting in a fining-up sequence.