HISTORICAL GEOLOGY LECTURE 2. SEDIMENTARY ENVIRONMENTS AND PALEOGEOGRAPHY

Harry Williams, Historical Geology 1

HISTORICAL GEOLOGYLECTURE 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….?


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.


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.


ENVIRONMENTS OF DEPOSITION:

Environmental conditions influence many aspects of sedimentary deposits such as texture, fossils and primary sedimentary structures.


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).


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.


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)


Stratification or layering indicates deposition in water e.g. rivers, glacial meltwater, lakes, oceans.


ripple marks -> dunes, tidal flats, river beds


cross bedding -> deltas, sand dunes, river deposits

Planar cross beds, Woodbine sandstone, Dallas.


Planar cross beds in modern and ancient sand dunes.


EXAMPLES OF SEDIMENTARY ROCKS RELATED TO DEPOSITIONAL ENVIRONMENTS. 1. SANDSTONES:

Sandstones vary in quartz content, grain rounding & matrix percentage.


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.


Clean quartz sandstone.


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.


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).


Hand specimen and thin section of graywacke.


d) lithic sandstone - typical of deltaic deposits e.g. Mississippi delta. Matrix < 15%. Transitional between quartz sandstones and graywackes.


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.


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.


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.


Timeunit Rock

units


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.


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.



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.


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


In a marine regression (falling sea-level) nearshore facies migrate out over offshore facies, resulting in a coarsening-up stratigraphic sequence.


The opposite occurs in a marine transgression, resulting in a fining-up sequence.

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HISTORICAL GEOLOGY LECTURE 2. SEDIMENTARY ENVIRONMENTS AND PALEOGEOGRAPHY