Structural relations or fossils may supplement the lithologic description on which a formation is based, but should not be the basis for a formation. wikipedia 5) If because of indistinct lithology, faunal or structural features appear more important, they must be very clear and distinctive – i.e. abundance vs. absence of fossils compared to over- and underlying 6) Boundaries between formations should be as sharp and as distinctive as possible. 7) Marker beds (distinct laterally traceable beds) may be good boundaries if they are laterally continuous, easily identifiable, and not repeated in a sequence. 8) Formation boundaries are not instants in time but unit should be about the same age throughout lateral extent. 9) Formations should be clearcut enough to be easily recognizable – and formations should be named in such a way to make them universally recognized by geologists – renaming should not be done unless it increases clarity 10)Distinct rules for naming – named from a locality – second part of the formation name is usually the dominant rock (or the word Formation) 11) Anything can be a formation but it should be primarily all one type or one group of types of rock throughout its lateral extent 12)Can be no duplication of names 13)Formations can be divided into named members or upper, lower, or middle 14)Formations can end laterally by pinching out, grading into another formations or be cut by a fault. 15)Upper and lower boundary should be easily recognizable. boundaries are called contacts and are normally drawn at horizons that involve marked lithologic change Now Do the Formations Exercise Facies Lithostratigraphic units are formed from lateral migration of facies over a period of time!! A. Facies = objective stratigraphic entities – the sum of all observable characteristics in a rock. Not the environment of deposition because this is an interpretation. Not reef facies but – coral sand facies. B. May include rock characteristics and fossil characteristics. Lithofacies vs. Biofacies. C. To most geologists, facies implies that different strata with different characteristics, are laterally contiguous and about the same age. a. Internationally, some geologists use this term also to interpret vertical changes in rock characteristic in a stratigraphic column. b. In the subsurface, we can make a map of facies that summarizes where lithologies are D. Facies distribution controlled by distribution of laterally coexisting paleoenvironments Sea level se e h t f o h c a e s in e i c a f e h t e b i descr u o y d l u o w s? H ow environment E. If environments were to remain in one place but subsidence allow for deposition, facies would stay in same place and build vertically – Would this be odd? F. If environments migrated laterally through time, the facies deposited would also migrate one on top of the other. This is what Walther’s principle (law) said back in 1893 – Law of correlation of facies = within a continuously deposited sequence of strata. The same succession of facies occurring laterally also occurs in vertical succession Slideshare.com Section through the Southwest Pass lobe of the Mississippi River delta, indicating growth of delta from 1764-1970. This is what makes formations – lateral migration of environments over time with deposition of their facies One location Another location This is what you see in the subsurface One location Another location This is what you interpret in cross section thickness horizontal distance One location Another location Facies distribution at time one when sea level is low downslope upslope Facies distribution at time two when sea level is higher downslope upslope Facies deposited after migration upward and laterally downslope upslope thickness horizontal distance Facies deposited after migration upward and laterally HERE IS WHAT YOU SEE IN YOUR OIL FIELD! thickness horizontal distance Facies deposited after migration outward as sea level falls downslope upslope thickness horizontal distance Facies deposited after migration outward as sea level falls downslope upslope thickness horizontal distance Facies deposited after shoreline migration landward and then seaward With subsidence providing accommodation thickness horizontal distance Facies deposited after shoreline migration landward and then seaward With subsidence providing accommodation The white lines show the time lines outlining depositional surfaces thickness horizontal distance Facies deposited after migration landward and then seaward HERE IS WHAT YOU SEE! thickness horizontal distance Facies deposited after migration landward and then seaward HERE IS WHAT YOU INTERPRET! thickness horizontal distance Transgression & Regression Now do the assignment on lithostratigraphic correlation Time and Stratigraphy Part I Local and regional correlation I. Knowing the age (or at least age equivalence) of the rocks is needed and wanted – Why? A. Study a reservoir in field but the next field is 300km away – Can we trust lithologic correlation over a 300km gap? B. Isolated reservoir or exposure We cannot physically correlate (rock correlation) so we need age of the rocks. C. Lithologic units are commonly diachronous (deposited at more than one time) on a large areal scale. (think about facies migration – one side is older than the other) Consequently, if we are to understand earth history, and predict the distribution of reservoir units, we must understand the time significance of rock units. II. Rock units vs. Time-Rock units vs. Time units. A. Rock units have little if any time significance – Fm. is just a three-dimensional body of rock, when and how it was deposited has nothing to do with time units. B. We relate time to rocks by measuring a type vertical section of strata somew

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