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Formation

FIGURE 4.10. Simplified stratigraphy of the Cambrian-Ordovician rocks in the Taconic allochthon. Adapted from Landing (1988), with permission.

nant or when productivity in the surface waters was increased, allowing the accumulation of organic matter. These intervals also contain beds of limestone breccia or angular conglomerates that represent debris flows broken from the edge of the continental shelf that avalanched down into deeper water. Actually, a rather diverse fauna of trilobites has been obtained from some of the limestone breccias or conglomerates.

The dark facies include the Lower Cambrian Browns Pond Formation and the Lower to Upper Cambrian Hatch Hill Formation (a 150-m interval of dark slaty shales) (Figures 4.6C and 4.11). Both units contain numerous interbeds of several types, including ripple cross-stratified sandstones that probably record turbidites. Turbidity currents (masses of suspended sediment that flow downslope under the influence of gravity) swept finegrained siliciclastic silt and sand off of shelf regions into the deeper water. Another common type of bed consists of light-gray weathering bands of very fine-grained limestone. Careful examination of some of these beds reveals that they contain very fine laminations or even cross-laminations and display sharp bases. Therefore, they have been inferred to have been deposited as relatively dilute turbidites of suspended carbonate silt and mud that were exported into the deeper water from the carbonate bank near the top of the slope. Such deposits rarely contain fossils, though trilobite fragments are known from some turbidites. Another type of accumulation within the Taconic dark shale consists of brecciated (fragmented) limestones. Perhaps the best known of these is the Lower Cambrian "Schodack Limestone" found near Castleton Cutoff in the Hudson Valley. Relatively few organisms could actually live in the low-oxygen deeper waters. But the remains of shallow-water organisms were abruptly transported into some of these environments as debris flows from the shallow-shelf regions above where these animals lived. The fossiliferous breccia beds are a key to the stratigraphy of the lower part of the Taconic rocks. These thin limestone-clast conglomerates contain a sandy matrix that yields abundant fossils of a variety of trilobites, including agnos-tids, brachiopods, and some of the world's oldest bivalves. These fossils, particularly the trilobites, are invaluable for dating the succession.

A few Lower Cambrian allochthonous rocks are rich with fragmentary, small trilobites, particularly agnostids and eodiscids. Rasetti (1946, 1952, 1966a, b, 1967) with Theokritoff (1967) reported the following:

MIDDLE GRANVILLE CaJodiscus Jobotus Fordaspis nana Serrodiscus speciosus

F ORM ATIO N EJliptocephaJa Kootcnia fordi asaphoides

The dark shaly intervals with more numerous interbeds represent a period of time either when the bottom was more stag-

LOWER CAMBRIAN

Acidiscus birdi

Acimctopus biJobatus AnaJox obtusa

Bathydiscus doJichometopus

BoJboparia superba

CaJodiscus fissifrons

CaJodiscus meeki

Acidiscus hexacanthus

AnaJox bipunctata Atops triJineatus

BoJboparia elongata

CaJodiscus agnostoides

CaJodiscus Jobatus

CaJodiscus occipitaJis

FIGURE 4.11. Lower Cambrian allochthonous beds of the low Taconic Mountains. Thin-striped white beds in the lower view are lime turbidites. A thin bed of broken or brecciated limestone (a) occurs (to the left of the hammer). Upper beds are black shaly Hatch Hill Formation (b). Rte. 9 south of Hudson, Columbia County.

FIGURE 4.11. Lower Cambrian allochthonous beds of the low Taconic Mountains. Thin-striped white beds in the lower view are lime turbidites. A thin bed of broken or brecciated limestone (a) occurs (to the left of the hammer). Upper beds are black shaly Hatch Hill Formation (b). Rte. 9 south of Hudson, Columbia County.

Calodiscus

reticulatus

Calodiscus

schucherti

Calodiscus

theokritofft

Calodiscus

walcotti

Chelediscus

chathamensis

Elliptoccphala

asaphoides

Eoagnostus

acrorhachis

Fordaspis

nana

Hyolithellus

micans

"Kochiella"

fitchi

Kootenia

fordi

Leptochilodiscus punctulatus

Litometopus

longispinus

Seopagetina

taconica

Oodiscus

binodosus

Oodiscus

subgranulatus

Pagetia bigra nulosa

Pagetia connexa

Pagetides

amplifrons

Pagetides

elegans

Pagetides

leiopygus

Pagetides

minutus

Pagetides

rupestris

Peronopsis

evansi

Peronopsis

cf. P. primigenea

Prozacanthoides eatoni

Rimouskia

typica

Serrodiscus

griswoldi

Serrodiscus

speciosus

Serrodiscus

spinulosis

Serrodiscus

subclovatus

Stigmadiscus

gibbosus

Stigmadiscus

stenometopus

Weymouthia

nobilis

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