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Abstract
Chapter from: M
64: Sequence Stratigraphy of Foreland Basin Deposits
Edited By
J.C. Van Wagoner and G.T. BertramAuthors:
David C. Jennette and Clive R. Jones Seismic/Sequence Stratigraphy
Published 1995 as
part of Memoir 64
Copyright © 1995 The American Association of Petroleum
Geologists. All Rights Reserved. |
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Chapter 10
*
Sequence Stratigraphy
of the Upper Cretaceous Tocito Sandstone: A Model for Tidally Influenced
Incised Valleys,
San Juan Basin, New Mexico David C. Jennette
Clive R. Jones
Exxon Production Research
Company
Houston, Texas, U.S.A.
*
ABSTRACT
In the subsurface of the San Juan basin,
New Mexico, the Coniacian Tocito Sandstone is composed of four high-frequency
sequences. Basal sequence boundaries are marked by erosional truncation
of underlying strata and have mappable axes of erosion which are typically
narrow and linear. These erosional features are interpreted to be incised
valleys cut by fluvial and estuarine processes during lowstands in relative
sea level. Two end-member facies are found in the valley fills: (1) a low-energy
mudstone-dominated facies with thin-bedded and bioturbated sandstones,
and (2) a high-energy facies chiefly consisting of medium- to coarse-grained
glauconitic sandstone. Tidal indicators in this facies include double clay
drapes, flaser and lenticular bedding, and large- to small-scale sigmoidal
cross-bedding. Iron-cemented shale rip-up clasts, quartz and phosphatic
pebbles, sharks' teeth, and detrital fragments of Inoceramus and
oyster shells also characterize the Tocito Sandstone. Paleophycus,
Thalassinoides,
and
Planolites
burrows with locally abundant
Ophiomorpha burrows typify the low-diversity,
low- to moderate-abundance trace-fossil suite. The combination of lithofacies,
sedimentary structures, and ichnofauna indicates lowstand deposition within
tide-dominated estuarine systems. The sandstone-prone fills are considered
proximal facies connected to fluvial systems, whereas the shalier facies
are interpreted to be more distal fills laid down as the estuaries expanded
to form large embayments.
Valley systems show a sequential change
in geometry and fill type in vertical profile: older valleys are narrow
and sandstone filled and give way to broader systems which are filled with
strata which are more shale-prone. The result is a retrogradational stacking
of the sequences or a transgressive sequence set. This stacking pattern,
coupled with the erosional juxtaposition of the valley fills, created many
stratigraphic traps for hydrocarbons. The two principal trap types are:
(1) bends in valley axes where valley-fill sandstones
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