The Lathrop Wells volcanic center: Status of field and geochronology studies Page: 4 of 19
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Special caution must be used in interpreting the vent
configurations and geometry of basaltievolcanic centers.
The pulsating dynamics of eruption columns and the
variability in magma effusion rates can produce
conspicuous unconformities and complexities in the
stratigraphic relationships of volcanic centers. These
complications can develop from eruptions over periods
of hours, days, months or years. For example,
eruptions of the Pu Oo vent on the flank of Kilauea
opened first with a fountaining eruption along a fissure,
then focused to a central vent. During the first few
years of eruptive activity, a symmetrical spatter cone
was formed, followed in time by an asymmetrical scoria
cone, shaped by the prevailing trade winds.
What we have discovered, through detailed multi-
disciplinary studies, is that some volcanic units at the
Lathrop Wells center are separated by soil-bounded
unconformities. The degree of horizon development in
the soils requires time between eruptions that far
exceed the cooling times of small volumes of basalt
magma in the shallow crust. It is these features that
lead to the inference of eruption of spatially and
temporally distinct batches of magma. We refer to
these eruptions as polycyclic and consider them to be a
subclass of polygenetic volcanoes. Polygenetic
volcanoes typically are large volume volcanoes, often
associated with shallow magma chambers. By contrast,
we use the term polycyclic to refer to intermittent
eruptions at small volume volcanoes (< 1 kn3) that are
normally classified as monogenetic.13
New field and stratigraphic evidence described above
suggests the original five lithostratigraphic units can
now be separated into at least seven lithostratigraphic
units (Fig. 2). These seven lithostratigraphic units can
be grouped into three chronostratigraphic units. We
infer that there is time significance to these units but
cannot define their temporal boundaries with
satisfactory confidence based on existing chronology
data. We recognize that the chronostratigraphic units
may change as additional geochronology and field data
are obtained. The current (December, 1991) division
of chronostratigraphic units for the Lathrop Wells
volcanic center are described below, from oldest to
youngest.
Chronostratigraphic Unit Three: The oldest
identified chronostratigraphic unit at the Lathrop Wells
center consists of three lavas, with minor vent scoria,
exposed at separate localities. The first lava is buried
largely by eolian sand and silt on the north flank of the
center (Fig. 2). It was correlated tentatively with the
Q15 lava.8 However, geochemical studies have not
confirmed this correlation.' Construction of shallow
trenches revealed that the buried flow rests upon-pyroclastic surge deposits.--The flow, in turn, is overlain
by primary and reworked surge and scoria-fall deposits
containing a soil with distinct horizon development.
This soil has been distmrbed discontinuously by
extensive bioturbation. Excavation of the buried flow
and overlying tephra/soil units showed that this
sequence underlies outcrops of the Ql lava. We thus
have identified an older lava of the center that is
separated from the Q14 lava sequence by a soil-bounded
unconformity. No age determinations or paleomagnetic
data have been obtained for the flow. We recently
drilled and sampled this unit and are now processing
the samples for paleomagnetic determinations.
The second lava unit of chronostratigraphic unit 3
consists of blocky aa lava flows and local cone scoria.
This unit was mapped originally as part of Ql
However, detailed petrology, and geochemical studies
have shown that one part of the Qs unit is
petrographically, geochemically, and isotopicallydistinct.
This unit has been labeled Qli on the revised geologic
map (Fig. 2). Unmodified exposures of the unit are
exposed along a narrow band at the southwest edge of
the center. Here the Ql6 unit consists of aa lava flows
and cone scoria. However, much of the original
distribution of the unit has been either removed or
covered by scoria excavation at the commercial quarry
site. The basal contact of the Q4 lava is elevated
above the modern pavement surface and base level. It
is the only volcanic unit in the Lathrop Wells volcanic
center that shows this relation. Additionally, exposure
edges for this unit are not flow margins. There has
been sufficient erosional stripping of the lava margin to
expose the massive aa flow interior which is underlain
and overlain by flow breccia and clinker. No K-Ar or
paleomagnetic data have been published for this unit.
We have sampled the unit for isotopic dating using the
U-Th disequilibrium method. Paleomagnetic studies of
this unit are in progress.
The third lava unit of chronostratigraphic unit 3 is
Qls, a series of lavas that crop out along the southern
part of the Lathrop Wells center (Fig 2). These lavas
were inferred to be derived from a fissure (Oss) that
extends northwestward, along the east base of the main
cone.8 This interpretation was based on the reported
uniformity of the field magnetic directions for these
units.9 Several new observations indicate that this
conclusion may be only partly correct. First, careful
examination of the paleomagnetic sample sites has
shown that the Qls lavas were not sampled or analyzed
for the paleomagnetic studies.' The basis for their
correlation with the northwest-trending fissure cannot
be confirmed. Second, petrologic studies suggest that
the Qls lavas are distinct compositionally from most of
the northwest-trending fissure.u Finally, we have
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Crowe, B.; Morley, R.; Wells, S.; Geissman, J.; McDonald, E.; McFadden, L. et al. The Lathrop Wells volcanic center: Status of field and geochronology studies, report, March 1, 1993; New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc619170/m1/4/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.