<html><head><style type="text/css"><!-- DIV {margin:0px;} --></style></head><body><div style="font-family:arial,helvetica,sans-serif;font-size:12pt"><div style="font-family: arial,helvetica,sans-serif; font-size: 12pt;"><div style="font-family: times new roman,new york,times,serif; font-size: 12pt;"><strong><em>La Asociación
Argentina de Sedimentología y el Departamento de Ciencias Geológicas<br>de la
UBA tienen el agrado de invitar a la conferencia del Dr. Allen W.
Archer:<br><br></em></strong><br><strong>"TIDAL RHYTHMITES IN MODERN AND ANCIENT
HYPERTIDAL ESTUARIES</strong>"<br><br> Aula Amos del Departamento de
Ciencias Geológicas, FCEyN, UBA. Ciudad<br>Universitaria, Pabellón II. Ciudad
Autónoma de Buenos Aires.<br><br> <strong>Viernes 19 de Marzo, 14
hs.<br><br></strong><br>El Dr. Allen W. Archer (PhD University of Indiana) es
actualmente profesor de<br>Estratigrafía y Sedimentología en la Univesidad de
Kansas, USA. Se ha especializado<br>en ciclicidad y ritmicidad en depósitos
originados por mareas (tidal rhythmites), de<br>edades que van desde el
Precámbrico hasta el presente, particularmente en estuarios<br>con rangos de
mareas extremos.<br><br><br>Resumen de la conferencia:<br><br>Tidal rhythmites,
which exhibit systematic variations in lamina thickness, are most<br>common in
hypertidal estuaries. Modern examples include the Bay of Fundy in
eastern<br>Canada (tidal range >13 m), Ungava Bay in northeastern Canada
(>12 m), Bay of Mont<br>Saint Michel in France (>11.5 m), estuaries along
the Atlantic coast of southern<br>Argentina (>11 m), and Cook Inlet in
south-central Alaska , USA (>10 m). These<br>modern examples provide analogs
for ancient rhythmites, which have been documented<br>from the Proterozoic and
Phanerozoic eons. Within the spectrum of astronomically<br>forced cycles,
short-term periods exhibited by tidal rhythmites include semi-daily,<br>daily,
half lunar month (neap-spring), and several lunar monthly cycles. All
of<br>these periodicities have been reported from modern and ancient of tidal
rhythmites.<br>Short-term tidal periods have quite consistent magnitude and
frequency and thus can<br>directly affect sedimentation. Over geologic time,
however,<br>tidal periods have changed. Examples of very ancient rhythmites
indicate that the<br>paleotidal day was significantly shorter and that there
were more solar days per<br>year. The rates of sedimentation within hypertidal
estuaries can be very high,<br>particularly in the fluvio-estuarine transitional
zones. Because of the high<br>turbidity and high sedimentation rates, sediment
reworking by the biota can be<br>limited. Thus, bioturbation is minimal. These
deposits are dominated by physical<br>sedimentary structures, such as tidal
bedding, ripples, and thin laminae.<br><br><br><br></div></div>
</div><br>
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